Cloud climate impact

The interaction processes in the aerosol-cloud-radiation system that determine the indirect impact of aerosol on climate remain poorly studied, though they are an important factor in RF formation (the respective estimates vary between OWm-2 and —4.8Wm-2). The contribution to indirect climatic impact by aerosol due to lower-level stratus clouds is important since... 

To analyze the formation and variability of the indirect climatic impact of aerosol within the second field experiment on studies of aerosol (ACE-2) and the PACE program to substantiate parameterization of this impact, Menon et al. (2003) undertook a comparison of six 1-D numerical models of the processes in the aerosol-cloud-radiation system that determine the climatic impact of aerosol under... 

The indirect climatic impact of aerosol at the ABL is determined by numerous interactions between aerosol and the dynamics of the microphysical and optical properties of clouds. The input to the atmosphere of anthropogenic aerosol particles functioning as CCN favors an increase in cloud droplet number density. As mentioned above, the related increase in the optical thickness and albedo of clouds, with their constant water content, was called the first indirect effect , which characterizes the climatic impact of aerosol. 

Brenguier et al. (2003) discussed the ACE-2 CC project, one of the five projects accomplished within the ACE-2 program of the second field experiment on studies of aerosol characteristics with the aim of understanding the indirect climatic impact of aerosol for marine stratocumulus clouds and to substantiate the strategy of closed aerosol-cloud-radiation experiments. Observations within the CC project were made in June-July 1997 in the Canary Islands using instruments onboard three flying laboratories and installed at a surface network. 

Clouds cover roughly two-thirds of our earth s surface and play an important role in influencing global climate by affecting the radiation budget. Cirrus clouds are one example of a cloud type whose optical properties are not accurately known. Cirrus clouds form in the upper troposphere and are composed almost exclusively of non-spherical ice crystal particles. The impact of cloud coverage on dispersion of pollution in the atmosphere is an area of great concern and intensive study. 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

An increase in aerosol particles that can act as CCN can increase the number of cloud droplets and their size distribution, both of which can affect the light scattering properties of clouds and hence climate. We first briefly discuss the effects of clouds on climate and then the potential impacts of anthropogenic aerosols on the formation and properties of clouds. 

An important problem is to provide an interactive consideration of the three types of aerosol impact on the ABL, clouds, and the indirect RF mentioned above. Solution of this problem was one of the main objectives of the second field observational experiment (ACE-2) on studies into aerosol characteristics in 1997 in the Canary Islands. Part of the program of this experiment (called CloudColumn ) was especially dedicated to a study of the indirect impact of anthropogenic aerosol on climate. In 1999, the European Commission supported further studies in this direction within the PACE project on the development of parameterization schemes for the impact of aerosol on climate. 

Brenguier et al. (2003) discussed the results of eight series of aircraft measurements of the microphysical characteristics of marine stratocumulus clouds in a broad range of observation conditions (different physico-chemical properties of aerosol, number density values in the interval 50 cm 3-25 cm-3, etc.)- The unique complex of synchronous observations of the microphysical and radiative characteristics of cloud cover obtained can be used to assess the indirect impact of aerosol on clouds and climate based on analysis of the ratio between the cloud optical thickness and effective radius of cloud droplets. Correlation between these values is usually negative, but in a heavily polluted atmosphere it can be positive. From the observational data obtained during ACE-2, the polluted systems of clouds turned out to be somewhat drier and therefore thinner, resulting in the positive correlation between the indirect impact of aerosol on climate and the effective radius of droplets. 

The impact of sea salt aerosol determines the decrease in droplet number concentration in marine clouds (CDNC) by 20%-60%, with a maximum decrease in the NH Roaring Forties (40%-70%) and NH mid-latitudes (20%-40%), characterized by the high concentration of sea salt aerosol. Some increase in CDNC due to sea salt aerosol was also observed in the equatorial band. The impact of changes in aerosol content and cloud cover on global climate will be estimated in future. 

Many uncertainties also remain about the impact of present changes in extra-atmospheric insolation on climate. Soon et al. (2000) detected, for instance, the super-sensitivity of the climate system to changes in UV insolation whose impact is enhanced by the feedback due to the statistical stability of clouds, influence of tropical cirrus clouds, and stratospheric ozone (the ozone-climate problem needs special analysis) (Kondratyev and Varotsos, 2000). 

The main difficulty in understanding the causes of climate change is connected with the impossibility of considering climatic feedbacks sufficiently reliably. Primarily, this refers to cloud-radiation feedback, direct and indirect (by the effect on radiation properties of clouds) impacts of atmospheric aerosols on climate, and the impact of the atmosphere-ocean interaction on climate formation. 

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