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Author: Alexandre Jousse
Publisher:
ISBN:
Category :
Languages : en
Pages : 102
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Marine Stratocumulus clouds are prevalent over the eastern boundary of the subtropical oceans (e.g. northeast and southeast Pacific). Due to their shortwave properties, these low clouds significantly impact the regional and global climate. However marine stratocumulus clouds are subject to modeling approximations as well as, numerous uncertainties on the factors contributing to their radiative properties, variability and possible future changes. In this dissertation, we present three regional modeling studies that intend to provide some more understanding to these issues. We first analyze the sensitivity of marine stratocumulus to parameterizations in the Weather Research and Forecasting (WRF) model. We use the southeast Pacific as a testbed region and compare the simulated surface energy fluxes to those measured during VOCALS-REx. Our results show that errors in shortwave fluxes are traceable to errors in liquid water path (LWP). Two mechanisms controlling the LWP in our simulations are diagnosed. The first mechanism involves boundary layer and shallow cumulus schemes, which control moisture available for cloud by regulating boundary layer height. The second mechanism involves microphysics schemes, which control LWP through the production of drizzle. This study demonstrates that when parameterizations are appropriately chosen, the stratocumulus deck and the related surface energy fluxes are reasonably well represented in WRF. In s second study, we take advantage of these advancements to evaluate the importance of aerosol indirect effects on clouds shortwave properties in the northeast Pacific. Satellite retrievals (e.g. MODIS) show that the cloud droplet number concentration is generally high along the U.S. west coast (~300cm-3), while it drops to smaller values further offshore (~50cm-3). Our results highlight the importance of representing accurately this aerosol spatial variability and the associated indirect effects on LWP for realistic shortwave fluxes simulations in the northeast Pacific. Finally, we analyze the marine stratocumulus variability and their possible anthropogenic changes using a suite of dynamically downscaled experiments in the California region. In particular, we develop a methodology that enables a clear identification of the factors contributing to low cloud cover anthropogenic changes. Our results show a systematic reduction in low cloud cover, which is mostly imputable to a reduction of the coupling between boundary layer top and surface. Our analysis suggests that the enhanced decoupling conditions might be at least partially driven by the drying of the free troposphere in comparison to the boundary layer in future climate.
Author: Alexandre Jousse
Publisher:
ISBN:
Category :
Languages : en
Pages : 102
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Book Description
Marine Stratocumulus clouds are prevalent over the eastern boundary of the subtropical oceans (e.g. northeast and southeast Pacific). Due to their shortwave properties, these low clouds significantly impact the regional and global climate. However marine stratocumulus clouds are subject to modeling approximations as well as, numerous uncertainties on the factors contributing to their radiative properties, variability and possible future changes. In this dissertation, we present three regional modeling studies that intend to provide some more understanding to these issues. We first analyze the sensitivity of marine stratocumulus to parameterizations in the Weather Research and Forecasting (WRF) model. We use the southeast Pacific as a testbed region and compare the simulated surface energy fluxes to those measured during VOCALS-REx. Our results show that errors in shortwave fluxes are traceable to errors in liquid water path (LWP). Two mechanisms controlling the LWP in our simulations are diagnosed. The first mechanism involves boundary layer and shallow cumulus schemes, which control moisture available for cloud by regulating boundary layer height. The second mechanism involves microphysics schemes, which control LWP through the production of drizzle. This study demonstrates that when parameterizations are appropriately chosen, the stratocumulus deck and the related surface energy fluxes are reasonably well represented in WRF. In s second study, we take advantage of these advancements to evaluate the importance of aerosol indirect effects on clouds shortwave properties in the northeast Pacific. Satellite retrievals (e.g. MODIS) show that the cloud droplet number concentration is generally high along the U.S. west coast (~300cm-3), while it drops to smaller values further offshore (~50cm-3). Our results highlight the importance of representing accurately this aerosol spatial variability and the associated indirect effects on LWP for realistic shortwave fluxes simulations in the northeast Pacific. Finally, we analyze the marine stratocumulus variability and their possible anthropogenic changes using a suite of dynamically downscaled experiments in the California region. In particular, we develop a methodology that enables a clear identification of the factors contributing to low cloud cover anthropogenic changes. Our results show a systematic reduction in low cloud cover, which is mostly imputable to a reduction of the coupling between boundary layer top and surface. Our analysis suggests that the enhanced decoupling conditions might be at least partially driven by the drying of the free troposphere in comparison to the boundary layer in future climate.
Author: Christopher R. Jones
Publisher:
ISBN:
Category : Climatic changes
Languages : en
Pages : 142
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Book Description
Subtropical stratocumulus clouds are important part of the Earth's energy budget. The response of low clouds to Earth's changing climate is one of the dominant uncertainties in global warming projections, due primarily to unresolved parameterized cloud processes in global climate models (GCMs). Improving our understanding of the role stratocumulus clouds play in response to climate change requires both a better understanding of the underlying physical mechanisms that govern stratocumulus clouds, and also better representing these processes in GCMs. This work addresses both aspects, using a range of models, from an idealized mixed-layer model (MLM) to a high-resolution large eddy simulation (LES). The aerosol indirect effect (AIE) for nondrizzling stratocumulus clouds is strongly dependent on an entrainment-sedimentation feedback that increases the entrainment efficiency for higher droplet concentrations, thereby decreasing the cloud amount. However, a single column model (SCM) derived from the CAM5 GCM exhibits the opposite sign of this response mechanism. Using an SCM we find that a combination of issues contribute to this, but the primary cause is due to a representation of cloud condensate within the PBL parameterization that is inconsistentwith other microphysical parameterizations that affect the cloud liquid water, rendering the entrainment efficiency insensitive to droplet concentration. The response of a representative stratocumulus to a variety of idealized climate-change perturbations is used in conjunction with an identically-forced LES to interpret the underlying mechanisms behind the observed sensitivity. The MLM and LES agree remarkably well for all cases where the boundary layer doesn't decouple in the LES. For doubling CO2 forcing perturbations estimated from the CMIP3 multimodel mean, the MLM predicts a positive shortwave cloud feedback, like most CMIP3 global climate models. The cloud remains overcast but thins in the warmer, moister, CO2-enhanced climate, due to the combined effects of an increased lower-tropospheric vertical humidity gradient and an enhanced free tropospheric greenhouse effect that reduces the radiative driving of turbulence. Reduced subsidence due to weakening of tropical overturning circulations and a strengthening of the capping inversion partly counteract these two factors by raising the inversion and allowing the cloud layer to deepen. These compensating mechanisms may explain the large scatter in low cloud feedbacks predicted by climate models. The rapidity with which a stratocumulus cloud can respond to perturbations is important for understanding its response to perturbations that occur across a range of characteristic time scales. Using a MLM and LES, I show that there are three separate timescales: a slow adjustment timescale associated with boundary layer deepening (several days), an intermediate thermodynamic timescale (approximately 1 day), and a hitherto unidentified fast timescale (6-12 hours) for cloud water path adjustment associated with internal entrainment rate feedbacks. The fast timescale response is elicited by perturbations to overlying humidity and surface and atmosphere temperature, whereas purely radiative perturbations do not elicit an entrainment-liquid water path feedback. A range of MLM entrainment closures are shown to support a fast timescale, provided the entrainment rate is sensitive to the integrated buoyancy flux. The underlying entrainment-liquid flux adjustment mechanism suggests a cloud-thinning response to a uniformly warmed climate perturbation.
Author: J. T. Kiehl
Publisher: Cambridge University Press
ISBN: 9781139453233
Category : Science
Languages : en
Pages : 410
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Book Description
The physics and dynamics of the atmosphere and atmosphere-ocean interactions provide the foundation of modern climate models, upon which our understanding of the chemistry and biology of ocean and land surface processes are built. Originally published in 2006, Frontiers of Climate Modeling captures developments in modeling the atmosphere, and their implications for our understanding of climate change, whether due to natural or anthropogenic causes. Emphasis is on elucidating how greenhouse gases and aerosols are altering the radiative forcing of the climate system and the sensitivity of the system to such perturbations. An expert team of authors address key aspects of the atmospheric greenhouse effect, clouds, aerosols, atmospheric radiative transfer, deep convection dynamics, large scale ocean dynamics, stratosphere-troposphere interactions, and coupled ocean-atmosphere model development. The book is an important reference for researchers and advanced students interested in the forces driving the climate system and how they are modeled by climate scientists.
Author:
Publisher:
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Category :
Languages : en
Pages :
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Marine low clouds are a major determinant of the Earth?s albedo and are a major source of uncertainty in how the climate responds to changing greenhouse gas levels and anthropogenic aerosol. Marine low clouds are particularly difficult to simulate accurately in climate models, and their remote locations present a significant observational challenge. A complex set of interacting controlling processes determine the coverage, condensate loading, and microphysical and radiative properties of marine low clouds. Marine low clouds are sensitive to atmospheric aerosol in several ways. Interactions at microphysical scales involve changes in the concentration of cloud droplets and precipitation, which induce cloud dynamical impacts including changes in entrainment and mesoscale organization. Marine low clouds are also impacted by atmospheric heating changes due to absorbing aerosols. The response of marine low clouds to aerosol perturbations depends strongly upon the unperturbed aerosol-cloud state, which necessitates greater understanding of processes controlling the budget of aerosol in the marine boundary layer. Entrainment and precipitation mediate the response of low clouds to aerosols but these processes also play leading roles in controlling the aerosol budget. The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility and Atmospheric System Research (ASR) program are making major recent investments in observational data sets from fixed and mobile sites dominated by marine low clouds. This report provides specific action items for how these measurements can be used together with process modeling to make progress on understanding and quantifying the key cloud and aerosol controlling processes in the next 5-10 years. Measurements of aerosol composition and its variation with particle size are needed to advance a quantitative, process-level understanding of marine boundary-layer aerosol budget. Quantitative precipitation estimates that combine radar and lidar measurements are becoming available, and these could be used to test process models, quantify precipitation responses to aerosol, and constrain climate models. Models and observations can be used to constrain how clouds respond dynamically to changing precipitation. New measurements of turbulence from ground-based remote sensing could be used to attempt to relate entrainment to the vertical and horizontal structure of turbulence in the boundary layer. Cloud-top entrainment plays a major role in modulating how low clouds respond to both aerosols and to greenhouse gases, so investment in promising new observational estimates would be beneficial. Precipitation formation and radiative cooling both help marine low clouds to organize on the mesoscale. More work is needed to develop metrics to characterize mesoscale organization, to elucidate mechanisms that determine the type and spatial scale of mesoscale cellular convection, and to understand the role of mesoscale structures in the stratocumulus-to-cumulus transition.
Author: National Academies of Sciences, Engineering, and Medicine
Publisher: National Academies Press
ISBN: 0309467578
Category : Science
Languages : en
Pages : 717
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Book Description
We live on a dynamic Earth shaped by both natural processes and the impacts of humans on their environment. It is in our collective interest to observe and understand our planet, and to predict future behavior to the extent possible, in order to effectively manage resources, successfully respond to threats from natural and human-induced environmental change, and capitalize on the opportunities â€" social, economic, security, and more â€" that such knowledge can bring. By continuously monitoring and exploring Earth, developing a deep understanding of its evolving behavior, and characterizing the processes that shape and reshape the environment in which we live, we not only advance knowledge and basic discovery about our planet, but we further develop the foundation upon which benefits to society are built. Thriving on Our Changing Planet presents prioritized science, applications, and observations, along with related strategic and programmatic guidance, to support the U.S. civil space Earth observation program over the coming decade.
Author: United States. Department of Energy. Office of Health and Environmental Research
Publisher:
ISBN:
Category : Atmospheric physics
Languages : en
Pages : 436
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Author: Xiaoli Zhou
Publisher:
ISBN:
Category :
Languages : en
Pages :
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"The extensive shallow marine stratocumulus over the eastern subtropical oceans is one of the most important cloud systems owning to their significant cooling effect on the Earth's radiation budget. Due to insufficient understanding of the key processes governing its behavior, stratocumulus remains misrepresented in climate models. The response of stratocumulus to changes in atmospheric aerosols modifies cloud microphysics, modulates precipitation, entrainment, and dynamics, which in turn regulates cloud macrophysical processes. This is referred to as the microphysical-macrophysical interactions. A better understanding of this process helps determine cloud radiative properties that impact climate. This dissertation provides a process-level understanding of microphysical-macrophysical interactions of marine stratocumulus on the synoptic scale and on the mesoscale via numerical simulations. On the synoptic scale, when advected by the trade winds from the subtropics toward the equator, stratocumulus thins and eventually breaks up to low-coverage cumulus as a result of warming sea surface temperature (SST). The trade winds also transport continental air masses downstream which often contain solar-absorbing aerosols from biomass burning. In this dissertation, we employ large-eddy simulations to investigate the impact of solar-absorbing aerosol layers on such cloud transition. On the mesoscale, stratocumulus clouds have been consistently observed to organize in either closed-cellular (dim walls, bright centers) or open-cellular (bright walls, dim centers) structures on horizontal scales of 10-50 km, yet no complete theory on the mechanism for the scale of open and closed cells has appeared in the literature. Here numerical experiments are designed to investigate the relation between cloud scale and aerosol loading, as well as the mechanism behind it. " --
Author: A. D. Marrs
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author:
Publisher:
ISBN:
Category : Climatic changes
Languages : en
Pages : 260
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Author: Constantin Andronache
Publisher: Elsevier
ISBN: 012810550X
Category : Science
Languages : en
Pages : 302
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Book Description
Mixed-Phase Clouds: Observations and Modeling presents advanced research topics on mixed-phase clouds. As the societal impacts of extreme weather and its forecasting grow, there is a continuous need to refine atmospheric observations, techniques and numerical models. Understanding the role of clouds in the atmosphere is increasingly vital for current applications, such as prediction and prevention of aircraft icing, weather modification, and the assessment of the effects of cloud phase partition in climate models. This book provides the essential information needed to address these problems with a focus on current observations, simulations and applications. - Provides in-depth knowledge and simulation of mixed-phase clouds over many regions of Earth, explaining their role in weather and climate - Features current research examples and case studies, including those on advanced research methods from authors with experience in both academia and the industry - Discusses the latest advances in this subject area, providing the reader with access to best practices for remote sensing and numerical modeling
