Browsing by Author "Rasmussen, Kristen L., committee member"
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Item Open Access An investigation of an east Pacific easterly wave genesis pathway and the impact of the Papagayo and Tehuantepec wind jets on the east Pacific mean state and easterly waves(Colorado State University. Libraries, 2021) Whitaker, Justin W., author; Maloney, Eric D., advisor; Bell, Michael M., committee member; Rasmussen, Kristen L., committee member; Niemann, Jeffrey D., committee memberPart one of this dissertation investigates the transition of a Panama Bight mesoscale convective system (MCS) into the easterly wave (EW) that became Hurricane Carlotta (2012). Reanalysis, observations, and a convective-permitting Weather Research and Forecasting (WRF) model simulation are used to analyze the processes contributing to EW genesis. A vorticity budget analysis shows that convective coupling and vortex stretching are very important to the transition in this case, while horizontal advection is mostly responsible for the propagation of the system. In the model, the disturbance is dominated by stratiform vertical motion profiles and a mid-level vortex, while the system is less top-heavy and is characterized by more prominent low-level vorticity later in the transition in reanalysis. The developing disturbance starts its evolution as a mesoscale convective system in the Bight of Panama. Leading up to MCS formation the Chocó jet intensifies, and during the MCS to EW transition the Papagayo jet strengthens. Differences in the vertical structure of the system between reanalysis and the model suggest that the relatively more bottom-heavy disturbance in reanalysis may have stronger interactions with the Papagayo jet. Field observations like those collected during the Organization of Tropical East Pacific Convection (OTREC) campaign are needed to further our understanding of this east Pacific EW genesis pathway and the factors that influence it, including the important role for the vertical structure of the developing disturbances in the context of the vorticity budget. In parts two and three of this dissertation, the Weather Research and Forecasting (WRF) model is used to quantify the impact that the Papagayo and Tehuantepec wind jets have on the east Pacific mean state and east Pacific easterly waves. Specifically, a control run simulation is compared with a gaps filled simulation, where mountain gaps in the Central American mountains are "filled in" to block the Papagayo and Tehuantepec wind jets. In the absence of these wind jets, the northern half of the east Pacific mean state becomes drier, supporting a reduction in convective activity and precipitation there. Further, a 700 hPa positive vorticity feature that is linked to the Papagayo jet is reduced. An easterly wave tracking algorithm is developed and shows that easterly wave track density and genesis density are generally reduced in the eastern half of the basin for the gaps filled run. An eddy kinetic energy (EKE) budget is also calculated and highlights that EKE, barotropic conversion, and eddy available potential energy (EAPE) to EKE conversion all decrease for easterly waves when the wind jets are blocked. A composite analysis reveals that there are slight horizontal structural changes between waves in the simulations, while the waves have surprisingly similar strengths. Overall, the Papagayo and Tehuantepec wind jets are shown to be supportive influences on east Pacific easterly waves.Item Open Access Assessing the impact of stratospheric aerosol injection on convective weather environments in the United States(Colorado State University. Libraries, 2023) Glade, Ivy, author; Hurrell, James W., advisor; Rasmussen, Kristen L., committee member; Anderson, Brooke, committee memberContinued climate warming, together with the overall development and implementation of climate mitigation and adaptation approaches, has prompted increasing research into the potential of proposed solar climate intervention (SCI) methods, such as stratospheric aerosol injection (SAI). SAI would reflect a small amount of incoming solar radiation away from the Earth to reduce warming due to increasing greenhouse gas concentrations. Research into the possible risks and benefits of SAI relative to the risks from climate change is emerging. There is not yet, however, an adequate understanding of how SAI might impact human and natural systems. To date, little or no research has been done to examine how SAI might impact environmental conditions critical to the formation of severe convective weather over the United States (U.S.), for instance. We use parallel ensembles of Earth system model simulations of future climate change, with and without hypothetical SAI deployment, to examine possible future changes in thermodynamic and kinematic parameters critical to the formation of severe weather during convectively active seasons over the U.S. Southeast and Midwest. We find that simulated forced changes in thermodynamic parameters are significantly reduced under SAI relative to a no-SAI world, while simulated changes in kinematic parameters are more difficult to distinguish. We also find that unforced internal climate variability may significantly modulate the projected forced climate changes over large regions of the U.S.Item 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 memberSupercell 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.Item 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 memberNearly 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.Item Open Access From surface to tropopause: on the vertical structure of the tropical cyclone vortex(Colorado State University. Libraries, 2024) DesRosiers, Alexander J., author; Bell, Michael M., advisor; Barnes, Elizabeth A., committee member; Rasmussen, Kristen L., committee member; Davenport, Frances V., committee memberThe internal vortex structure of a tropical cyclone (TC) influences intensity change. Beneficial structural characteristics that allow TCs to capitalize on favorable environmental conditions are an important determinant as to whether a TC will undergo rapid intensification (RI) or not. Accurately forecasting RI is a significant challenge and past work identified characteristics of radial and azimuthal structure of the tangential winds which favor RI, but vertical structure has received less attention. This dissertation aims to define vertical structure in a consistent manner to improve our understanding of how it influences intensity change in observed and modeled TCs, as well as discern when strong winds are more likely to reach the surface with potential for greater impacts. Part 1 investigates the height of the vortex (HOV) in observed TCs and its potential relationships with intensity and intensification rate. As a TC intensifies, the tangential wind field expands vertically and increases in magnitude. Past work supports the notion that vortex height is important throughout the TC lifecycle. The Tropical Cyclone Radar Archive of Doppler Analyses with Recentering (TC-RADAR) dataset provides kinematic analyses for calculation of HOV in observed TCs. Analyses are azimuthally-averaged with tangential wind values taken along the radius of maximum winds (RMW). A threshold-based technique is used to determine the HOV. A fixed-threshold HOV strongly correlates with current TC intensity. A dynamic HOV (DHOV) metric quantifies vertical decay of the tangential wind normalized to its maximum at lower levels with reduced intensity dependence. DHOV exhibits a statistically significant relationship with TC intensity change with taller vortices favoring intensification. A tall vortex is always present in observed cases meeting a pressure-based RI definition in the following 24-hr period, suggesting DHOV may be useful to intensity prediction. In Part 2, numerical modeling simulations are utilized to discern mechanisms responsible for the observed relationships in Part 1. Vertical wind shear (VWS) can tilt the TC vortex by misaligning the low- and mid-level circulation centers which prevents intensification until realignment occurs. Both observed and simulated TCs with small vortex tilt magnitudes possess DHOV values consistent with those observed prior to RI. In aligned TC intensification, DHOV and intensity have a mutually increasing relationship, indicating the metric provides useful information about vertical structure in both tilted and aligned TCs. Vertical vortex growth during RI is sensitive to internal processes which strengthen the TC warm core in the upper-levels of the troposphere. Comparison of a TC simulated in the presence of a concentrated upper-level jet of VWS to a control simulation in quiescent flow indicates that disruption of intensification in the upper levels limits vortex height and intensity without appreciable low- to mid-level tilt. Part 3 focuses on decay of the TC wind field as it encounters friction near the surface in the planetary boundary layer (PBL). Surface winds are important to operational TC intensity estimation, but direct observations within the PBL are rare. Forecasters use reduction factors formulated with wind ratios (WRs) from winds observed by aircraft in the free troposphere and surface winds. WRs help reduce stronger winds aloft to their expected weaker values at the surface. Asymmetries in the TC wind field such as those induced by storm motion can limit the accuracy of static existing WR values employed in operations. A large training dataset of horizontally co-located wind measurements at flight level and the surface is constructed to train a neural network (NN) to predict WRs. A custom loss function ensures the model prioritizes accurate prediction of the strongest wind observations which are uncommon. The NN can leverage relevant physical relationships from the observational data and predict a surface wind field in real-time for forecasters with greater accuracy than the current operational method, especially in high winds.Item Open Access Impact of forced and internal climate variability on changes in convective environments over the eastern United States(Colorado State University. Libraries, 2022) Franke, Megan E., author; Hurrell, James W., advisor; Rasmussen, Kristen L., committee member; Mueller, Nathan D., committee memberHazards from convective weather and severe storms pose a serious threat to the continental United States (CONUS). Previous studies have examined how future projected changes in climate might impact the frequency and intensity of severe weather using simulations with both convection permitting regional models and coarser-grid Earth system models. However, most of these studies have been limited to single representations of the future climate state with little insight into the uncertainty of how the population of convective storms may change. To more thoroughly explore this aspect, we utilize a large-ensemble of climate model simulations to investigate the forced response and how it may be modulated by internal variability. Specifically, we use daily data from an ensemble of 50 climate simulations with the most recent version of the Community Earth System Model (CESM) to examine changes in the severe weather environment over the eastern CONUS during boreal spring from 1870-2100. Our results indicate that the large-scale convective environment changed little between 1870 and 1990, but from then throughout the 21st century, convective available potential energy increases while 0-6 km vertical wind shear and convective inhibition decreases (increased stability). While the forced changes in these variables are robust both in space and time, we show that they are likely to be modified significantly by internal climate variability. This effect can either act to significantly enhance the forced response or conversely, suppress it in such a way that produces changes in the convective environment that are opposite to the forced response. The time evolution of bivariate distributions of convective indices illustrates that future springtime convective environments over the eastern CONUS will be characterized by relatively less frequent, but deeper and more intense convection. Future convective environments will also be less supportive of the most severe convective modes and their associated hazards.Item Open Access Investigation of the dynamics of tropical cyclone precipitation structure using radar observations and numerical modeling(Colorado State University. Libraries, 2023) Cha, Ting-Yu, author; Bell, Michael M., advisor; Rasmussen, Kristen L., committee member; Schumacher, Russ S., committee member; Lee, Wen-Chau, committee member; Reising, Steven C., committee memberPrecipitation from tropical cyclones (TCs) produces significant damage and causes fatalities worldwide. Forecast skill of the structure of precipitation in a TC remains challenging, due in part to limited fundamental understanding of the underlying complex dynamics and limitations in our observational capability. This dissertation seeks to improve our understanding of the underlying dynamics of TC precipitation structure by using and improving radar retrieval techniques and numerical modeling. In Part 1, the vortex dynamics of TC polygonal eyewall structure during rapid intensification (RI) of Hurricane Michael (2018) are examined from ground-based single‐Doppler radar analysis. Although the organization of polygonal precipitation asymmetries has been theorized to be related to vortex Rossby wave (VRW) dynamics, prior studies have had observational limitations that prevent a detailed description of the phenomena. Here, we present the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the axisymmetric and asymmetric winds at 5‐min intervals. Novel single Doppler radar retrievals show that both tangential wind and reflectivity asymmetries rotate at speeds that are consistent with linear VRW theory. Dual‐Doppler winds from airborne radar provide further evidence of the vortex structure that supports growth of asymmetries during RI. In Part 2, the relationship between VRWs and the polygonal precipitation structure is further explored through a simple modeling framework. A two-layer model consisting of a shallow water fluid on top of a slab boundary layer is used to understand the dynamical relationship. The model maintains an approximate gradient wind balance in the free atmospheric layer and parameterizes the diabatic heating produced by convection from the vertical motion out of the boundary layer. The two-layer model provides insight into the essential dynamics of Hurricane Michael's intensification and precipitation structure observed by radar in Part 1. The results show that the convective maxima located at the vertices of an elliptical vortex are due to boundary layer processes and not the free atmospheric convergence. The simulations further show that continuous intensification of the vortex can happen in the presence of elliptical asymmetries and even after the potential vorticity ring breakdown when the diabatic heating is continuously maintained by boundary layer processes. When TCs approach land they can produce voluminous rainfall totals, especially when interacting with complex terrain. Doppler radar can provide the capability to monitor extreme rainfall events over land, but our understanding of airflow modulated by orographic interactions remains limited. In Part 3, a new Doppler radar technique is developed to retrieve three-dimensional wind fields in precipitation over complex terrain. New boundary conditions are implemented in a variational multi-Doppler radar technique to represent the topographic forcing and surface impermeability. A series of observing simulation sensitivity experiments using a full-physics model and radar emulator simulating rainfall from Typhoon Chanthu (2021) over Taiwan are conducted to evaluate the retrieval accuracy and parameter settings. Analysis from real radar observations from Chanthu demonstrates that the improved retrieval technique can advance scientific analyses for the underlying dynamics of orographic precipitation using radar observations.Item Open Access Objective analysis of extreme precipitation events in diverse geographic regions(Colorado State University. Libraries, 2018) Kelly, Nathan Robert, author; Schumacher, Russ S., advisor; Rasmussen, Kristen L., committee member; Nelson, Peter A., committee memberExtreme precipitation events are a focus of much research in the atmospheric science community today. These events are extraordinarily impactful to society, damaging critical infrastructure and in the worst cases taking lives. The factors that lead to these destructive events are not the same everywhere, dependent on each regions unique geography and climatology. There are two critical ingredients to precipitation: moisture and lift. However, there are many synoptic patterns that can combine these two ingredients in the right proportions, resulting in an extreme storm. This thesis addresses the relationship between lift and moisture, and relates these two variables to the patterns that produce them, in a way that can be applied to any region of the world. To accomplish this task the synoptic patterns must be categorized. This is done in an objective way, using a global reanalysis product (namely MERRA-2) so as to be applicable to any area around the globe. The period 1980 to 2016 is analyzed, and an extreme precipitation event is defined here as an event that exceeds the 99.9th percentile of running 24-hour precipitation sums. Two domains are analyzed, one covering Argentina, and another covering northeast Colorado and part of the high plains to the north and east. Principal Component Analysis (PCA) is the objective method employed to investigate the variability within extreme precipitation events. PCA gives an indication as to what variables input into the analysis have the most impact on the variability of the dataset as a whole. This allows for an analysis of what variables are most different in different extreme events and what variables are about the same across events. PCA is performed on two different sets of variables at each grid point in both the northern Colorado and Argentina. Two points are selected for further analysis herein; these are 40.5N 104.375W (near Greeley, Colorado) and 31.5S 63.75W (near Córdoba, Argentina). At the northern Colorado gridpoint it is clear that there are two very distinct modes of extreme 24 hour precipitation. The first is a convective mode that is characterized at upper levels by a large ridge aloft with a small embedded shortwave. The second is a synoptic mode commonly associated with the most intense snowstorms in the region; a cutoff low approaching from the southwest. The convective mode is associated with more precipitable water than the synoptic mode, whereas in the synoptic mode the upper air features are able to contribute significantly to the lifting of air and cause extreme precipitation with a relative dearth of moisture. In Argentina, the primary variability seems to be in the position of a surface trough in the lee of the Andes as a large scale upper level trough impinges on the Andes crest. The first mode has this lee trough more directly contributing to lift and allowing the low level jet and associated moisture to reach farther south. The second involves the position of the lee trough farther north, which allows the south Atlantic high to push flow from the Atlantic upslope into the Sierra de Córdoba, initiating convection. The overlap between 1-hour and 24-hour extremes is also explored for Argentina, confirming the convective nature of much of this precipitation and illustrating just how important these convective episodes are to the production of extreme precipitation.Item Embargo Recent and future Colorado water: snow drought, streamflow, and winter recreation(Colorado State University. Libraries, 2023) Pfohl, Anna K. D., author; Fassnacht, Steven R., advisor; Barnard, David M., committee member; Kampf, Stephanie K., committee member; Rasmussen, Kristen L., committee memberWater in the western United States is a crucial resource for ecosystems, the abiotic environment, and people (for industrial, agricultural, and residential purposes). A majority of this water originates in the seasonal snowpack in the mountains. The snowpack is responsible for maintaining the water supply, and changes to this system have broad and severe implications. Various metrics have been used to quantify these patterns when snow is less than normal, often referred to as a snow drought or a low snow year. In recent decades, the number of years with low snow have increased, and this will continue and intensify into the future. With observed decreases in long-term snow and modeled decreases for the future, high snow years become more critical to support the water supply. Beyond supplying water for downstream use, the seasonal snowpack also sustains the winter recreation industry, which is a large component of many local and state economies. The Weather Research and Forecasting Model (WRF) is a 4-km mesoscale model that can capture orography and convective processes over complex terrain. WRF includes two time periods: the control (CTL) based on historic conditions and the future under pseudo-global warming (PGW) conditions. This dataset was used to drive SnowModel (WRF-SM) to produce 100-m, daily snow water equivalent (SWE), total precipitation, solid precipitation, snowmelt, runoff, and air temperature. Using these datasets, this research examines past and future snow and streamflow in Colorado. We evaluated 1) common metrics and trends for snow drought; 2) used WRF data to drive the Ages hydrologic model to examine changes (snow, streamflow, and flow partitioning) in two high snow years; and 3) ski opportunities at nine different resorts. To evaluate methods of defining snow drought, we used SWE and winter precipitation data from Snow Telemetry stations and the WRF-SM dataset described above. Classifying drought with the ratio of SWE to winter precipitation resulted in drought occurrence for more than 50% of station-years from 1981 to 2020. Using percentiles of long-term peak SWE indicated that occurrence of low or very low years increased from 2001 to 2020 compared with the previous 20 years. Under PGW conditions, elevations between 1800 and 2400 m shifted drought classification towards low or very low, with higher elevations (3200 m and above) remaining relatively unchanged. To examine changes in snow, streamflow, and flow partitioning under a PGW scenario for two high snow years (2008 and 2011), we used Ages, a spatially distributed watershed model, in the Upper Blue River watershed in central Colorado. Changes in snow (snowmelt and solid precipitation) were greatest in magnitude at high elevations. Timing of peak streamflow shifted to nearly two months earlier under a PGW scenario. To examine ski opportunities, we developed metrics to quantify ski conditions. The number of opportunities for snowmaking in the future will decrease throughout the season, but especially in October and November. Ski days (snow depth greater than 50 cm) will decrease in early and late season and increase at lower elevations from January through March. Powder days (fresh depth greater than 15 cm and fresh density greater than 125 kg/m3) follow a similar pattern. Ski resorts at low elevations will generally be more susceptible to changes under a PGW scenario. Additionally, using a fine-resolution dataset allowed investigation of smaller study areas to understand the changes that are not captured with coarser resolutions.Item Open Access Response of MCSs and low-frequency gravity waves to vertical wind shear and nocturnal thermodynamic environments(Colorado State University. Libraries, 2019) Groff, Faith, author; Schumacher, Russ S., advisor; Adams-Selin, Rebecca D., advisor; Rasmussen, Kristen L., committee member; Nelson, Peter A., committee memberLow-frequency gravity waves have been found to both increase and decrease environmental favorability ahead of mesoscale convective systems (MCSs) based on their associated vertical motions. The strength and timing of these waves is determined by the internal dynamics of the MCS. This study investigates the sensitivities of MCSs to changes in the vertical wind and thermodynamic profiles through idealized cloud model simulations, highlighting how internal MCS processes impact low-frequency gravity wave generation, propagation and environmental influence. A common feature among all of the simulations is that fluctuations within the internal latent heating profile, the generation mechanism behind n = 1 (N1) waves, display concurrent cellularity with the MCS updrafts. Spectral analysis is performed on the rates of latent heat release, updraft velocity, and deep-tropospheric descent ahead of the convection as a signal for N1 wave passage. Results strongly suggest that perturbations in mid-level descent up to 100 km ahead of the MCS occur at the same frequency as N1 gravity wave generation due to fluctuations of latent heat release caused by the cellular variations of MCS updrafts. The introduction of deep vertical wind shear does not change this connection nor impact the lifecycle of daytime MCS updrafts and associated N1 wave generation, however within a nocturnal environment, the frequency of the cellularity of the updrafts increases, subsequently increasing the frequency of N1 wave generation. In response to surges of latent cooling within the lower half of the troposphere, n = 2 (N2) low-frequency gravity waves are generated, however this only occurs with cooling contributions from both evaporation and melting of hydrometeors. Results indicate that in environments with minimal upper-level wind shear atop more pronounced shear below, the N2 wave generation mechanisms and environmental influence behave similarly among daytime and nocturnal MCSs. Within environments that incorporate deep vertical wind shear, many of the N2 waves are strong enough to support cloud development ahead of the MCS as well as sustain and support convection within the domain.Item Open Access The influence of cloud radiative effects on hydrologic sensitivity and variability(Colorado State University. Libraries, 2021) Naegele, Alexandra Claire, author; Randall, David A., advisor; Betsill, Michele M., committee member; Rasmussen, Kristen L., committee member; van den Heever, Susan C., committee memberThe global-mean precipitation change in response to CO2-forced warming, normalized by global-mean surface warming, is referred to as the hydrologic sensitivity. It is estimated at 1-3% K-1, much lower than the rate of increasing atmospheric moisture availability. Here, we study the role of cloud radiative effects (CREs) in constraining the hydrologic sensitivity. Often, the change in clear-sky atmospheric radiative cooling (ARC) is used to constrain the change in precipitation, but this constraint is incomplete. CMIP5 model data are analyzed to show that although the all-sky ARC increases at a lower rate than the clear-sky ARC, the smaller change in ARC due to CREs is balanced by the change in the surface sensible heat flux. Together, the change in the all-sky ARC with the change in the surface sensible heat flux provide a more accurate and complete energetic constraint on hydrologic sensitivity than by using the clear-sky radiative cooling alone. Idealized aquaplanet simulations using SP-CAM are analyzed to assess the temperature dependence of the hydrologic cycle and the large-scale circulation responses to CREs. We examine the response of the hydrologic cycle and the large-scale circulation to CREs at a range of sea surface temperatures (SSTs), including a cool (280 K) SST that is representative of the mid-latitudes; typically, the extratropics have been less studied than the tropics in similar idealized simulations. We use simulations with uniform SSTs to test the hypothesis that CREs enhance precipitation variability at cool temperatures, and reduce precipitation variability at warm temperatures. In these simulations, our hypothesis is confirmed. In less idealized simulations with a more realistic SST pattern, the influence of CREs on precipitation variability is obscured by other circulation changes. Can the hydrologic response to CREs be explained by the large-scale circulation response to CREs? Using the same idealized simulations, the vertical velocity —used here as an indicator of the circulation response to CREs—is compared to precipitation. We find that the influence of CREs on vertical velocity variability is very similar to the influence of CREs on precipitation variability.Item Open Access The unconventional eyewall replacement cycle of Hurricane Ophelia (2005)(Colorado State University. Libraries, 2018) Razin, Muhammad Naufal Bin, author; Bell, Michael M., advisor; Rasmussen, Kristen L., committee member; Bangerth, Wolfgang, committee memberOne of the mechanisms proposed for the spin-up of the tropical cyclone (TC) mean tangential circulation is the convergence of absolute angular momentum above the boundary layer. This mechanism is important for the outer primary circulation and results in the broadening of the TC wind field. We hypothesize that the mid-level inflow associated with the stratiform precipitation in TC rainbands may be instrumental in spinning up the broader circulation, and may be important in the development of secondary eyewalls. Hurricane Ophelia (2005) underwent an unconventional eyewall replacement cycle (ERC) as it was a Category 1 storm located over cold sea surface temperatures near 23°C. The ERC was observed using airborne radar observations during the Hurricane Rainband and Intensity Change Experiment (RAINEX). Data was collected from the single-parabolic X-band radar aboard the National Oceanic and Atmospheric Administration (NOAA) P-3 aircraft and from the dual-beam X-band Electra Doppler Radar (ELDORA) aboard the Naval Research Laboratory (NRL) P-3 aircraft. The two aircraft flew simultaneously along Ophelia's primary rainband during a research flight beginning around 1700 UTC on 11 September 2005, allowing for quad-Doppler wind retrievals along the rainband. Analyses were conducted using a spline-based three-dimensional variational wind synthesis technique. Results showed a broadened tangential wind field associated with the ERC was observed in the stratiform-dominant rainbands of Ophelia. The broadening of the tangential wind field was collocated with the strongest radial advection of angular momentum through the stratiform mid-level inflow and is consistent with the proposed mechanism for TC intensity change.