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Author: Gallia Painter
Publisher:
ISBN:
Category : Boundary layer (Meteorology)
Languages : en
Pages : 47
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Book Description
Low clouds lie at the heart of climate feedback uncertainties. The representation of clouds in global climate models relies on parameterization of many sub-grid scale processes that are crucial to understanding cloud responses to climate; low clouds in particular exist as a result of tightly coupled microphysical, mesoscale, and synoptic mechanisms. The influence of anthropogenic aerosols on cloud properties could have important ramifications for our understanding of how clouds respond to a changing climate. The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS REx) sampled the persistent stratocumulus cloud deck located off the coast of Peru and Chile in the southeastern Pacific ocean. Several cloud features found in the stratocumulus deck during VOCALS exhibit signs of interesting aerosol-cloud interactions, including pockets of open cells (POCs). POCs are regions of open-cellular convection surrounded by closed cell stratocumulus, exhibiting not only a marked transition in mesoscale organization and cloud morphology, but also sharp microphysical gradients (especially in droplet concentration) across the boundary between open-cellular and closed cellular convection. In addition, precipitation is often higher at the POC boundaries, hinting at the importance of precipitation in driving their formation. In order to evaluate the microphysical characteristics of POCs prior cloud breakup, we use Lagrangian trajectories coupled with geostationary satellite imagery and cloud retrievals, as well as observational data from VOCALS REx and model data. In three of our case studies, we found regions of anomalously low droplet concentration 18-24 hours prior to POC formation (coupled with liquid water path similar to or higher than surrounding cloud), supporting a precipitation driven mechanism for POC formation. Another group of features with interesting aerosol-cloud interactions observed during VOCALS were mesoscale hook-like features of high droplet concentration which extend far offshore into regions of normally very clean cloud. We use Lagrangian trajectories to investigate the source of the high droplet concentrations of the mesoscale "hooks," and evaluate whether boundary layer transport of coastal pollutants alone can account for their extent. We find that boundary layer trajectories past 85 W do not pass sufficiently close to the coastline to explain high aerosol concentrations offshore.
Author: Gallia Painter
Publisher:
ISBN:
Category : Boundary layer (Meteorology)
Languages : en
Pages : 47
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Book Description
Low clouds lie at the heart of climate feedback uncertainties. The representation of clouds in global climate models relies on parameterization of many sub-grid scale processes that are crucial to understanding cloud responses to climate; low clouds in particular exist as a result of tightly coupled microphysical, mesoscale, and synoptic mechanisms. The influence of anthropogenic aerosols on cloud properties could have important ramifications for our understanding of how clouds respond to a changing climate. The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS REx) sampled the persistent stratocumulus cloud deck located off the coast of Peru and Chile in the southeastern Pacific ocean. Several cloud features found in the stratocumulus deck during VOCALS exhibit signs of interesting aerosol-cloud interactions, including pockets of open cells (POCs). POCs are regions of open-cellular convection surrounded by closed cell stratocumulus, exhibiting not only a marked transition in mesoscale organization and cloud morphology, but also sharp microphysical gradients (especially in droplet concentration) across the boundary between open-cellular and closed cellular convection. In addition, precipitation is often higher at the POC boundaries, hinting at the importance of precipitation in driving their formation. In order to evaluate the microphysical characteristics of POCs prior cloud breakup, we use Lagrangian trajectories coupled with geostationary satellite imagery and cloud retrievals, as well as observational data from VOCALS REx and model data. In three of our case studies, we found regions of anomalously low droplet concentration 18-24 hours prior to POC formation (coupled with liquid water path similar to or higher than surrounding cloud), supporting a precipitation driven mechanism for POC formation. Another group of features with interesting aerosol-cloud interactions observed during VOCALS were mesoscale hook-like features of high droplet concentration which extend far offshore into regions of normally very clean cloud. We use Lagrangian trajectories to investigate the source of the high droplet concentrations of the mesoscale "hooks," and evaluate whether boundary layer transport of coastal pollutants alone can account for their extent. We find that boundary layer trajectories past 85 W do not pass sufficiently close to the coastline to explain high aerosol concentrations offshore.
Author: Johannes Karel Christiaan Mohrmann
Publisher:
ISBN:
Category :
Languages : en
Pages : 104
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Book Description
Marine low clouds are an important feature of the climate system, cooling the planet due to their high albedo and warm temperatures. They display a variety of different mesoscale organizations, which are tied to the varying environmental conditions in which they occur. This dissertation explores the drivers of marine low cloud variability using an observational perspective that draws upon aircraft, satellite, and reanalysis data, and uses the application of a number of machine learning techniques. The focus is largely though not exclusively on mesoscale organization. In the first part of this work, data from the Cloud System Evolution over the Trades (CSET) campaign over the Pacific stratocumulus-to-cumulus transition are organized into 18 Lagrangian cases suitable for study and future modeling, made possible by the use of a track-and-resample flight strategy. Analysis of these cases shows that 2-day Lagrangian coherence of long-lived species (CO and O3) is high (r=0.93 and 0.73, respectively), but that of subcloud aerosol, MBL depth, and cloud properties is limited. Although they span a wide range in meteorological conditions, most sampled air masses show a clear transition when considering 2-day changes in cloudiness (-31%averaged over all cases), MBL depth (1560 m), estimated inversion strength (EIS; 22.2K), and decoupling, agreeing with previous satellite studies and theory. Changes in precipitation and droplet number were less consistent. The aircraft-based analysis is augmented by geostationary satellite retrievals and reanalysis data along Lagrangian trajectories between aircraft sampling times, documenting the evolution of cloud fraction, cloud droplet number concentration, EIS, and MBL depth. An expanded trajectory set spanning the summer of 2015 is used to show that the CSET-sampled air masses were representative of the season, with respect to EIS and cloud fraction. Two Lagrangian case studies attractive for future modeling are presented with aircraft and satellite data. The first features a clear Sc-Cu transition involving MBL deepening and decoupling with decreasing cloud fraction, and the second undergoes a much slower cloud evolution despite a greater initial depth and decoupling state. Potential causes for the differences in evolution are explored, including free-tropospheric humidity, subsidence, surface fluxes, and microphysics. The remaining work focuses on the mesoscale organization of marine low clouds. A convolutional neural network (CNN) model is trained to classify 128 km by 128 km scenes of marine low clouds into six categories: stratus, open-cellular mesoscale cellular convection (MCC), closed-cellular MCC, disorganized MCC, clustered cumulus, and suppressed cumulus. Overall model test accuracy was approximately 90%. This model is applied to three years of data in the southeast Pacific, as well as the 2015 northeast Pacific summer for comparison with the CSET campaign. Meteorological variables related to marine low cloud processes are composited by mesoscale cloud type, allowing for the identification of distinct meteorological regimes. Presentation of MCC is largely consistent with previous literature, both in terms of geographic distribution boundary layer structure, and cloud-controlling factors. The two more novel types, clustered and suppressed cumulus, are examined in more detail. The patterns in precipitation, circulation, column water vapor, and cloudiness are consistent with the presentation of marine shallow mesoscale convective self-aggregation found in previous large eddy simulations of the boundary layer. Although they occur under similar large-scale conditions, the suppressed and clustered low cloud regimes are found to be well-separated by variables associated with a low-level mesoscale circulation, with surface wind divergence being the clearest discriminator between them, whether reanalysis or satellite observations are used. Divergence is consistent with near-surface inflow into clustered regimes and outflow from suppressed regimes. To further understand the dependencies of mesoscale cloud type on environmental factors, a second classification model is built. This uses a random forest of decision trees to predict cloud type, but instead of using an image of a cloud scene, mesoscale averages of meteorological variables are used as inputs. The model uses the three-year dataset output from the CNN model for training, and overall accuracy is approximately 50%. Rotated principal component analysis of the meteorological variables is used to create a set of decorrelated features on which to train the model, allowing for the application of certain statistical analyses which rely on uncorrelated data. Permutation feature importance is used to quantify which variables are most important for correct prediction of cloud mesoscale organization. Overall, temperature and stability are approximately equally important; for correctly distinguishing between open-MCC and closed-MCC, stability is the most important feature, and for correctly distinguishing between suppressed and clustered cumulus, surface divergence is the most important variable. Partial dependence analysis is used to show the relationship between each input variable and the likelihood of observing each cloud type, and 2-dimensional partial dependence analysis shows bimodal distributions of MCC types, consistent with their subtropical and midlatitude incarnations. The random forest model is able to reproduce the geographic distributions of cloud type occurrences.
Author: Efthymios Serpetzoglou
Publisher:
ISBN:
Category : Boundary layer (Meteorology)
Languages : en
Pages : 180
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Author: Xue Zheng
Publisher:
ISBN:
Category :
Languages : en
Pages :
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This dissertation investigates the impacts of meteorological factors and aerosol indirect effects on the costal marine stratocumulus (Sc) variations in the southeast Pacific, a region that has been largely unexplored and is a major challenge of the modeling community, through both observational and numerical studies. This study provides a unique dataset for documenting the characteristics of the marine Sc-topped BL off the coast of Northern Chile. The observational study shows that the boundary layer (BL) over this region was well mixed and topped by a thin and non-drizzling Sc layer on days synoptically-quiescent with little variability between this region and the coast. The surface wind, the surface fluxes and the BL turbulence appeared to be weaker than those over other ocean regions where stratocumulus clouds exist. The weaker turbulence in the BL may contribute to a relatively low entrainment rate calculated from the near cloud top fluxes. This in-situ data set can help us better understand cloud processes within this coastal regime, and also be valuable for the calibration of the satellite retrievals and the evaluation of numerical models operating at a variety of scales. A strong positive correlation between the liquid water path (LWP) and the cloud condensation nuclei (CCN) was observed under similar boundary layer conditions. This correlation cannot be explained by some of the hypotheses based on previous modeling studies. The satellite retrievals obtained upstream one day prior to the flight observations reveal some sign that the clouds under the high CCN concentrations have minimal LWP loss due to precipitation suppression effects. The results from large eddy simulations with a two-momentum bulk microphysics scheme under different idealized environment scenarios based on aircraft observations indicate that 1) the simulated Sc responds more quickly to changes in large-scale subsidence than to those changes in surface fluxes, free-tropospheric humidity, and the BL-top stability; 2) large-scale vertical wind shear clearly induces cloud-top mixing and enhances entrainment rate; 3) the solar radiation could weaken the BL turbulence, reduce the entrainment rate and decouple the BL; and 4) the impact of the reduced cloud sedimentation due to increasing aerosol on the cloud is small.
Author:
Publisher:
ISBN:
Category : Meteorology
Languages : en
Pages : 1652
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Author: Robert Pincus
Publisher: Springer
ISBN: 3319772732
Category : Science
Languages : en
Pages : 396
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Book Description
This volume presents a series of overview articles arising from a workshop exploring the links among shallow clouds, water vapor, circulation, and climate sensitivity. It provides a state-of-the art synthesis of understanding about the coupling of clouds and water vapor to the large-scale circulation. The emphasis is on two phenomena, namely the self-aggregation of deep convection and interactions between low clouds and the large-scale environment, with direct links to the sensitivity of climate to radiative perturbations. Each subject is approached using simulations, observations, and synthesizing theory; particular attention is paid to opportunities offered by new remote-sensing technologies, some still prospective. The collection provides a thorough grounding in topics representing one of the World Climate Research Program’s Grand Challenges. Previously published in Surveys in Geophysics, Volume 38, Issue 6, 2017 The aritcles “Observing Convective Aggregation”, “An Observational View of Relationships Between Moisture Aggregation, Cloud, and Radiative Heating Profiles”, “Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations”, “A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment”, “Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review”, “Mechanisms and Model Diversity of Trade-Wind Shallow Cumulus Cloud Feedbacks: A Review”, “Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere”, “Emerging Technologies and Synergies for Airborne and Space-Based Measurements of Water Vapor Profiles”, “Observational Constraints on Cloud Feedbacks: The Role of Active Satellite Sensors”, and “EUREC4A: A Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation” are available as open access articles under a CC BY 4.0 license at link.springer.com.
Author: M.K. Yau
Publisher: Elsevier
ISBN: 0080570941
Category : Science
Languages : en
Pages : 308
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Book Description
Covers essential parts of cloud and precipitation physics and has been extensively rewritten with over 60 new illustrations and many new and up to date references. Many current topics are covered such as mesoscale meteorology, radar cloud studies and numerical cloud modelling, and topics from the second edition, such as severe storms, precipitation processes and large scale aspects of cloud physics, have been revised. Problems are included as examples and to supplement the text.
Author: Ryan Maxwell Eastman
Publisher:
ISBN:
Category :
Languages : en
Pages : 138
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This work analyzes cloud processes on daily timescales in stratocumulus (Sc) cloud decks over eastern subtropical oceans. These clouds have a significant effect on climate. Subtropical Sc decks reflect abundant solar radiation, but emit infrared radiation at a temperature nearly as warm as the surface, causing a net cooling to the climate system. Climate models tend to poorly represent subtropical Sc, often producing too few clouds, which are too bright. The size and significance of these cloud decks combined with their poor representation in models motivates further research into how clouds are responding to their environment, and what internal processes drive observed behavior. To address this knowledge gap, a substantial Lagrangian framework is developed here, where thousands of cloudy parcels are followed for several days over the eastern subtropical oceans. These parcels are repeatedly sampled at 12-hour intervals, creating a large-scale dataset of environmental and cloud variables that can incorporate a time dimension with a variety of lead times. This framework is applied here in several ways: The first chapter incorporates the Lagrangian framework in two climate models as well as in observations for the same year. The model comparison shows which daily-scale cloud processes are well simulated and which are poorly simulated. The final two chapters address transitions in cellular structure in Sc cloud decks, first assessing which variables are associated with 24-hour transitions from classic, closed cell Sc to more broken open cells or disorganized cells, showing that transitions in cellularity are associated with separate mechanisms. In chapter 3, the results from chapter 2 are put in context using an expanded set of trajectories to study multi-step processes that incorporate interplay between environmental variables, cloud processes, and observed changes in cellular structure. The following paragraphs introduce the three chapters. A Lagrangian framework is developed and applied to two GCMs (CAM5 and HadGEM, a.k.a. UKMET) and an observational dataset in order to compare the daily-scale evolution of cloud cover and cloud variables between models and observations in the eastern subtropical oceans. Cloud cover in both models is less extensive than cloud cover seen by MODIS. Observed rain rates, as estimated by CloudSat-tuned AMSR/E 89 GHz brightness temperatures, is comparable to the HadGEM rain rates, while CAM5 rain rates appear too heavy. Inversion height estimated by MODIS and CALIPSO-observed cloud tops falls between the too-shallow CAM5 inversion and the too-deep HadGEM inversion. Lagrangian decorrelation timescales are similar in the modeled and observed environments, with e-folding times on the order of 12-36 hours for most cloud variables, shorter for cloud water path and cloud cover. Predictor variables, both meteorological and internal to the boundary layer, are tested as drivers of changes to cloud variables. Increased subsidence is modeled and observed to decrease cloud cover, inversion height, precipitation, and cloud water content. Modelled clouds tended to be oversensitive to changes in SST, while cloud microphysical and precipitation processes were poorly simulated by both models, indicating a need for improvement for the simulation of these processes. In chapter 2, mesoscale cellular convection (MCC) is classified by daytime MODIS L2 cloud liquid water path in 256km square boxes spaced 128 km apart in stratocumulus decks in the southeastern subtropical oceans. A Lagrangian framework is applied to MCC observations taken 24 hours apart in order to assess meteorological conditions and cloud properties associated with transitions from closed cell MCC to open cells, or from closed cells to disorganized cells more akin to trade cumulus. Results suggest that higher rain rates, observed by the CloudSat-tuned AMSR/E 89 GHz brightness temperatures, are associated with the closed-to-open MCC transition along with reduced cloud drop concentration as seen by MODIS and strong wind speeds, sourced from the ERA5 reanalysis. Strongly contrasting with the closed-to-open MCC transition, the closed-disorganized MCC transition is associated with entrainment warming and drying in a rapidly deepening boundary layer, observed by CALIPSO-tuned MODIS cloud top temperatures. In the closed-disorganized transition, the boundary layer appears to deepen in response to declining subsidence and reduced humidity in the lower troposphere, as well as a warmer sea surface. In chapter 3 the ability of wind speed to induce the closed-to-open MCC transition in subtropical marine stratocumulus is further investigated. This Lagrangian framework is expanded to use trajectories that span 96 hours. A new analysis is created to assess the power of a variable to predict a 24-hour closed-to-open or closed-to-disorganized MCC transition relative to a closed-to-closed case. Predicting power is compared for a large set of variables at various lead times up to 72 hours prior to the MCC transition. Results show that strong wind speeds precede heavy drizzle as a predictor of the closed-to-open transition. Further Lagrangian analysis shows that strong winds are associated with heavier rain in closed cell Sc and increasing rain rates over the 12 hours past the strong wind observation. The positive relationship between wind and rain is explained by rearranging the relationship between latent heating, humidity, and wind speed from LHF ~ wind/RH to wind ~ RH x LHF. This means that in a capped boundary layer, wind speed represents a combination of the moisture flux and moisture content, so that wind is pumping moisture by evaporating seawater into a closed system, which drives increased rainfall. The rainfall can initiate the closed-to-open transition through cold-pool convergence processes that have been shown to sustain open cells in prior modeling work. The closed-to-open MCC transition is compared to the closed-to-disorganized transition: Two different systems emerge, where the closed-to-open transition occurs when the boundary layer is overloaded with moisture, while the closed-to-disorganized transition occurs when the boundary layer dries due to excess entrainment. These results indicate that closed cell stratocumulus clouds rely on a balance between moisture input from wind and entrainment drying from the incorporation of free-tropospheric air. Excesses of moisture or drying can break apart the closed cells.
Author: Alexei Lyapustin
Publisher: Frontiers Media SA
ISBN: 2832500757
Category : Science
Languages : en
Pages : 312
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Author: Sylvia C. Sullivan
Publisher: John Wiley & Sons
ISBN: 1119700345
Category : Science
Languages : en
Pages : 371
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Book Description
Clouds and Their Climatic Impacts Clouds are an influential and complex element of Earth’s climate system. They evolve rapidly in time and exist over small spatial scales, but also affect global radiative balance and large-scale circulations. With more powerful models and extensive observations now at our disposal, the climate impact of clouds is receiving ever more research attention. Clouds and Their Climatic Impacts: Radiation, Circulation, and Precipitation presents an overview of our current understanding on various types of clouds and cloud systems and their multifaceted role in the radiative budget, circulation patterns, and rainfall. Volume highlights include: Interactions of aerosol with both liquid and ice clouds Surface and atmospheric cloud radiative feedbacks and effects Arctic, extratropical, and tropical clouds Cloud-circulation coupling at global, meso, and micro scales Precipitation efficiency, phase, and measurements The role of machine learning in understanding clouds and climate The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.
