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Languages : en
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Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterizes aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (Ta) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at least over the ocean. The relationship between Ta and liquid water path is simulated much too strongly by the models. It is shown that this is partly related to the representation of the second aerosol indirect effect in terms of autoconversion. A positive relationship between total cloud fraction (fcld) and Ta as found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them. In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strong fcld - Ta relationship, our results indicate that none can be identified as unique explanation. Relationships similar to the ones found in satellite data between Ta and cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR - Ta relationship show a strong positive correlation between Ta and fcld The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of Ta, and parameterisation assumptions such as a lower bound on Nd. Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of -1.5+-0.5 Wm-2. An alternative estimate obtained by scaling the simulated clear- and cloudy-sky forcings with estimates of anthropogenic Ta and satellite-retrieved Nd - Ta regression slopes, respectively, yields a global annual mean clear-sky (aerosol direct effect) estimate of -0.4+-0.2 Wm-2 and a cloudy-sky (aerosol indirect effect) estimate of -0.7+-0.5 Wm-2, with a total estimate of -1.2+-0.4 Wm-2.
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Languages : en
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Book Description
Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterizes aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (Ta) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at least over the ocean. The relationship between Ta and liquid water path is simulated much too strongly by the models. It is shown that this is partly related to the representation of the second aerosol indirect effect in terms of autoconversion. A positive relationship between total cloud fraction (fcld) and Ta as found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them. In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strong fcld - Ta relationship, our results indicate that none can be identified as unique explanation. Relationships similar to the ones found in satellite data between Ta and cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR - Ta relationship show a strong positive correlation between Ta and fcld The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of Ta, and parameterisation assumptions such as a lower bound on Nd. Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of -1.5+-0.5 Wm-2. An alternative estimate obtained by scaling the simulated clear- and cloudy-sky forcings with estimates of anthropogenic Ta and satellite-retrieved Nd - Ta regression slopes, respectively, yields a global annual mean clear-sky (aerosol direct effect) estimate of -0.4+-0.2 Wm-2 and a cloudy-sky (aerosol indirect effect) estimate of -0.7+-0.5 Wm-2, with a total estimate of -1.2+-0.4 Wm-2.
Author: Johannes Quaas
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Languages : en
Pages : 0
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Author: Avey Lance A.
Publisher:
ISBN:
Category : Atmospheric aerosols
Languages : en
Pages : 122
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Languages : en
Pages :
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Evidence of aerosol-cloud interactions are evaluated using satellite data from MODIS, CERES, AMSR-E, reanalysis data from NCEP and data from the NASA Goddard Institute for Space Studies climate model. We evaluate a series of model simulations: (1) Exp N- aerosol direct radiative effects; (2) Exp C- Like Exp N but with aerosol effects on liquid-phase cumulus and stratus clouds; (3) Exp CN- Like Exp C but with model wind fields nudged to reanalysis data. Comparison between satellite-retrieved data and model simulations for June to August 2002, over the Atlantic Ocean indicate the following: a negative correlation between aerosol optical thickness (AOT) and cloud droplet effective radius (R{sub eff}) for all cases and satellite data, except for Exp N; a weak but negative correlation between liquid water path (LWP) and AOT for MODIS and CERES; and a robust increase in cloud cover with AOT for both MODIS and CERES. In all simulations, there is a positive correlation between AOT and both cloud cover and LWP (except in the case of LWP-AOT for Exp CN). The largest slopes are obtained for Exp N, implying that meteorological variability may be an important factor. The main fields associated with AOT variability in NCEP/MODIS data are warmer temperatures and increased subsidence for less clean cases, not well captured by the model. Simulated cloud fields compared with an enhanced data product from MODIS and AMSR-E indicate that model cloud thickness is over-predicted and cloud droplet number is within retrieval uncertainties. Since LWP fields are comparable this implies an under-prediction of R{sub eff} and thus an over-prediction of the indirect effect.
Author: Ken S. Carslaw
Publisher: Elsevier
ISBN: 0128231726
Category : Science
Languages : en
Pages : 856
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Book Description
The ever-diversifying field of aerosol effects on climate is comprehensively presented here, describing the strong connection between fundamental research and model applications in a way that will allow both experienced researchers and those new to the field to gain an understanding of a wide range of topics. The material is consistently presented at three levels for each topic: (i) an accessible "quick read" of the essentials, (ii) a more detailed description, and (iii) a section dedicated to how the processes are handled in models. The modelling section in each chapter summarizes the current level of knowledge and what the gaps in this understanding mean for the effects of aerosols on climate, enabling readers to quickly understand how new research fits into established knowledge. Definitions, case studies, reference data, and examples are included throughout. Aerosols and Climate is a vital resource for graduate students, postdoctoral researchers, senior researchers, and lecturers in departments of atmospheric science, meteorology, engineering, and environment. It will also be of interest to those working in operational centers and policy-facing organizations, providing strong reference material on the current state of knowledge. - Includes a section in each chapter that focuses on the treatment of relevant aerosol processes in climate models - Provides clear exposition of the challenges in understanding and reducing persistent gaps in knowledge and uncertainties in the field of aerosol-climate interaction, going beyond the fundamentals and existing knowledge - Authored by experts in modeling and aerosol processes, analysis or observations to ensure accessibility and balance
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Category :
Languages : en
Pages : 56
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Author: Yuan Wang
Publisher: Springer
ISBN: 3662471752
Category : Science
Languages : en
Pages : 100
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The studies in this dissertation aim at advancing our scientific understandings about physical processes involved in the aerosol-cloud-precipitation interaction and quantitatively assessing the impacts of aerosols on the cloud systems with diverse scales over the globe on the basis of the observational data analysis and various modeling studies. As recognized in the Fifth Assessment Report by the Inter-government Panel on Climate Change, the magnitude of radiative forcing by atmospheric aerosols is highly uncertain, representing the largest uncertainty in projections of future climate by anthropogenic activities. By using a newly implemented cloud microphysical scheme in the cloud-resolving model, the thesis assesses aerosol-cloud interaction for distinct weather systems, ranging from individual cumulus to mesoscale convective systems. This thesis also introduces a novel hierarchical modeling approach that solves a long outstanding mismatch between simulations by regional weather models and global climate models in the climate modeling community. More importantly, the thesis provides key scientific solutions to several challenging questions in climate science, including the global impacts of the Asian pollution. As scientists wrestle with the complexities of climate change in response to varied anthropogenic forcing, perhaps no problem is more challenging than the understanding of the impacts of atmospheric aerosols from air pollution on clouds and the global circulation.
Author: Huilin Huang
Publisher:
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Category :
Languages : en
Pages : 61
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Aerosols come from both natural and anthropogenic sources and contribute large uncertainties to estimates of the Earth's changing energy budget. It is thus of great importance to understand the mechanism through which aerosols play a role on global climate. In this thesis, we investigate the direct and indirect effect of aerosols on global and regional climate variability (e.g. West Africa, South Asia and East Asia) using an atmospheric general circulation model, GFS (Global Forecast System) coupled with SSiB2 (the second version of Si mplified Simple Biosphere Model). The three-dimensional aerosol data from the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model has been adopted in this study. We first analyze the direct effect of aerosols, especially absorption aerosols, on global and regional energy budget, precipitation, and surface temperature and the mechanism involved. For instance, we find the dust aerosol in North Africa produces a heating in the atmosphere, which generates a cyclonic circulation in middle layer over Sahel region, which further brings about upward motion in the lower level and results in precipitation increase by 0.96 mm/day in June-July-August. We also examine the impact of aerosols on ice clouds effective radius by applying an advanced ice cloud parameterization in the GCM. We find that increased aerosol loading reduces ice crystal size due to aerosol first indirect effect, with the maxima occurs in South Asia and North Indian Ocean. Ice clouds with smaller crystal sizes can absorb both shortwave and longwave radiation, thus resulting in less downward solar flux and less outgoing longwave on top of atmosphere (TOA). Global mean net radiation change on TOA is about 0.5W/m2 and its sign is largely dependent on the relative magnitude of shortwave and longwave change and precipitation changes primarily respond to cooling/warming of the atmosphere. Lastly, we use sulfate data in both pre-industrial and present-day case to test the impact of aerosols on liquid cloud effective radius. We find aerosols can act as cloud condensation nuclei, and hence change the shortwave optical properties of liquid clouds. Radiative cooling occurs globally because smaller droplets size leads to increased cloud albedo. The mean value is about 2.5W/m2. Moreover, most radiative cooling occurs in North Hemisphere, where anthropogenic sulfate aerosols locate, such as East Asia and North America. We also compare the radiative forcing of aerosol direct and indirect effect on both ice clouds and liquid clouds on global as well as the three monsoon regions. All aerosol effects result in radiative cooling on global scale. However, surface net radiation changes are different in West Africa, East and South Asia, which relates to local atmospheric conditions such as cloud cover and convection. Aerosol direct effect and aerosol indirect effect for liquid clouds have comparable impact on surface net radiation change (more than -3 W/m2), while aerosol indirect effects for ice cloud are smaller (~1 W/m2) because of the negative feedback from cloud cover. Decreased land surface temperature can be found over North Hemisphere continent in all three effects, especially over higher latitude, with varied magnitude. The precipitation changes are less predictable. Aerosol indirect effects on averaged global precipitations are close to zero because the precipitation changes are different or even opposite in different regions. The effect of aerosol on precipitations can be influenced by convection strength, topography, and even the relative location of aerosols and monsoon system.
Author: Shunlin Liang
Publisher: Elsevier
ISBN: 0128032219
Category : Science
Languages : en
Pages : 3183
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Book Description
Comprehensive Remote Sensing, Nine Volume Set covers all aspects of the topic, with each volume edited by well-known scientists and contributed to by frontier researchers. It is a comprehensive resource that will benefit both students and researchers who want to further their understanding in this discipline. The field of remote sensing has quadrupled in size in the past two decades, and increasingly draws in individuals working in a diverse set of disciplines ranging from geographers, oceanographers, and meteorologists, to physicists and computer scientists. Researchers from a variety of backgrounds are now accessing remote sensing data, creating an urgent need for a one-stop reference work that can comprehensively document the development of remote sensing, from the basic principles, modeling and practical algorithms, to various applications. Fully comprehensive coverage of this rapidly growing discipline, giving readers a detailed overview of all aspects of Remote Sensing principles and applications Contains ‘Layered content’, with each article beginning with the basics and then moving on to more complex concepts Ideal for advanced undergraduates and academic researchers Includes case studies that illustrate the practical application of remote sensing principles, further enhancing understanding
Author: A. Pier Siebesma
Publisher: Cambridge University Press
ISBN: 1107061075
Category : Mathematics
Languages : en
Pages : 421
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Book Description
Comprehensive overview of research on clouds and their role in our present and future climate, for advanced students and researchers.
