Mountain Scholar :: Browsing by Author "van den Heever, Susan C., advisor"

Browsing by Author "van den Heever, Susan C., advisor"

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Open Access
Aerosol impacts on deep convective storms in the tropics: a combination of modeling and observations
(Colorado State University. Libraries, 2012) Storer, Rachel Lynn, author; van den Heever, Susan C., advisor; Stephens, Graeme L., committee member; Johnson, Richard H., committee member; Eykholt, Richard, committee member
It is widely accepted that increasing the number of aerosols available to act as cloud condensation nuclei (CCN) will have significant effects on cloud properties, both microphysical and dynamical. This work focuses on the impacts of aerosols on deep convective clouds (DCCs), which experience more complicated responses than warm clouds due to their strong dynamical forcing and the presence of ice processes. Several previous studies have seen that DCCs may be invigorated by increasing aerosols, though this is not the case in all scenarios. The precipitation response to increased aerosol concentrations is also mixed. Often precipitation is thought to decrease due to a less efficient warm rain process in polluted clouds, yet convective invigoration would lead to an overall increase in surface precipitation. In this work, modeling and observations are both used in order to enhance our understanding regarding the effects of aerosols on DCCs. Specifically, the area investigated is the tropical East Atlantic, where dust from the coast of Africa frequently is available to interact with convective storms over the ocean. The first study investigates the effects of aerosols on tropical DCCs through the use of numerical modeling. A series of large-scale, two-dimensional cloud-resolving model simulations was completed, differing only in the concentration of aerosols available to act as CCN. Polluted simulations contained more deep convective clouds, wider storms, higher cloud tops and more convective precipitation across the entire domain. Differences in the warm cloud microphysical processes were largely consistent with aerosol indirect theory, and the average precipitation produced in each DCC column decreased with increasing aerosol concentration. A detailed microphysical budget analysis showed that the reduction in collision and coalescence largely dominated the trend in surface precipitation; however the production of rain through the melting of ice, though it also decreased, became more important as the aerosol concentration increased. The DCCs in polluted simulations contained more frequent, stronger updrafts and downdrafts, but the average updraft speed decreased with increasing aerosols in DCCs above 6 km. An examination of the buoyancy term of the vertical velocity equation demonstrates that the drag associated with condensate loading is an important factor in determining the average updraft strength. The largest contributions to latent heating in DCCs were cloud nucleation and vapor deposition onto water and ice, but changes in latent heating were, on average, an order of magnitude smaller than those in the condensate loading term. It is suggested that the average updraft is largely influenced by condensate loading in the more extensive stratiform regions of the polluted storms, while invigoration in the convective core leads to stronger updrafts and higher cloud tops. The goal of the second study was to examine observational data for evidence that would support the findings of the modeling work. In order to do this, four years of CloudSat data were analyzed over a region of the East Atlantic, chosen for the similarity (in meteorology and the presence of aerosols) to the modeling study. The satellite data were combined with information about aerosols taken from the output of a global transport model, and only those profiles fitting the definition of deep convective clouds were analyzed. Overall, the cloud center of gravity, cloud top, rain top, and ice water path were all found to increase with increased aerosol loading. These findings are in agreement with what was found in the modeling work, and are suggestive of convective invigoration with increased aerosols. In order to separate environmental effects from that due to aerosols, the data were sorted by environmental convective available potential energy (CAPE) and lower tropospheric static stability (LTSS). The aerosol effects were found to be largely independent of the environment. A simple statistical test suggests that the difference between the cleanest and most polluted clouds sampled are significant, lending credence to the hypothesis of convective invigoration. This is the first time evidence of deep convective invigoration has been demonstrated within a large region and over a long time period, and it is quite promising that there are many similarities between the modeling and observational results.
Open Access
Assessing the impacts of cloud condensation nuclei on cumulus congestus clouds using a cloud resolving model
(Colorado State University. Libraries, 2011) Sheffield, Amanda M., author; van den Heever, Susan C., advisor; Kreidenweis, Sonia, committee member; Eykholt, Richard, committee member
Cumulus congestus clouds are mid-level clouds that form part of the trimodal tropical cloud distribution. They act to moisten the atmosphere and may become mixed-phase in their lifetime. Congestus typically surpass the tropical trade wind inversion from where they may either develop into deeper convection, or alternatively remain as terminal congestus. Such growth is dependent on multiple factors, including those which alter the local environment and the microphysical structure of the cloud. This study investigates the impacts of cloud condensation nuclei (CCN) on cumulus congestus clouds through the use of large domain, cloud-resolving model (CRM) simulations in radiative convective equilibrium (RCE). Previous studies have focused on the convective invigoration of congestus and their subsequent growth to deep convection in association with ice processes. This study will focus on the response of congestus clouds to more polluted conditions, with particular emphasis on the development and growth of congestus from the warm phase to beyond the freezing level. It is found that convection is invigorated in the more polluted cases in association with the enhanced latent heat released during the vapor diffusional growth of cloud droplets in the warm phase. Such invigoration results in stronger updraft speeds, enhanced vertical lofting of cloud water, and a subsequent increase in the number of clouds growing to above the freezing level. The lofted cloud water is available to form more ice, however the ice water produced is smaller in magnitude compared to cloud water amounts above the freezing level. The low amounts of ice result in relatively insignificant contributions of the latent heat of freezing to the updraft strength. The impacts of enhanced CCN concentrations on various other cloud characteristics and microphysical processes are also investigated.
Open Access
Assessing the impacts of microphysical and environmental controls on simulated supercell storms
(Colorado State University. Libraries, 2018) Freeman, Sean William, author; van den Heever, Susan C., advisor; Rasmussen, Kristen L., committee member; Eykholt, Richard, committee member
Supercell thunderstorms are some of the most dangerous single-cell storms on the planet. These storms produce many hazards to life and property, including tornadoes, floods, damaging straightline winds, strong updrafts and downdrafts, and lightning. Although these hazards are not unique to supercells, some of them are often at their strongest when supercell-produced. Because of the destructive power of supercell hazards, supercells have been the subject of scientific research for decades. In this thesis, two of these hazards will be examined: supercell rainfall and supercell tornadoes, with the overarching goal to improve both our process-level understanding and forecasts of these hazards. The first part of this study focuses on supercell rainfall forecasts. Rainfall prediction by weather forecasting models, including supercell rainfall prediction, is strongly dependent on the microphysical parameterization being utilized in the model. As forecasting models have become more advanced, they are more commonly using double moment bulk microphysical parameterizations, which typically predict the hydrometeor number concentration and mass mixing ratio. While these double moment schemes are more sophisticated and require fewer a priori parameters than single moment parameterizations, a number of parameter values must still be fixed for quantities that are not prognosed or diagnosed. Two such parameters, the width of the drop size distribution and the choice of liquid collection efficiencies, are examined in Chapter 2. Simulations of a supercell were performed in which the collection efficiency dataset and the a priori width of the rain drop size distribution (DSD) were independently and simultaneously modified. Analysis of the results show that the a priori width of the DSD was a larger control on the total accumulated precipitation (a change of up to 130%) than the choice of the collection efficiency dataset used (a change of up to 10%). While the total precipitation difference when changing collision efficiency is relatively small, it does have a larger control on the warm rain process rates (including autoconversion and liquid accretion) than changing the rain DSD width does. The decrease in rainfall as the DSD width narrows is due to a combination of three main factors: (a) decreased rain production due to increased evaporation, (b) decreased rain production due to decreased ice melting, and (c) slower raindrop fall speeds which leads to longer residency times and changes in rain self-collection. The decreasing precipitation rate and accumulated precipitation with narrower DSD is consistent with observations of continental convection. This part of the study emphasizes that, in order to improve rainfall and flooding forecasts, the number of a priori parameters required by microphysical parameterizations should be reduced. Improvements in rainfall forecasts can be made immediately through the further development and implementation of triple-moment microphysical schemes, which do not require an a priori specified DSD width. The second part of this study focuses on supercell tornado forecasts. Supercell-produced tornadoes make up a majority of the most violent tornadoes and result in 90% of tornado-related deaths. Improving lead times and reducing false alarm rates is therefore critical. However, this requires an enhanced understanding of the controls that environmental conditions have on supercell tornadogenesis as well as improved observational platforms that are able to better detect environments that can produce tornadic supercells in advance. Therefore, the goals of the research presented in Chapter 3 are to (1): understand the storm processes that change as environmental conditions of supercells are perturbed and (2): determine how sensitive platforms, especially space based platforms, would need to be in order to distinguish between environments that can produce tornadic supercells from those that will produce nontornadic supercells. To address the goals, a suite of experiments were performed with a numerical model where the Convective Available Potential Energy (CAPE), Lifted Condensation Level (LCL), and low level wind shear are independently perturbed. The presented research shows that a platform with high accuracy in temperature and wind shear measurements can add value to supercell tornado forecasting. Further, several processes that influenced tornadogenesis, including cold pool strength and the role of horizontal vorticity, are found to have an impact on tornadogenesis. This part of the study emphasizes the need for new observational platforms that can more accurately observe environmental conditions in order to improve supercell tornado forecasting. Overall, the research presented here highlights supercell flooding and tornado forecast improvements that can be made with forecasting models and observational systems. Careful selection of a priori parameters, such as the width of the rain DSD, or reducing the number of those parameters required by microphysical parameterizations could improve supercell rainfall forecasts, therefore improving flooding forecasts. Supercell tornado forecasts can be improved by the addition of accurate space-based observational platforms which can help to distinguish between tornadic and nontornadic environmental conditions.
Open Access
Cold pool processes in different environments
(Colorado State University. Libraries, 2018) Grant, Leah Danielle, author; van den Heever, Susan C., advisor; Randall, David A., committee member; Rutledge, Steven A., committee member; Niemann, Jeffrey D., committee member
Cold pools are localized regions of dense air near Earth's surface. They form in association with precipitating clouds in many environments ranging from moist tropical to semi-arid continental conditions, and they play important roles in weather in climate. The overarching goal of this dissertation research is to improve our process-level understanding of cold pool interactions with different components of the Earth system, focusing on two key knowledge gaps: (1) interactions with Earth's surface in continental environments; and (2) interactions with organized convective systems in tropical oceanic environments. The primary goal of the first study conducted in this dissertation is to evaluate how surface sensible heat fluxes impact cold pool dissipation in dry continental environments via two pathways: (a) by directly heating the cold pool, and (b) by changing mixing rates between cold pool air and environmental air through altering turbulence intensity. Idealized 2D simulations of isolated cold pools are conducted with varying sensible heat flux formulations to determine the relative importance of these two mechanisms. The results demonstrate that the impact of sensible heat fluxes on mixing, i.e. mechanism (b), contributes most significantly to cold pool dissipation. Cold pool – land surface interactions in semi-arid continental conditions are investigated in the second study. Two questions are addressed: (1) how does the land surface respond to the cold pool; and (2) to what extent do land surface feedbacks modulate the cold pool evolution? Idealized 3D simulations of a cold pool evolving in a turbulent boundary layer are conducted to answer these questions. The land surface cools in response to the cold pool, resulting in suppressed sensible heat fluxes in the center of the cold pool. However, sensible heat fluxes are enhanced near the edge of the cold pool in association with higher wind speeds, leading to cold pool dissipation from the edge inwards. The land surface interactions are shown to strongly affect the cold pool, reducing its lifetime, size, and intensity by up to 50%. Preliminary analysis of a cold pool that was observed in northeastern Colorado on 17 May 2017 ("The Bees Day") during the C3LOUD-Ex field campaign is presented in the third study. The observed case exhibits similar environmental and cold pool characteristics to the first two numerical studies, thereby providing observational context for their hypotheses and conclusions. The objective of the fourth study presented in this dissertation is to determine the role of cold pools in organized tropical oceanic convective systems. To address this goal, two convective systems embedded in a weakly sheared cloud population approaching radiative-convective equilibrium are simulated at high resolution. The cold pools are weakened in the sensitivity tests by suppressing evaporation rates below cloud base. Both of the convective systems respond in a consistent manner as follows: (a) when cold pools are weakened, the convective intensity increases; and (b) the mesoscale structure, propagation speeds, and system lifetimes are insensitive to the changes in the cold pools, in contrast to the prevailing (RKW) theory that cold pools are critical to the mesoscale organization of convective systems. In summary, these high-resolution modeling and observational studies demonstrate new insights into cold pool – surface – convection interactions. The results suggest that cold pool interactions with different components of the Earth system are not all created equally; rather, these interactions depend on the environment in which the cold pools find themselves.
Open Access
Cold pool train dynamics and transport
(Colorado State University. Libraries, 2023) Neumaier, Christine Allison, author; van den Heever, Susan C., advisor; Grant, Leah D., advisor; Kreidenweis, Sonia M., committee member; Venayagamoorthy, Subhas K., committee member
Convectively generated cold air outflows, referred to as cold pools, can initiate new convection and loft aerosols, such as dust or pollen. In the BioAerosols and Convective Storms Phase I (BACS-I) field campaign, we observed multiple cold pools passing over the same location on the same day, without colliding, which we refer to as a "cold pool train". The goals of this study are to examine how the dynamics of cold pools in a cold pool train differ, how cold pools in a cold pool train affect the vertical distribution of aerosols, and how the results may change if the properties of the second cold pool change. We utilize idealized simulations of a cold pool train composed of two cold pools to investigate the dynamics of the cold pools in the train and how cold pool trains loft and transport aerosols. We test the sensitivity of the second cold pool's evolution and aerosol lofting to its initial temperature deficit and timing relative to the first cold pool, based on the cold pool trains observed during BACS-I. Passive tracers are initialized at different times to represent the background aerosols present before cold pools, aerosols newly emitted after the passage of the first cold pool in the train, and aerosols within and ahead of each cold pool, to distinguish between how cold pools loft their own air compared to distinct environmental air. We find that the first cold pool (CP1) in the cold pool train stably stratifies the environment ahead of the downshear side of the second cold pool (CP2) in the train. All else equal, this stabilization acts to decrease the height of CP2's head and increase its propagation speed. However, the stratification also increases the horizontal wind shear ahead of CP2 by decreasing the lower level wind speeds, which opposes the stability effects and acts to deepen the head of CP2. In the CONTROL case, where CP2 is initialized two hours after CP1 and with the same temperature deficit as CP1, we find that the wind profile plays a more dominant role for the dynamics of CP2 because overall, CP2's head is deeper and propagates slower compared to CP1. In the temperature deficit sensitivity experiments, we find that CP2's head depth and propagation speed decreases with decreasing temperature deficit. Finally, in the timing sensitivity tests of CP2, we find CP2 initiated 90 minutes after CP1 had the deepest head, while CP2 in the CONTROL (120 minutes) experiment propagated the slowest. Our analysis of the tracer lofting mechanisms in the simulations shows that the downshear leading edge of CP1 lofts the highest concentration of background aerosol, while the downshear leading edge of the CONTROL CP2 lofts less than half of the amount of background aerosol as CP1. However, the downshear leading edge of CP2 lofts more than double the concentration of newly emitted aerosol compared to the background aerosol lofted by CP1. The atmospheric stratification left behind by CP1 acts to trap the newly emitted aerosol near the surface, leading to greater concentrations lofted compared to the background aerosol which is well mixed in the boundary layer. Analysis of the tracers initialized within and ahead of the cold pools demonstrates that the lofted aerosol primarily originates from the air ahead of the cold pools, while the aerosol originating in the cold pools remains trapped within the cold pools. The CONTROL CP2 lofts the most aerosol of the temperature deficit sensitivity tests, and the CONTROL CP2, released the farthest apart temporally from CP1, lofts the most aerosol out of the timing sensitivity tests. Therefore, while the wind profile change ahead of CP2 plays a dominant role in its dynamics, atmospheric static stabilization plays a dominant role for the aerosol concentration lofted by CP2.
Open Access
Convective cold pools: characterization and soil moisture dependence
(Colorado State University. Libraries, 2016) Drager, Aryeh Jacob, author; van den Heever, Susan C., advisor; Davis, Christopher A., committee member; Kirby, Michael J., committee member; Schubert, Wayne H., committee member
Convective cold pools play an important role in Earth's climate system. However, a common framework does not exist for conceptually defining and objectively identifying convective cold pools in observations and models. The first part of this thesis begins with a review of the identification methods used in previous works. This is followed by an investigation of convective cold pools within a high-resolution simulation of rainforest convection simulated using the Regional Atmospheric Modeling System (RAMS), an open-source cloud-resolving model with a coupled land-surface model. Multiple variables are assessed for their potential for identifying convective cold pool boundaries, and a novel technique is developed and tested for identifying and tracking convective cold pools in numerical model simulations. This algorithm is based on surface rainfall rates and radial gradients in the density potential temperature field. The algorithm successfully identifies near-surface cold pool boundaries and is able to distinguish between connected cold pools. Once cold pools have been identified and tracked, composites of cold pool evolution are then constructed, and average cold pool properties are investigated. One novel result is the presence of moist patches that develop within the centers of cold pools where the ground has been soaked with rainwater. These moist patches help to maintain cool temperatures and prevent cold pool dissipation, which has implications for the development of subsequent convection. The second part of this thesis explores how the properties of convective cold pools are modulated by soil moisture. Three high-resolution simulations of tropical rainforest convection are performed using the RAMS, and the initial soil moisture is varied between 25% and 75% saturation. The cold pool identification algorithm developed in the first part of the thesis is used to construct composites of cold pools within each simulation, and the composites are compared. When soil moisture is decreased, stronger convective cold pools result. These stronger cold pools are also smaller because increased sensible heat fluxes in the reduced soil-moisture simulations cause the cold pools to dissipate more quickly as they expand. Finally, the rings of enhanced water vapor that have been documented in previous studies of tropical cold pools disappear when soil moisture is reduced. These results emphasize the role that land surface properties can have in modulating convective cold pool properties.
Open Access
Environmental controls and aerosol impacts on tropical sea breeze convection
(Colorado State University. Libraries, 2020) Park, Jungmin, author; van den Heever, Susan C., advisor; Cooley, Daniel S., committee member; Kreidenweis, Sonia M., committee member; Miller, Steven D., committee member; Rasmussen, Kristen L., committee member
Nearly half of the world's human population resides within 150 km of the ocean, and this coastal population is expected to continue increasing over the next several decades. The accurate prediction of convection and its impacts on precipitation and air quality in coastal zones, both of which impact all life's health and safety in coastal regions, is becoming increasingly critical. Thermally driven sea breeze circulations are ubiquitous and serve to initiate and support the development of convection. Despite their importance, forecasting sea breeze convection remains very challenging due to the coexistence, covariance, and interactions of the thermodynamic, microphysical, aerosol, and surface properties of the littoral zone. Therefore, the overarching goal of this dissertation research is to enhance our understanding of the sensitivity of sea breeze circulation and associated convection to various environmental parameters and aerosol loading. More specifically, the objectives are the following: (1) to assess the relative importance of ten different environmental parameters previously identified as playing critical roles in tropical sea breeze convection; and (2) to examine how enhanced aerosol loading affects sea breeze convection through both microphysical and aerosol-radiation interactions, and how the environment modulates these effects. In the first study, the relative roles of five thermodynamic, one wind, and four land/ocean-surface properties in determining the structure and intensity of sea breeze convection are evaluated using ensemble cloud-resolving simulations combined with statistical emulation. The results demonstrate that the initial zonal wind speed and soil saturation fraction are the primary controls on the inland sea breeze propagation. Two distinct regimes of sea breeze-initiated convection, a shallow and a deep convective mode, are also identified. The convective intensity of the shallow mode is negatively correlated by the inversion strength, whereas the boundary layer potential temperature is the dominant control of the deep mode. The processes associated with these predominant controls are analyzed, and the results of this study underscore possible avenues for future improvements in numerical weather prediction of sea breeze convection. The sea breeze circulation and associated convection play an important role in the transport and processing of aerosol particles. However, the extent and magnitude of both direct and indirect aerosol effects on sea breeze convection are not well known. In the second part of this dissertation, the impacts of enhanced aerosol concentrations on sea breeze convection are examined. The results demonstrate that aerosol-radiation-land surface interactions produce less favorable environments for sea breeze convection through direct aerosol forcing. When aerosol-radiation interactions are eliminated, enhanced aerosol loading leads to stronger over-land updrafts in the warm-phase region of the clouds through increased condensational growth and latent heating. This process occurs irrespective of the sea breeze environment. While condensational invigoration of convective updrafts is therefore robust in the absence of aerosol direct effects, the cold-phase convective responses are found to be environmentally modulated, and updrafts may be stronger, weaker, or unchanged in the presence of enhanced aerosol loading. Surface precipitation responses to aerosol loading also appear to be modulated by aerosol-radiation interactions and the environment. In the absence of the aerosol direct effect, the impacts of enhanced aerosol loading may produce increased, decreased, or unchanged accumulated surface precipitation, depending on the environment in which the convection develops. However, when aerosols are allowed to interact with the radiation, a consistent reduction in surface precipitation with increasing aerosol loading is observed, although the environment once again modulated the magnitude of this aerosol-induced reduction.
Open Access
Examining the impacts of convective environments on storms using observations and numerical models
(Colorado State University. Libraries, 2022) Freeman, Sean William, author; van den Heever, Susan C., advisor; Bell, Michael M., committee member; Kreidenweis, Sonia M., committee member; Eykholt, Richard, committee member
Convective clouds are significant contributors to both weather and climate. While the basic environments supporting convective clouds are broadly known, there is currently no unifying theory on how joint variations in different environmental properties impact convective cloud properties. The overaching goal of this research is to assess the response of convective clouds to changes in the dynamic, thermodynamic and aerosol properties of the local environment. To achieve our goal, two tools for examining convective cloud properties and their environments are first described, developed and enhanced. This is followed by an examination of the response of convective clouds to changes in the dynamic, thermodynamic and aerosol properties using these enhanced tools. In the first study comprising this dissertation, we assess the performance of small temperature, pressure, and humidity sensors onboard drones used to sample convective environments and convective cloud outflows by comparing them to measurements made from a tethersonde platform suspended at the same height. Using 82 total drone flights, including nine at night, the following determinations about sensor accuracy are made. First, when examining temperature, the nighttime flight temperature errors are found to have a smaller range than the daytime temperature errors, indicating that much of the daytime error arises from exposure to solar radiation. The pressure errors demonstrate a strong dependence on horizontal wind speed with all of the error distributions being multimodal in high wind conditions. Finally, dewpoint temperature errors are found to be larger than temperature errors. We conclude that measurements in field campaigns are more accurate when sensors are placed away from the drone's main body and associated propeller wash and are sufficiently aspirated and shielded from incoming solar radiation. The Tracking and Object-Based Analysis of Clouds (tobac) tracking package is a commonly used tracking package in atmospheric science that allows for tracking of atmospheric phenomena on any variable and on any grid. We have enhanced the tobac tracking package to enable it to be used on more atmospheric phenomena, with a wider variety of atmospheric data and across more diverse platforms than before. New scientific improvements (three spatial dimensions and an internal spectral filtering tool) and procedural improvements (enhanced computational efficiency, internal re-gridding of data, and treatments for periodic boundary conditions) comprising this new version of tobac (v1.5) are described in the second study of this dissertation. These improvements have made tobac one of the most robust, powerful, and flexible identification and tracking tools in our field and expanded its potential use in other fields. In the third study of this dissertation, we examine the relationship between the thermodynamic and dynamic environmental properties and deep convective clouds forming in the tropical atmosphere. To elucidate this relationship, we employ a high-resolution, long-duration, large-area numerical model simulation alongside tobac to build a database of convective clouds and their environments. With this database, we examine differences in the initial environment associated with individual storm strength, organization, and morphology. We find that storm strength, defined here as maximum midlevel updraft velocity, is controlled primarily by Convective Available Potential Energy (CAPE) and Precipitable Water (PW); high CAPE (>2500 J kg-1) and high PW (approximately 63 mm) are both required for midlevel CCC updraft velocities to reach at least 10 m s-1. Of the CCCs with the most vigorous updrafts, 80.9% are in the upper tercile of precipitation rates, with the strongest precipitation rates requiring even higher PW. Furthermore, vertical wind shear is the primary differentiator between organized and isolated convective storms. Within the set of organized storms, we also find that linearly-oriented CCC systems have significantly weaker vertical wind shear than nonlinear CCCs in low- (0-1 km, 0-3 km) and mid-levels (0-5 km, 2-7 km). Overall, these results provide new insights into the joint environmental conditions determining the CCC properties in the tropical atmosphere. Finally, in the fourth study of this dissertation, we build upon the third study by examining the relationship between the aerosol environment and convective precipitation using the same simulations and tracking approaches as in the third study. As the environmental aerosol concentrations are increased, the total domain-wide precipitation decreases (-3.4%). Despite the overall decrease in precipitation, the number of tracked terminal congestus clouds increases (+8%), while the number of tracked cumulonimbus clouds is decreased (-1.26%). This increase in the number of congestus clouds is accompanied by an overall weakening in their rainfall as aerosol concentration increases, with a decrease in overall rain rates and an increase in the number of clouds that do not precipitate (+10.7%). As aerosol particles increase, overall cloud droplet size gets smaller, suppressing the initial generation of rain and leading to clouds evaporating due to entrainment before they are able to precipitate.
Open Access
Exploring post-cold frontal moisture transport in an idealized extratropical cyclone study
(Colorado State University. Libraries, 2016) Sheffield, Amanda Marie, author; van den Heever, Susan C., advisor; Eykholt, Richard, committee member; Johnson, Richard, committee member; Kreidenweis, Sonia, committee member
Moisture transport in extratropical cyclones (ETCs) has been studied in the past in the context of the warm conveyor belt (WCB), a 'conveyor belt' transferring moisture from the warm sector boundary layer to the free troposphere both eastward and poleward of the warm front. Recent research has highlighted a different, potentially important mechanism of transporting water vapor in ETCs by post-cold frontal (PCF) clouds. PCF clouds are typically boundary layer cumulus clouds located in the cold sector of an ETC that transfer moisture to the free troposphere through convective-evaporative processes. Recent studies have suggested that these PCF cumuli may vertically transport nearly equivalent amounts of moisture as the WCB. Therefore, not only are these PCF cumuli important in venting the PCF boundary layer, they also play a role in limiting the amount of moisture available for convergence in the source region of the WCB. This limitation can have important consequences for regional weather and climate through its impact on the timing and location of precipitation, the three-dimensional redistribution of water vapor, and the distribution of clouds within ETCs. The goal of this study is to investigate the role of PCF clouds in the moisture transport of an ETC, and the impacts of environmental factors such as SST and aerosol loading on this transport role. We have achieved this goal through the use of numerical simulations of such a storm system. Previous studies have utilized model simulations with relatively coarse grid resolutions and convective parameterization schemes. Here, we simulate a wintertime ETC over the Pacific Ocean using high spatial and temporal resolution, advanced microphysics and explicitly resolved convection. The results of this research demonstrate that PCF cumuli are found to vertically ventilate BL moisture over an expansive region behind the cold front. The free tropospheric moisture contents and stability profile of the cold sector exert a strong control over the size, depth and frequency of the PCF clouds, and varies with distance from the cold front. Increased aerosol loading results in the invigoration of the PCF clouds. This is associated with an increase in the upward vertical moisture flux, increased cloud condensate formation, and reduced precipitation rates. Sea surface temperature is found to be a significantly more important factor in the development of PCF cumuli than aerosol loading, where increasing SSTs are associated with increased cloud fraction, cloud top heights, and precipitation rates. The impact of PCF clouds on vertically redistributing water vapor from the cold sector is found to depend in varying degrees on the large-scale advection of water vapor by the ETC system, the surface evaporation rates, the updraft velocities, the precipitation rates, and the cloud fraction within the PCF region. The pathways of the vertically redistributed water vapor within the ETC were then examined through the use of massless, passive tracers. The results of these experiments show that the water vapor lofted out of the PCF BL by the cumulus clouds is advected hundreds of kilometers eastward within 8-12 hours of release of tracers in the PCF BL. Furthermore, cross frontal transport from behind the cold front to the WCB source region appears to be small, in contradiction to previously hypothesized results. This is due to the fact that the cold frontal boundary provides a zone of strong vertical lifting that does not allow tracers to converge further east.
Open Access
Exploring precipitation processes in stratocumulus clouds from satellite-derived cloud properties
(Colorado State University. Libraries, 2021) Murakami, Yasutaka, author; Kummerow, Christian D., advisor; van den Heever, Susan C., advisor; Chiu, Christine, committee member; Venkatachalam, Chandrasekaran, committee member
Marine stratocumulus clouds are low-level convective clouds that develop within the marine atmospheric boundary layer and have a large impact on the global radiation budget and hydrological cycle. Drizzle plays an important but complicated role in their longevity and microphysical properties. Many studies have examined the response of cloud base rain rate to varying cloud droplet number concentrations and cloud thickness, as well as liquid water path (LWP), and found that cloud base rain rates are enhanced with lower cloud droplet number concentrations and greater cloud thickness or LWP. In warm stratocumulus clouds, cloud base rain rate is a combination of raindrop embryo production through collision coalescence (i.e. autoconversion) and raindrop embryo growth by collecting cloud droplets (i.e. accretion). Previous studies have shown that cloud base rain rate depends on LWP or cloud thickness and the geographical location of stratocumulus clouds, but the dependence of the autoconversion process on these variables is not well known because cloud base rain rate represents the effects of both autoconversion and accretion. This two-part dissertation explores the dependence of stratocumulus cloud precipitation processes on cloud thickness and geographical location by examining the cloud properties retrieved by A-Train satellite observations from CloudSat's Cloud Profiling Radar (CPR), CALIPSO's Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and Aqua's Moderate Resolution Imaging Spectroradiometer (MODIS). In the first part, the relations between cloud top properties (radar reflectivity, LWC and cloud droplet number concentration) and cloud geometrical thickness are investigated for subtropical stratocumulus clouds. Satellite-observations show that cloud top LWC and effective radius increase as clouds become thicker. The data also suggest that autoconversion may be more efficient in thicker clouds. These findings are consistent with previous studies that have shown that thicker clouds have larger cloud droplets and thus produce more rain embryos. However, it is also found that clouds separate into two sub-groups as they transition from thick (i.e. geometrical thickness of 384-480m) to very thick clouds (i.e. geometrical thickness of 624-720m). Drizzling clouds have higher LWC and their drops have larger effective radii, whereas non-drizzling clouds have lower cloud top LWC and smaller effective radii. In the second part, the climatology of satellite-derived cloud top properties (radar reflectivity, LWC and cloud droplet number concentration) for 8 stratocumulus cloud regions are presented. While LWP tends to be larger for midlatitude clouds, cloud top LWC tends to be larger at subtropical stratocumulus clouds. Since midlatit0ude stratocumulus clouds are thicker, these results suggests that effective condensation rates are larger for subtropical stratocumulus clouds. Both cloud top and cloud base radar reflectivity also tend to be larger for subtropical stratocumulus clouds. Based on these findings, the sensitivity of cloud top radar reflectivity on LWC and cloud droplet number concentration are examined. Cloud top radar reflectivity is more (less) sensitive to changes in LWC and cloud droplet number concentration for clouds with stronger (weaker) cloud top radar reflectivity. This is consistent with previous findings that collision-coalescence efficiency between liquid water droplets (i.e. approximately 20 μm in diameter) increases non-linearly with droplet size. The overall results presented in this dissertation indicate that the autoconversion process can be represented with a globally applicable function of cloud top LWC and cloud droplet number concentration for all stratocumulus clouds regardless of their geolocation and geometrical thickness. It is also demonstrated that cloud top raindrop embryo generation rate is an important factor for determining the precipitation generation rate for stratocumulus clouds as a whole. In general, accretional growth is controlled by both the total cross-sectional area of rain drops and LWP. By comparing spatial patterns of cloud top radar reflectivity (i.e. total cross-sectional area of rain drops) and radar reflectivity increase from cloud top to bottom (i.e. accretional growth), it is found that accretional growth depends more on total cross-sectional area of rain drops and less on LWP in stratocumulus clouds. These conclusions can explain the findings of previous studies that cloud base rain rate depends on LWP (or cloud thickness) and geographical location of stratocumulus clouds. Cloud base rain rate is dependent on geometrical thickness because cloud top LWC increases as cloud become thicker. Subtropical stratocumulus clouds tend to have stronger precipitation at a given LWP compared to midlatitude stratocumulus clouds because the effective condensation rate of subtropical stratocumulus clouds is greater and so is the cloud top LWC. In this study, the effect of Cloud Condensation Nuclei on warm rain processes is represented by varying cloud droplet number concentration. The results presented in this dissertation represent more than one hundred thousand independent pixels and provide a statistically robust benchmark that numerical models should reproduce.
Open Access
Interactions between the Madden-Julian oscillation and mesoscale to global scale phenomena
(Colorado State University. Libraries, 2019) Toms, Benjamin A., author; van den Heever, Susan C., advisor; Barnes, Elizabeth A., committee member; Maloney, Eric D., committee member; Cooley, Daniel, committee member
The Madden-Julian Oscillation (MJO) influences and interacts with atmospheric phenomena across the globe, from the tropics to the poles. In this two-part study, the interactions of the MJO with other phenomena across a broad range of scales are considered, including mesoscale convective structures within the tropics and global teleconnection patterns. While the two studies are distinct in the scales of the interactions they discuss, each highlights an aspect of the importance of interactions between the MJO and variability across a broad range of scales within the climate system. The study of such cross-scale interactions is important for understanding our climate system, as these interactions can transfer energy between phenomena of starkly different spatial and temporal scales. Part one of the study uses a cloud-resolving model, the Regional Atmospheric Modeling System, to consider the relationship between mesoscale convective structures within the Indo-Pacific region and the regional, intraseasonal anomalies associated with the MJO. The simulation captures the entirety of a canonical boreal summertime MJO event, spanning 45 days in July and August of 2016, during which the convective anomaly associated with the MJO propagated over the Maritime Continent. The convective cloud structures, or cells, within the simulation were tracked and logged according to their location relative to the regional convective anomaly of the MJO. Using both spectral analysis and phase compositing, it was found that a progressive relationship exists between the boreal summertime MJO and mesoscale deep convective structures within the Indo-Pacific region, specifically within the convectively enhanced region of the MJO, as follows: increased cell longevity in the initial phases of the MJO, followed by increased cell number in the intermediate phases, progressing into increased cell expanse in the terminal phases. This progressive relationship is connected back to the low-frequency atmospheric response to the MJO. It is suggested that the bulk thermodynamic and kinematic anomalies of the MJO are closely related to the convective cell expanse and longevity, although the number of convective cells appears to be tied to another source of variability not identified within this study. These findings emphasize that while the MJO is commonly defined as an intraseasonal-scale convective anomaly, it is also intrinsically tied to the mesoscale variability of the convective systems that constitute its existence. The second part of the study quantifies the prevalence of the MJO within the overall climate system, along with the dependence of its teleconnections on variability in another tropical phenomena on a larger scale than itself. It is well known that the MJO exhibits pronounced seasonality in its tropical and global signature, and recent research has suggested that its tropical structure also depends on the state of the Quasi-Biennial Oscillation (QBO). We therefore first quantify the relationship between 300-mb geopotential anomalies and the MJO across the globe, then test the dependence of the relationship on both the meteorological season and the QBO phase using a derivative of cross-spectral analysis, magnitude-squared coherence Coh2. It is found that the global upper-tropospheric signature of the MJO exhibits pronounced seasonality, but also that the QBO significantly modulates the upper-tropospheric tropical and extratropical anomalies associated with the MJO. Globally, variability in upper tropospheric geopotential linked to the MJO is maximized during the boreal summertime and wintertime of easterly QBO phases, which is consistent with previous research that has shown easterly QBO phases to enhance the persistence of tropical convection associated with the MJO. Additional features are identified, such as the global maximum in upper-tropospheric variability associated with the MJO occurring during boreal summertime, rather than boreal wintertime. Overall, the MJO explains seven to thirteen percent of intraseasonal atmospheric variability in 300-mb geopotential, depending on season and QBO phase. These results highlight the importance of considering the phase of the QBO in analyses related to either global or local impacts of the MJO, along with the importance of cross-scale relationships, such as those between the MJO and QBO, in governing the coupling between the MJO and teleconnections across the globe. This thesis considers the relationship between the MJO and processes that operate on both longer and shorter timescales than itself, including tropical convection and the Quasi-Biennial Oscillation. In doing so, this work highlights the importance of considering relationships between the MJO and atmospheric phenomena on different spatial and temporal scales and with origins distinct from the MJO itself. While theories exist describing the MJO as its own distinct entity, this research corroborates the idea that it is at its core fundamentally linked to the rest of the climate system, both modulating and being modulated by a broad range of atmospheric processes.
Open Access
Latent heating and aerosol-precipitation interactions within mesoscale convective systems
(Colorado State University. Libraries, 2016) Marinescu, Peter James, author; van den Heever, Susan C., advisor; Kreidenweis, Sonia M., advisor; Eykholt, Richard, committee member; Schumacher, Russ S., committee member
Two studies are presented in this thesis that focus on understanding cloud processes within simulations of two mesoscale convective system (MCS) events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E). Simulations are conducted with the Regional Atmospheric Modeling System (RAMS) and are compared with a suite of observations obtained during MC3E. It is concluded that the simulations reasonably reproduce the two MCS events of interest. Both studies provide information that can assist in the advancement of cloud process parameterizations in atmospheric models. The first study details the microphysical process contributions to latent heating profiles within MCS convective and stratiform regions and the evolution of these profiles throughout the MCS lifetime. Properly representing the distinctions between the latent heating profiles of MCS convective and stratiform regions has significant implications for the atmospheric responses to latent heating on various scales. The simulations show that throughout the MCSs, condensation and deposition are the primary contributors to latent warming, as compared to riming and nucleation processes. In terms of latent cooling, sublimation, melting, and evaporation all play significant roles. Furthermore, it is evident that throughout the MCS lifecycle, convective regions demonstrate an approximately linear decrease in the magnitudes of latent heating rates, while the evolution of latent heating within stratiform regions is associated with transitions between MCS flow regimes. The second study addresses the relative roles of middle-tropospheric and lower-tropospheric aerosol particles on MCS precipitation during the mature stage. A suite of sensitivity simulations for each MCS event is conducted, where the simulations are initialized with different aerosol profiles that vary in the vertical location of the peak aerosol particle number concentrations. Importantly, the total integrated aerosol mass remains constant between the different initialization aerosol profiles, and therefore, differences between the simulated MCS precipitation characteristics can be more directly attributed to the varied vertical location of the aerosol particles. The simulations from both MCS events demonstrate that during the mature stage, the concentrations of lower-tropospheric aerosol particles are the primary factor in determining the intensity of precipitation near the cold pool leading edge, while middle-tropospheric aerosol particles were entrained within convective updrafts, thus altering the cloud droplet properties. However, the aerosol effects on total surface precipitation is not consistent between the two simulated MCS events, suggesting that the MCS structure and environmental conditions play important roles in regulating the impacts of middle-tropospheric and lower-tropospheric aerosol particles on MCS precipitation. Lastly, changes in precipitation processes can result in dynamical feedbacks that further modify, and hence complicate, the net effect of aerosol particles on MCS precipitation. One such feedback process involving the MCS cold pool intensity and updraft tilt is discussed.
Open Access
Morphology, lifecycles, and environmental sensitivities of tropical trimodal convection
(Colorado State University. Libraries, 2022) Sokolowsky, George Alexander, author; van den Heever, Susan C., advisor; Maloney, Eric D., committee member; Kreidenweis, Sonia M., committee member; Jathar, Shantanu, committee member
Convective clouds are ubiquitous in the tropics and typically follow a trimodal distribution of cumulus, congestus, and cumulonimbus clouds. Due to the crucial role each convective mode plays in tropical and global transport of heat and moisture, there has been both historical and recent interest in the characteristics, sensitivities, and lifecycles of these clouds. However, designing novel studies to further our knowledge has been challenging due to several limitations: the extensive computing resources needed to conduct modeling studies at sufficient resolution and scale to capture the trimodal distribution in detail; the lack of analysis tools which can objectively detect and track these clouds throughout their lifetime; and a need for more observational and modeling data of the tropical convective environments that produce these clouds. In this dissertation, three distinct but related studies that address these problems to advance the knowledge of our field on the morphology, lifecycles, and environmental sensitivities of tropical trimodal convection are presented. The first study examines the sensitivities of the tropical trimodal distribution and the convective environment to initial aerosol loading and low-level static stability. The Regional Atmospheric Modeling System (RAMS) configured as a Large Eddy Simulation (LES) is utilized to resolve all three modes in detail through two full diurnal cycles. Three initial static stabilities and three aerosol profiles are independently and simultaneously varied for a suite of nine simulations. This research found that (1) large aerosol loading and strong low-level static stability suppress the bulk environment and the intensity and coverage of convective clouds; (2) cloud and environmental responses to aerosol loading tend to be stronger than those from static stability; (3) the effects of aerosol and stability perturbations modulate each other substantially; (4) the deepest convection and highest dynamical intensity occur at moderate aerosol loading, rather than at low or high loading; and (5) most of the strongest feedbacks due to aerosol and stability perturbations are seen in the boundary layer (the latter being applied within the boundary layer themselves), though some are stronger above the freezing level. The second study presented seeks to further enhance an artificial intelligence analysis tool, the Tracking and Object-Based Analysis of Clouds (tobac) Python package, from both a scientific and procedural standpoint to enable a wider variety of research uses, including process-level studies of tropical trimodal convection. Scientific improvements to tobac v1.5 include an expansion of the tool from 2D to 3D analyses and the addition of a new spectral filtering tool. Procedural enhancements added include greater computational efficiency, data regridding capabilities, and treatments for processing data with singly or doubly periodic boundary conditions (PBCs). My distinct contributions to this work focused on the 2D to 3D expansion and the PBC treatment. These new capabilities are presented through figures, schematics, and discussion of the new science that tobac v1.5 facilitates, such as the analysis of large basin-scale datasets and detailed simulations of layered clouds, that would have been impossible before. Finally, the last study in this dissertation is a process-focused modeling study on the sensitivities of upscale growth of tropical trimodal convection to environmental aerosol loading. This project was enabled by the scientific and procedural improvements to tobac discussed in the second study, in particular the new abilities of tobac to detect and track objects in 3D and with model PBCs. Here, we used a subset of RAMS simulations from the first study, where only aerosol loading was changed and the upscale growth from shallow cumulus through congestus and cumulonimbus during the nighttime hours was investigated. This study revealed that moderately increasing aerosol loading enhances collision-coalescence processes in the middle of the cloud, which delays initial glaciation but promotes it later in the growth period. Greatly increasing aerosol, however, produces a cloud structure with a more extreme aspect ratio and greater entrainment aloft that rapidly loses buoyancy and vertical velocity with height, as well as exhibiting a greater amount of condensate loading towards the top of the cloud. We also found the relative timing of these processes to be especially important, with more rapid initial growth and lofting of condensate often inhibiting deeper convective growth.
Open Access
Observations of aerosol particles and deep convective updrafts and the modeling of their interactions
(Colorado State University. Libraries, 2020) Marinescu, Peter James, author; van den Heever, Susan C., advisor; Kreidenweis, Sonia M., advisor; Bell, Michael M., committee member; Eykholt, Richard, committee member
Within cloud updrafts, cloud droplets form on aerosol particles that serve as cloud condensation nuclei (CCN). Varying the concentrations of CCN alters the concentrations of cloud droplets, which in turn modifies subsequent microphysical processes within clouds. In this dissertation, both observational and modeling studies are presented that reduce the uncertainties associated with these aerosol-induced feedback processes in deep convective clouds. In the first study, five years of observations of aerosol particle size distributions from central Oklahoma are compared, and useful metrics are provided for implementing aerosol size distributions into models. Using these unique, long-term observations, power spectra analyses are also completed to determine the most relevant cycles (from hours to weeks) for different aerosol particle sizes. Diurnal cycles produce the strongest signals in every season, most consistently in the accumulation mode and the smallest (diameters < 30 nm) particles. The latter result suggests that these smallest particles may play a more important role in the CCN budget than previously thought. Ultimately, in understanding which, when and why different aerosol particles are present in the atmosphere, we can better assess the impacts that they have on clouds. The types and number of aerosol particles that can serve as CCN depend on the amount of supersaturation, and thus the magnitude of the cloud updraft vertical velocities. However, in situ updraft observations in deep convective clouds are scarce, and other vertical velocity estimates often have uncertainties that are difficult to characterize. In the next study, novel, in situ observations of deep convective updraft vertical velocities from targeted radiosonde launches during the CSU Convective Cloud Outflows and Updrafts Experiment (C3LOUD-Ex) are presented. Vertical velocities of over 50 m s-1 are estimated from radiosonde observations taken in Colorado. Radar data are used to contextualize the radiosonde measurements and to provide an independent estimate of the updraft magnitudes for comparison. These observations are valuable in that they: 1) contribute novel estimates of the vertical velocities within deep convective clouds, 2) demonstrate that in situ observations of vertical velocities complement estimates from other platforms and 3) will allow for better assessments of the supersaturation magnitudes, and thus the amount of CCN that are present within deep convective clouds. While the first two studies focus on observing aerosol particles and updrafts separately, the third study within this dissertation presents simulations of their interactions from an international model intercomparison project. Seven models from different institutions simulated the same case study of isolated deep convective clouds with both high and low CCN concentrations. The range of the responses in updrafts to varying CCN concentrations are calculated for this model suite. Despite the various physical parameterizations that these models utilize, all the models simulate stronger updrafts in the High-CCN simulations from near cloud base through ~8 km AGL, with diverging results above this altitude. The vertical velocity tendency equation is analyzed to explain which processes are causing the consistent and inconsistent updraft responses to varying CCN concentrations amongst the models. The three studies in this dissertation each reduce the uncertainties related to aerosol effects on deep convective cloud updrafts. This work also assisted in motivating the DOE Tracking Aerosol Convection Interactions Experiment (TRACER), which will further connect observational and modeling research to reduce the uncertainties in aerosol-cloud interactions.
Open Access
On the relation between satellite observed liquid water path, cloud droplet number concentration and cloud base rain rate and its implication for the auto-conversion rate in stratocumulus clouds
(Colorado State University. Libraries, 2020) Murakami, Yasutaka, author; Kummerow, Christian D., advisor; van den Heever, Susan C., advisor; Venkatachalam, Chandrasekaran, committee member
Stratocumulus clouds are low-level convective clouds that develop within the atmospheric boundary layer. Their persistence and broad coverage of the earth's surface produces important impacts on the global radiation energy budget and hydrological cycle. Precipitation processes of these stratocumulus clouds play a large role in their longevity and spatial distribution through their interaction with the vertical profiles of humidity and temperature within the atmospheric boundary layer. This has led to a number of field campaigns to understand the precipitation processes of stratocumulus clouds. However, because of the limited field campaign domains and limited amount of these observations, it is difficult to draw statistically significant conclusions on the precipitation processes of global stratocumulus clouds from these data. In this study, space-borne observations from A-Train satellites are utilized to obtain robust relations among the liquid water path, cloud droplet number concentration and cloud base rain rate for three geographical regions with similar large-scale environments, namely the north east Pacific off the coast of California, the south east Pacific off the coast of Peru and the south east Atlantic off the coast of Namibia, where strong subsidence flow from the subtropical-high is observed. Radar reflectivity from CloudSat's Cloud Profiling Radar (CPR) is employed to estimate the cloud base rain rate (Rcb). Liquid water path (LWP) and cloud droplet number concentration (Nd) are estimated from Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical thickness and effective radius. The relation between cloud base rain rate (Rcb) and the ratio of liquid water path to cloud droplet number concentrations (LWP/Nd) are obtained from a large number of A-train observations that show similar probability density distribution for all three target areas in this study. Rcb has a positive correlation with LWP/Nd and the increase in Rcb becomes larger as LWP/Nd increases, which is consistent with the results from previous ground-based observations. The research presented here also shows that the increase of Rcb in respect to LWP/Nd become more gradual in larger Nd regions, which suggests that the relation between Rcb and LWP/Nd changes with different cloud droplet number concentrations. These findings are consistent with our theoretical understanding of cloud physics processes in that 1) auto-conversion and accretion growth of rain embryos becomes more effective as cloud droplet number concentrations near cloud top decrease, and 2) auto-conversion is suppressed when the cloud droplet radius is small enough. The sensitivity of the auto-conversion rate to cloud droplet number concentration is investigated by examining pixels with small LWP in which the accretion process is assumed to have little influence on Rcb. The upper limit of the dependency of auto-conversion on the cloud droplet number concentration is assessed from the relation between cloud base rain rate and cloud top droplet number concentration since the sensitivity is exaggerated by the accretion process. The upper limit of the sensitivity of auto-conversion found in this study was found to be a cloud droplet number concentration to the power of -1.44±0.12. This study demonstrates that satellite observations are capable of detecting the average manner in which precipitation processes are modulated by the liquid water path and drop number concentrations.
Open Access
Processes driving shallow convective development and their interactions with aerosols: aerosol transport and aerosol breezes
(Colorado State University. Libraries, 2022) Leung, Gabrielle R., author; van den Heever, Susan C., advisor; Kreidenweis, Sonia M., committee member; Jathar, Shantanu, committee member
In this two-part thesis we investigate the development of tropical shallow convective clouds (i.e. shallow cumulus and cumulus congestus) and their interactions with the aerosol environment using idealized large-eddy simulations (LES). Although much about shallow convection is well-understood, we specifically focus on three facets of shallow convection that remain understudied: (1) the factors governing the development of congestus extending above the 0ºC stable layer; (2) the detrainment of aerosol particles and water vapor from congestus clouds into the mid-troposphere; and (3) the impacts of strong horizontal gradients in aerosol concentration on mesoscale circulations. Part one of this study explores environmental controls on congestus development and the implications of that development on aerosol lofting and transport. Congestus is the middle mode of tropical convection, with cloud tops around or exceeding the 0ºC level (~5km AGL). While some congestus are terminal, meaning capped by the 0ºC stable layer, others are transient and may develop into deep convection. Although this distinction impacts the congestus-to-deep convection transition and the convective transport of water vapor and aerosols into the mid-troposphere, there is still much to be understood about the processes causing congestus to overshoot the 0℃ level and continue growing. We find that terminal and transient congestus updrafts are characterized by a similar overturning circulation between the updraft and subsiding shell. However, transient congestus have stronger updrafts, and importantly, the downward branch of their corresponding circulations is constrained by the 0ºC level. Our findings support previous results suggesting buoyancy as a control on congestus height, and we specifically demonstrate that congestus developing in more humid midlevel environments are more likely to be transient. We additionally determine that terminal congestus regenerate more aerosol via evaporation along their cloud edges, while transient congestus create stronger midlevel detrainment layers of aerosol and water vapor due to the trapping of the regenerated aerosol above the 0ºC level. Such midlevel detrainment layers are important for the formation of altocumulus clouds. Part two of this study introduces and explores the concept of an "aerosol breeze", a thermally-driven circulation resulting from mesoscale gradients in aerosol loadings. We call the resulting circulation an aerosol breeze so as to be analogous to well-documented circulations associated with heterogenous surfaces, like sea breezes. The aerosol-induced circulation sets up a gradient in convection and precipitation that is opposite in direction to that of the aerosol gradient. Clouds in the presence of an aerosol gradient precipitate sooner and more intensely than those in the same integrated aerosol loading distributed horizontally homogeneously. These results suggest unrepresented sub-grid scale heterogeneity in aerosol emissions may lead to biases in simulated cloudiness and precipitation. We also present two observational case studies of aerosol breezes that are similar to our model results in scale and cloud distribution. Further study of the aerosol breeze phenomena is warranted, especially in regions where strong aerosol gradients may be expected, such as along the edges of wildfire plumes or urbanized regions.
Open Access
Response of convective cold pools and precipitation to changes in soil moisture
(Colorado State University. Libraries, 2020) Drager, Aryeh Jacob, author; van den Heever, Susan C., advisor; Bell, Michael M., committee member; Davis, Christopher A., committee member; Kirby, Michael J., committee member; Schubert, Wayne H., committee member
In Part 1 of this dissertation, we examine the role of soil moisture in modulating convective cold pool properties. This investigation is performed within an idealized modeling framework featuring a cloud-resolving model coupled to an interactive land surface model. Five high-resolution simulations of tropical continental convection are conducted in which the initial soil moisture is varied. The hundreds of cold pools forming within each simulation are identified and composited across space and time using an objective cold pool identification algorithm. Several important findings emerge from this analysis. Lower initial soil moisture results in greater daytime heating of the surface, which produces a deeper, drier subcloud layer. As a result, latent cooling through the evaporation of precipitation is enhanced, and cold pools are stronger and deeper. Increased gust front propagation speed, combined with wider rain shafts, results in wider cold pools. Finally, the "water vapor rings" that surround each cold pool under wet-soil conditions disappear under dry-soil conditions, due to the suppression of surface latent heat fluxes. Instead, when soils are dry, short-lived "puddles" of enhanced water vapor permeate the interiors of the cold pools. The results are nonlinear in that the properties of the cold pools in the two driest-soil simulations depart substantially from the cold pool properties in the three simulations initialized with wetter soil. The dividing line between the resulting wet-soil and dry-soil regimes is the permanent wilting point (PWP), below which transpiration is subdued. Land surface-boundary layer-cloud interactions are found overall to play a key role in governing the properties of cold pools. During Part 1 of this dissertation, we identify a novel "intermediate-soil moisture disadvantage" regime in which soils whose initial liquid water content slightly exceeds the PWP receive the least rainfall. In Part 2, we investigate the physical mechanisms behind this result. Four suites of ten idealized, high-resolution numerical experiments are conducted using the same modeling system used in Part 1. Each suite uses a distinct combination of soil type and vegetation, and within each suite, each simulation is initialized with a different amount of soil moisture. The "intermediate soil-moisture disadvantage" from Part 1 is reproduced. This result is found to stem from differing amounts of subcloud rain evaporation across the simulations, as well as from divergent balances between the level of free convection and the strength of boundary layer vertical motions. However, the result only holds for vegetated surfaces; bare-soil surfaces are instead found to exhibit a pure "wet-soil advantage" relationship. These results have important implications for the design of future process-level studies and large-scale model parameterizations.
Open Access
Strong and weak cold pool collisions
(Colorado State University. Libraries, 2022) Falk, Nicholas Michael, author; van den Heever, Susan C., advisor; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee member
Collisions between convective cold pools commonly initiate new convective storms. This occurs through enhancements to the vertical velocity through mechanical forcing, and increased water vapor content via thermodynamic forcing. The goal of this study is to investigate the impact of the following four parameters on the mechanical and thermodynamic forcing associated with cold pool collisions: (1) the initial temperature perturbation of cold pools, (2) the initial distance between cold pools, (3) the environment in which cold pools exist, and (4) the strength of atmospheric diffusion. To achieve this goal, the dynamical and thermodynamical processes of colliding pairs of cold pools is investigated using a two-dimensional, high- resolution non-hydrostatic anelastic model. The four parameters of interest were varied across a wide range of values in a model suite comprised of 11,200 large eddy simulations in total. To facilitate our analysis, a classification of cold pool collisions into categories of "mechanically strong" and "mechanically weak" is proposed. "Mechanically strong" cold pool collisions occur when the updraft velocities resulting from the collisions are greater than those produced by the flow of air forced up the leading edges of individual cold pools. In "mechanically weak" collisions, the updraft velocities produced by individual cold pools are greater than those from cold pool collisions. An analogous classification of "thermodynamically strong/weak" collisions is also proposed. The results of this analysis show that the initial temperature perturbation of the cold pools has the largest impact on mechanical and thermodynamic forcing from cold pool collisions. Colder cold pools have greater horizontal wind velocities at their heads, leading to greater near- surface horizontal convergence when they collide. This in turn leads to greater updraft velocities which are also more effective at advecting water vapor upwards. The second largest impact on mechanical and thermodynamic cold pool forcing is from the environment in which the cold pools exist. Due to a decreased vertical gradient of potential temperature, weaker low-level static stability increases mechanical forcing as the air lofted by the collisions is decelerated less by negative buoyancy. Environments with larger low-level vertical moisture gradients are associated with increased thermodynamic forcing through enhanced vertical moisture advection. The initial edge-to-edge distance between the cold pools has the third largest impact on the proxies for convective initiation. Mechanical forcing is found to peak at an optimal initial distance between cold pools of ~2.5 km due to a balance between the creation and dissipation of kinetic energy. Thermodynamic forcing, on the other hand, peaks for much greater initial cold pool distances than those associated with the mechanical forcing. This is likely a result of the faster updraft winds generated during collisions for closely spaced initial cold pools also being more effective at advecting moisture away during the collision, thereby decreasing the thermodynamic forcing. The smallest impact on the proxies for convective initiation comes from the atmospheric diffusion rate which impacts cold pool strength through mixing. Thus, this work finds that convective initiation becomes increasingly likely from a cold pool collision when the cold pools are colder, the environment is less stable and has a greater vertical water vapor gradient, the cold pools start close to some optimal separation distance, and the atmospheric diffusion rate is low.
Open Access
The effects of land surface-atmosphere interactions within two convective storm regimes
(Colorado State University. Libraries, 2024) Ascher, Benjamin D., author; van den Heever, Susan C., advisor; Schumacher, Russ, committee member; McGrath, Daniel, committee member
Convective storms, which are driven in part by atmospheric thermodynamic instability, come in a range of shapes and sizes and bring a variety of impacts both at the surface and throughout the atmosphere. Often these storms initiate as a result of lifting within the Planetary Boundary Layer (PBL), the behavior of which is strongly affected by the characteristics of the land surface below them. To examine the effects of land surface properties on convective storm behavior and impacts, I have conducted two high-resolution mesoscale modeling studies. The first study examined the impact of Lake Huron on convective lake-effect snow over Lake Erie, while the second analyzed the effects of heterogeneous vegetation cover on deep convection in an idealized coastal environment. Our findings in the first study revealed that Lake Huron initiates lake-effect snow bands which persist over land between Lake Huron and Lake Erie and then reintensify after moving over Lake Erie. The persistent band "kickstarts" convection over Lake Erie and increases snowfall over and downwind of Lake Erie compared to when Lake Huron is not present. I also found that areas of snow-free land can act as a "brown lake" and initiate lake effect-like convection on their own. An area of snow-free land upwind of Lake Erie fulfilled a similar role to Lake Huron in enhancing convection and snowfall downwind of Lake Erie. Such findings may have important implications for improved short-term forecasting of the location and intensity of heavy snowfall. The results in our second study indicated that heterogeneous land surfaces enhance convective storm activity over certain vegetation types and suppress it over others. In particular, I found an increase in precipitation over forests surrounded by pasture lands and suburban regions, while the precipitation over the pasture and suburban regions is suppressed. I also discovered that circulations induced by these heterogeneous land surfaces appear to be more important to the location and timing of convection initiation than a sea breeze which forms in the simulations. Finally, I concluded that cold pools produced by convective storms reinforce the land surface-induced circulations, thereby allowing these circulations to collide in the center of the forested region, where they initiate intense convection which subsequently produces heavy rainfall.
Open Access
The impacts of mineral dust on organized mesoscale deep convection
(Colorado State University. Libraries, 2012) Seigel, Robert Brian, author; van den Heever, Susan C., advisor; Kreidenweis, Sonia M., committee member; Schubert, Wayne H., committee member; Niemann, Jeffrey D., committee member
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