The relevance of aerosol optical depth to cumulus fraction changes: a five-year climatology at the ACRF SGP siteReportar como inadecuado

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1 PNNL - Pacific Northwest National Laboratory

Abstract : The objective of this study is to investigate, by observational means, the magnitude and sign of the actively discussed relationship between cloud fraction N and aerosol optical depth ?a. Collocated and coincident ground-based measurements and Terra-Aqua satellite observations at the Atmospheric Radiation Measurement ARM Climate Research Facility ACRF Southern Great Plains SGP site form the basis of this study. The N??a relationship occurred in a specific 5-year dataset of fair-weather cumulus FWC clouds and mostly non-absorbing aerosols. To reduce possible contamination of the aerosols on the cloud properties estimation and vice versa, we use independent datasets of ?a and N obtained from the Multi-filter Rotating Shadowband Radiometer MFRSR measurements and from the ARM Active Remotely Sensed Clouds Locations ARSCL value-added product, respectively. Optical depth of the FWC clouds ?cld and effective radius of cloud droplets re are obtained from the MODerate resolution Imaging Spectroradiometer MODIS data. We found that relationships between cloud properties N,?cld, re and aerosol optical depth are time-dependent morning versus afternoon. Observed time-dependent changes of cloud properties, associated with aerosol loading, control the variability of surface radiative fluxes. In comparison with pristine clouds, the polluted clouds are more transparent in the afternoon due to smaller cloud fraction, smaller optical depth and larger droplets. As a result, the corresponding correlation between the surface radiative flux and ?a is positive warming effect of aerosol. Also we found that relationship between cloud fraction and aerosol optical depth is cloud size dependent. The cloud fraction of large clouds larger than 1 km is relatively insensitive to the aerosol amount. In contrast, cloud fraction of small clouds smaller than 1 km is strongly positively correlated with ?a. This suggests that an ensemble of polluted clouds tends to be composed of smaller clouds than a similar one in a pristine environment. One should be aware of these time- and size-dependent features when qualitatively comparing N??a relationships obtained from the satellite observations, surface measurements, and model simulations.

Autor: E. I. Kassianov - L. K. Berg - C. Flynn - S. Mcfarlane -



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