Browsing by Author "Randall, David A., advisor"

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Open Access
A study of low cloud climate feedbacks using a generalized higher-order closure subgrid model
(Colorado State University. Libraries, 2013) Firl, Grant J., author; Randall, David A., advisor; Denning, Scott, committee member; Johnson, Richard, committee member; Evangelista, Paul, committee member
One of the biggest uncertainties in projections of future climate is whether and how low cloudiness will change and whether that change will feed back on the climate system. Much of the uncertainty revolves around the difference in scales between the processes that govern low cloudiness and the processes that can be resolved in climate models, a fact that relegates shallow convection to the parameterization realm with varying levels of success. A new subgrid-scale parameterization, named THOR, has been developed in an effort to improve the representation of low cloudiness via parameterization in climate models. THOR uses the higher-order closure approach to determine the statistics describing subgrid-scale processes. These statistics are used to determine a trivariate double-Gaussian PDF among vertical velocity, ice-liquid water potential temperature, and total water specific humidity. With this information, one can diagnose what portion of the grid cell is cloudy, subgrid-scale cloud water content, and subgrid-scale vertical cloud water flux. In addition, samples are drawn from the trivariate PDF in order to drive the microphysics and radiation schemes. Although schemes similar to THOR have been developed over the past decade, THOR includes several novel concepts, like the generalization of the saturation curve to include condensation over both ice and liquid substrates, the determination of the PDF parameters from the given turbulence statistics, the introduction of a stochastic parcel entrainment process for the turbulence length scale, and a sub-column approach for calculating radiative transfer using the PDF. The new model is validated by simulating five test cases spanning a wide range of boundary layer cloud types, from stratocumulus to cumulus and the transition between the two. The results are compared to an ensemble of LES models running the same cases, with particular attention paid to turbulence statistics and cloud structure. For all cloud types tested, THOR produces results that are generally within the range of LES results, indicating that the single-column THOR is able to reproduce the gross characteristics of boundary layer clouds nearly as well as three-dimensional LES. Sensitivity to vertical grid spacing, diagnostic/prognostic third- order moments, choice of turbulence length scale entrainment process, and whether or not PDF sampling is used to drive the microphysics and radiation schemes is assessed for all test cases. Simulation of the cumulus regime was degraded when vertical grid spacing exceeded 200 m, when more third-order moments were predicted, when higher parcel entrainment rates were assumed, and when PDF sampling for the microphysics scheme was omitted. Simulation of stratocumulus was degraded with grid spacing larger than 100 m, when PDF sampling for microphysics was omitted, and when PDF sampling for radiation was included. Lastly, THOR is used to study low cloud climate feedbacks in the northeastern Pacific Ocean in the context of the CGILS project. Initial conditions and forcings are supplied at 13 points along the GPCI cross-section that spans from the ITCZ northeast to the coast of California transecting regions of shallow cumuli and stratocumuli, for both the current climate and a climate with a +2K SST perturbation. A change in net cloud radiative forcing of 0-8 W/m2 was simulated along the cross-section for the perturbed climate, representing neutral to weak positive feedback. The responsible mechanism appeared to be increased boundary layer entrainment and stratocumulus decoupling leading to reduced maximum cloud cover in the cumulus regime and reduced liquid water path in the stratocumulus regime.
Open Access
Design of a nonhydrostatic atmospheric model based on a generalized vertical coordinate
(Colorado State University. Libraries, 2008) Toy, Michael Douglas, author; Randall, David A., advisor; Schubert, Wayne H., committee member; Kirkpatrick, Allan Thomson, committee member; Johnson, Richard H., committee member
The isentropic system of equations has particular advantages in the numerical modeling of weather and climate. These include the elimination of the vertical velocity in adiabatic flow, which simplifies the motion to a two-dimensional problem and greatly reduces the numerical errors associated with vertical advection. Vertical resolution is enhanced in regions of high static stability which leads to better resolving of features such as the tropopause boundary. Also, sharp horizontal gradients of atmospheric properties found along frontal boundaries in traditional Eulerian coordinate systems are nonexistent in the isentropic coordinate framework. The extreme isentropic overturning that can occur in fine-scale atmospheric motion presents a challenge to nonhydrostatic modeling with the isentropic vertical coordinate. This dissertation presents a new nonhydrostatic atmospheric model based on a generalized vertical coordinate. The coordinate is specified in a similar manner as Konor and Arakawa, but elements of arbitrary Eulerian-Lagrangian methods are added to provide the flexibility to maintain coordinate monotonicity in regions of negative static stability and return the coordinate levels to their isentropic targets in statically stable regions. The model is mass-conserving and implements a vertical differencing scheme that satisfies two additional integral constraints for the limiting case of z-coordinates. The hybrid vertical coordinate model is tested with mountain wave experiments which include a downslope windstorm with breaking gravity waves. The results show that the advantages of the isentropic coordinate are realized in the model with regards to vertical tracer and momentum transport. Also, the isentropic overturning associated with the wave breaking is successfully handled by the coordinate formulation.
Open Access
Diagnosing the angular momentum fluxes that drive the quasi-biennial oscillation
(Colorado State University. Libraries, 2023) Hughes, Ann-Casey, author; Randall, David A., advisor; Hurrell, James, committee member; Oprea, Iuliana, committee member
The quasi-biennial oscillation (QBO) is a descending pattern of alternating easterly and westerly equatorial stratospheric winds that is produced by the upward transport of momentum in multiple types of atmospheric waves. The discovery of the QBO and its role in the global circulation are discussed. The angular momentum budget of the QBO is analyzed using ERA-Interim isentropic analyses. We explain the benefits of isentropic coordinates and angular momentum as tools for analyzing atmospheric motion. We diagnose vertical motion utilizing continuity, allowing direct computation of the angular momentum fluxes due to vertical motion. The angular momentum fluxes due to unresolved convectively generated gravity waves are computed as a residual. These results are discussed with the goal of improving the representation of sub-grid scale motions in numerical models. We also discuss these results within the context of the reliability of reanalysis datasets and the downsides to treating reanalysis data as observations. We also revisit and discuss the seasonal dependence of the QBO transition.
Open Access
Examining chaotic convection with super-parameterization ensembles
(Colorado State University. Libraries, 2017) Jones, Todd R., author; Randall, David A., advisor; Kummerow, Christian D., committee member; Van den Heever, Susan S., committee member; Schumacher, Russ S., committee member; Eykholt, Richard E., committee member
This study investigates a variety of features present in a new configuration of the Community Atmosphere Model (CAM) variant, SP-CAM 2.0. The new configuration (multiple-parameterization-CAM, MP-CAM) changes the manner in which the super-parameterization (SP) concept represents physical tendency feedbacks to the large-scale by using the mean of 10 independent two-dimensional cloud-permitting model (CPM) curtains in each global model column instead of the conventional single CPM curtain. The climates of the SP and MP configurations are examined to investigate any significant differences caused by the application of convective physical tendencies that are more deterministic in nature, paying particular attention to extreme precipitation events and large-scale weather systems, such as the Madden-Julian Oscillation (MJO). A number of small but significant changes in the mean state climate are uncovered, and it is found that the new formulation degrades MJO performance. Despite these deficiencies, the ensemble of possible realizations of convective states in the MP configuration allows for analysis of uncertainty in the small-scale solution, lending to examination of those weather regimes and physical mechanisms associated with strong, chaotic convection. Methods of quantifying precipitation predictability are explored, and use of the most reliable of these leads to the conclusion that poor precipitation predictability is most directly related to the proximity of the global climate model column state to atmospheric critical points. Secondarily, the predictability is tied to the availability of potential convective energy, the presence of mesoscale convective organization on the CPM grid, and the directive power of the large-scale.
Open Access
Impacts of historic anthropogenic aerosol forcing on large climate ensembles through the lens of poleward energy transport
(Colorado State University. Libraries, 2024) Needham, Michael Robert, author; Randall, David A., advisor; Rugenstein, Maria, committee member; van Leeuwen, Peter Jan, committee member; Rugenstein, Jeremy, committee member
In discussions of the human impact on Earth's climate, aerosols receive much less attention than greenhouse gases. And yet, the change in the global mean effective radiative forcing from anthropogenic aerosols was roughly of the same magnitude (but of opposite sign) as the change in greenhouse gases throughout much of the twentieth century. Aerosols also represent the largest uncertainty in the effective radiative forcing, due to their complex interactions with clouds and solar radiation. Complicating this even further, aerosols are relatively short-lived within the atmosphere, and thus exhibit a large degree of variability in space and time. This dissertation presents a set of studies which investigate the ways in which historic anthropogenic aerosols may have impacted the Earth's weather and climate, through the analysis of a large number historic climate model simulations which comprise so-called large ensembles. Analysis of these ensembles allows for the isolation of some forced signal (e.g., the influence of aerosols) from the noise (i.e., the background variability of the model). This leads to conclusions through the analysis of summary statistics across members of the ensemble population which would be impossible to make based on only one or a few simulations. In particular, these studies show that the emission of aerosol precursors from Europe and North America increased the northward transport of heat from the southern into the northern hemisphere in an ensemble of simulations performed with version 2 of the Community Earth System Model (CESM2). The additional heat transport was in excess of 0.25 PW. This is an increase of at least 4-5% compared to the baseline maximum transport of between 5-6 PW which occurs in the mid-latitudes. At latitudes away from these maxima, the increase was a much larger percentage of the total. This anomalous northward energy transport was accomplished by changes in both atmospheric and oceanic processes. These include a southward shift of the Intertropical Convergence Zone (ITCZ) associated with changes in the Hadley cells; an increase in the frequency of extratropical cyclones in the north Atlantic; a strengthening of the Atlantic Meridional Overturning Circulation (AMOC); as well as changes to multiple ocean processes across the Indo-Pacific. Comparison of these results to the literature indicates that this modeled response to aerosols in CESM2 is likely too large. Furthermore, analysis of two additional large ensembles reveals that this over-sensitivity of CESM2 cannot be due to some deficiency in the model. Instead, it is demonstrated that the difference is the result of changes to the historical emission estimates between phase 5 and phase 6 of the Coupled Model Intercomparison Project (i.e., CMIP5 and CMIP6). This finding leads to the hypothesis that the higher interannual variability associated with a change from decadal-scale CMIP5 emissions to annual-scale CMIP6 emissions is the ultimate cause of the overzealous response of the model. Testing this hypothesis likely will provide the most fertile ground for future work.
Open Access
Interactions of arctic clouds, radiation, and sea ice in present-day and future climates
(Colorado State University. Libraries, 2016) Burt, Melissa Ann, author; Randall, David A., advisor; Kreidenweis, Sonia M., committee member; Kummerow, Christian D., committee member; Betsill, Michele M., committee member
The Arctic climate system involves complex interactions among the atmosphere, land surface, and the sea-ice-covered Arctic Ocean. Observed changes in the Arctic have emerged and projected climate trends are of significant concern. Surface warming over the last few decades is nearly double that of the entire Earth. Reduced sea-ice extent and volume, changes to ecosystems, and melting permafrost are some examples of noticeable changes in the region. This work is aimed at improving our understanding of how Arctic clouds interact with, and influence, the surface budget, how clouds influence the distribution of sea ice, and the role of downwelling longwave radiation (DLR) in climate change. In the first half of this study, we explore the roles of sea-ice thickness and downwelling longwave radiation in Arctic amplification. As the Arctic sea ice thins and ultimately disappears in a warming climate, its insulating power decreases. This causes the surface air temperature to approach the temperature of the relatively warm ocean water below the ice. The resulting increases in air temperature, water vapor and cloudiness lead to an increase in the surface downwelling longwave radiation, which enables a further thinning of the ice. This positive ice-insulation feedback operates mainly in the autumn and winter. A climate-change simulation with the Community Earth System Model shows that, averaged over the year, the increase in Arctic DLR is three times stronger than the increase in Arctic absorbed solar radiation at the surface. The warming of the surface air over the Arctic Ocean during fall and winter creates a strong thermal contrast with the colder surrounding continents. Sea-level pressure falls over the Arctic Ocean and the high-latitude circulation reorganizes into a shallow "winter monsoon." The resulting increase in surface wind speed promotes stronger surface evaporation and higher humidity over portions of the Arctic Ocean, thus reinforcing the ice-insulation feedback. In the second half of this study, we explore the effects of super-parameterization on the Arctic climate by evaluating a number of key atmospheric characteristics that strongly influence the regional and global climate. One aspect in particular that we examine is the occurrence of Arctic weather states. Observations show that during winter the Arctic exhibits two preferred and persistent states — a radiatively clear and an opaquely cloudy state. These distinct regimes are influenced by the phase of the clouds and affect the surface radiative fluxes. We explore the radiative and microphysical effects of these Arctic clouds and the influence on these regimes in two present-day climate simulations. We compare simulations performed with the Community Earth System Model, and its super-parameterized counterpart (SP-CESM). We find that the SP-CESM is able to better reproduce both of the preferred winter states, compared to CESM, and has an overall more realistic representation of the Arctic climate.
Open Access
Model evaluation using space-borne lidar observations
(Colorado State University. Libraries, 2008) Ahlgrimm, Maike, author; Randall, David A., advisor
In this study, the use of space-borne lidar observations for the comparison with, and evaluation of modeled clouds is explored. Four version of the ECMWF Integrated Forecast System and two versions of the Goddard Earth Observing System (GEOS-5) model are assessed for their ability to produce marine boundary layer clouds. The cause of some of the model deficiencies is investigated, and specific suggestions for improvements are made and tested. In order to do so, two cloud types are defined: a stratocumulus type (Sc), and a trade cumulus or transitional cumulus type (TCu). Samples in four oceanic regions are classified into those categories, and the frequency of occurrence, location, and properties of the samples compared between models and observations.
Open Access
Observations and simulations of the interactions between clouds, radiation, and precipitation
(Colorado State University. Libraries, 2016) Naegele, Alexandra Claire, author; Randall, David A., advisor; Kummerow, Christian D., committee member; Ramirez, Jorge A., committee member
The first part of this study focuses on the radiative constraint on the hydrologic cycle as seen in observations. In the global energy budget, the atmospheric radiative cooling (ARC) is approximately balanced by latent heating, but on regional scales, the ARC and precipitation are inversely related. We use precipitation data from the Global Precipitation Climatology Project and radiative flux data from the Clouds and the Earth's Radiant Energy System (CERES) project to investigate the radiative constraint on the hydrologic cycle and how it changes in both space and time. We find that the effect of clouds is to decrease the ARC in the tropics, and to increase the ARC in middle and higher latitudes. We find that, spatially, precipitation and the ARC are negatively correlated in the tropics, and positively correlated in middle and higher latitudes. In terms of the global mean, the precipitation rate and the ARC are temporally out-of-phase during the Northern Hemisphere winter. In the second part of this study, we use a cloud-resolving model to gain a deeper understanding of the relationship between precipitation and the ARC. In particular, we explore how the relationship between precipitation and the ARC is affected by convective aggregation, in which the convective activity is confined to a small portion of the domain that is surrounded by a much larger region of dry, subsiding air. We investigate the responses of the ARC and precipitation rate to changes in the sea surface temperature (SST), domain size, and microphysics parameterization. Both fields increase with increasing SST and the use of 2-moment microphysics. The precipitation and ARC show evidence of convective aggregation, and in the domain average, both fields increase as a result. While running these sensitivity tests, we observed a pulsation in the convective precipitation rate, once aggregation had occurred. The period of the pulsation is on the order of ten simulated hours for a domain size of 768 km. The sensitivity tests mentioned above were used to investigate the mechanism of the pulsation. We also performed an additional test with no evaporation of falling rain, which leads to no cold pools in the boundary layer. Our results show that the period of the pulsation is noticeably sensitive to microphysics and domain size. The pulsation disappears completely when cold pools are prevented from forming, which suggests a "discharge-recharge" mechanism.
Open Access
Paleo-feedbacks in the hydrological and energy cycles in the Community Climate System Model 3
(Colorado State University. Libraries, 2008) Burt, Melissa A., author; Randall, David A., advisor; Denning, Scott, committee member; Wohl, Ellen E., 1962-, committee member; Otto-Bliesner, Bette, committee member
This research focuses on the joint variability of the hydrological and energy cycles for the atmosphere and lower boundary and climate feedbacks associated with these changes at the Last Glacial Maximum. The LGM simulated climate experiences a global cooling of 4.9 K compared to the PI climate, with greatest cooling in the high latitudes of both hemispheres. Additional cooling also exists over the continental ice sheets in North America, Northern Europe, and Antarctica. Precipitation and evaporation are reduced by 10%, and precipitable water by 20%, compared to conditions at PI. Overall, from LGM to PI the changes in clouds are weak. The water vapor, ice-albedo, and cloud feedbacks act to amplify the climate change from LGM to PI. The positive water vapor and ice-albedo feedbacks account for 5.04 W m-2 K-1 and 2.38 W m-2 K-1, respectively of the climate change. The cloud feedbacks produces -2.83 of the change. An interesting and unexpected result was that the sign of the ice-albedo feedback changed regionally and is driven by changes in ocean basin size. Combined, the radiative feedbacks from LGM to PI act to amplify the climate change by 5.67 W m-2 K-1 and are balanced by an increase in surface evaporation.
Open Access
Quantifying and understanding current and future links between tropical convection and the large-scale circulation
(Colorado State University. Libraries, 2020) Jenney, Andrea M., author; Randall, David A., advisor; Barnes, Elizabeth A., advisor; Maloney, Eric, committee member; Rasmussen, Kristen, committee member; Anderson, Georgiana Brooke, committee member
Tropical deep convection plays an important role in the variability of the global circulation. The Madden Julian Oscillation (MJO) is a large tropical organized convective system that propagates eastward along the equator. It is a key contributor to weather predictability at extended time scales (10-40 days). For example, variability in the MJO is linked with variability in meteorological phenomena such as landfalling atmospheric rivers, tornado and hail activity over parts of North America, and extreme temperature and rainfall patterns across the Northern Hemisphere. Links between the MJO and atmospheric variability in remote locations are heavily studied. This is in part because the current skill of weather forecasts at extended time scales is mediocre, and because of evidence suggesting that the potential predictability offered by the MJO may not be fully captured in numerical prediction models. In the first part of this dissertation, I develop a tool for these types of studies. The "Sensitivity to the Remote Influence of Periodic Events" (STRIPES) index is a novel index that condenses the information obtained through composite analysis of variables after a periodic event (such as the MJO) into a single number, which includes information about the life cycle of the event, and for a range of lags with respect to each stage of the event. I apply the STRIPES index to surface observations and show that the MJO signal is detectable and significant at the level of individual weather stations over many parts of North America, and that the maximum strength of this signal exhibits regionality and seasonality. Tropical convection affects the extratropics primarily through the excitation of Rossby waves at the places where the upper-tropospheric divergent outflow associated with deep convection interacts with the background wind. In a future warmer climate, the strength of the mean circulation and convective mass flux is expected to weaken. A potential consequence is a weakening of Rossby wave excitation by tropical convective systems such as the MJO. In the second part of this study, I analyze a set of idealized simulations with specified surface warming and superparameterized convection and develop a framework to better understand why the mean circulation weakens with warming. I show that the decrease in the strength of the mean circulation can be explained by the slow rate at which atmospheric radiative cooling intensifies relative to the comparatively fast rate that the tropical dry static stability increases. I also show that despite a decrease in the mean convective mass flux, the warming tendency of the convective mass flux over the most deeply- convecting regions is not constrained to follow that of the global mean. In the final part of this dissertation, I consider how changes in the MJO and of the mean atmospheric state due to warming from increases in greenhouse gas concentrations may lead to changes in the MJO's impact over the North Pacific and North America. Specifically, I show that changes to the atmosphere's mean state dry static energy and winds have a larger impact on the MJO teleconnection than changes to MJO intensity and propagation characteristics.
Open Access
Seasonal, synoptic, and intraseasonal variability of the West African monsoon
(Colorado State University. Libraries, 2012) McCrary, Rachel Rose, author; Randall, David A., advisor; Denning, A. Scott, committee member; van den Heever, Susan C., committee member; Betsill, Michele, committee member
The simulation of the West African monsoon is examined in two coupled general circulation models (CGCMs). The first model is the standard Community Climate System Model (CCSM) which uses traditional parameterizations to represent convective processes. The second model is the superparameterized-CCSM (SP-CCSM), in which convective parameterizations have been replaced by embedding a two-dimensional cloud resolving model into each gridbox. Superparameterization is intended to improve simulation of the complex multiscale interactions that occur between the large-scale environment and clouds. Key features of West African climate are analyzed in both models including: the mean annual cycle of the monsoon, African easterly wave (AEW) activity and dynamics, and the intraseasonal modulation of precipitation. Adding superparameterization improves the position and intensity of the summer maximum in precipitation which is shifted from over the Gulf of Guinea in CCSM (not realistic), to over the continent in SP-CCSM which is in keeping with the observations. AEWs and their relationship with convection are also improved in the SP-CCSM: In the standard model, little to no easterly wave activity occurs over West Africa, and the relationship with convection is tenuous at best. SP-CCSM on the other hand produces strong AEWs over the region that exhibit similar horizontal and vertical structures to observations. AEWs in SP-CCSM are strongly coupled to convection, more so than is supported by observations. An examination of the energetics of the simulated AEWs suggests that convection drives the generation and propagation the waves in SP- CCSM. Consistent with observations, intraseasonal variations in West African precipitation in SP-CCSM appear to be linked to variations in convection in the Indo-Pacific region corresponding with the MJO and the Indian monsoon. Because of these physically-realistic relationships, SP-CCSM has potential to deepen our understanding of the teleconnections between the MJO and West Africa, helping to improve seasonal rainfall forecasts.
Open Access
The impacts of Amazon deforestation on Pacific climate
(Colorado State University. Libraries, 2016) Lindsey, Leah, author; Randall, David A., advisor; Denning, A. Scott, committee member; Kirkpatrick, Allan T., committee member
Variability in eastern Pacific sea surface temperatures (SSTs) associated with the El Niño Southern Oscillation are known to affect Amazonian precipitation, but to what extent do changing Amazonian vegetation and rainfall impact eastern Pacific SST? The Amazon rainforest is threatened by many factors including climate change and clearing for agricultural reasons. Forest fires and dieback are more likely due to increased frequency and intensity of droughts in the region. It is possible that extensive Amazon deforestation can enhance El Niño conditions by weakening the Walker circulation. Correlations between annual rainfall rates over the Amazon and other atmospheric parameters (global precipitation, surface air temperature, low cloud amount, 500 hPa vertical velocity, surface winds, and 200 hPa winds) over the eastern Pacific indicate strong relationships among these fields. Maps of these correlations (teleconnection maps) reveal that when the Amazon is rainy SSTs in the central and eastern Pacific are cold, rainfall is suppressed over the central and eastern Pacific, low clouds are prominent over the eastern and southeastern Pacific, and subsidence over the central and eastern Pacific is enhanced. Precipitation in the Amazon is also consistent with a strong Walker circulation (La Niña conditions), manifest as strong correlations with the easterly surface and westerly 200 hPa zonal winds. Coupling between Amazon rainfall and these fields are seen in observations and model data. Correlations were calculated using data from observations, reanalysis data, two models under the Coupled Model Intercomparison Project/Atmospheric Model Intercomparison Project (CMIP5/AMIP), and an AMIP run with the model used in this study, the Community Earth System Model (CESM1.1.1). Although the correlations between Amazon precipitation and the aforementioned fields are strong, they do not show causality. In order to investigate the impact of tropical South American deforestation on the Pacific climate, numerical experiments were performed using the CESM. Amazon deforestation was studied in an idealized world where a single continent was covered in forest and then, in a separate simulation, covered in grassland. Four different sets of simulations were carried out: 1) the baseline idealized set-up with prescribed SST, 2) another with an Andes-like mountain range, 3) a simulation with a slab ocean model rather than prescribed SST, and 4) a simulation repeated with the standard Community Atmosphere Model (CAM4) replaced by the Superparameterized version (SP-CAM). The continent in these simulations was compared to the Amazon, and the ocean to the west of the continent was compared to the eastern Pacific. All of the simulations showed a strong warming of around 3-4˚C over the continent going from forest to grassland. A notable decrease in precipitation over land of about 1-3 mm day-1 and increase to the west of the continent of about 1-2 mm day-1 was also observed in most of the simulations. The simulations with the slab ocean model showed enhanced precipitation changes with a corresponding decrease of 2-4 mm day-1 over land and increase of 3-5 mm day-1 west of the continent. Simulations that used the SP-CAM showed very small changes in precipitation, which was likely due to the decreased spin-up time allowed for these simulations. The decrease in the surface roughness and reduction in the evapotranspiration for the simulations with grassland contributed to these changes in surface temperature and precipitation. The conversion of forest to grassland in our experiments imply that deforestation can lead to weakening of the Walker circulation by weakening easterly surface winds and westerly upper tropospheric winds. These findings suggest that large-scale Amazon deforestation is capable of enhancing El Niño conditions.
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 member
The 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.