A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds

[1] In determining aerosol-cloud interactions, the properties of aerosols must be characterized in the vicinity of clouds. Numerous studies based on satellite observations have reported that aerosol optical depths increase with increasing cloud cover. Part of the increase comes from the humidification and consequent growth of aerosol particles in the moist cloud environment, but part comes from 3-D cloud-radiative transfer effects on the retrieved aerosol properties. Often, discerning whether the observed increases in aerosol optical depths are artifacts or real proves difficult. The paper only addresses the cloud-clear sky radiative transfer interaction part. It provides a simple model that quantifies the enhanced illumination of cloud-free columns in the vicinity of clouds that are used in the aerosol retrievals. This model is based on the assumption that the enhancement in the cloud-free column radiance comes from enhanced Rayleigh scattering that results from the presence of the nearby clouds. This assumption leads to a larger increase of AOT for shorter wavelengths, or to a "bluing" of aerosols near clouds. The assumption that contribution from molecular scattering dominates over aerosol scattering and surface reflection is justified for the case of shorter wavelengths, dark surfaces, and an aerosol layer below the cloud tops. The enhancement in Rayleigh scattering is estimated using a stochastic cloud model to obtain the radiative flux reflected by broken clouds and comparing this flux with that obtained with the molecules in the atmosphere causing extinction, but no scattering.

[1]  Brent N. Holben,et al.  An analysis of potential cloud artifacts in MODIS over ocean aerosol optical thickness products , 2005 .

[2]  Soon K. Cho,et al.  Electromagnetic Scattering , 2012 .

[3]  E. Kassianov.  Stochastic radiative transfer in multilayer broken clouds. Part I: Markovian approach , 2003 .

[4]  E. Vermote,et al.  The MODIS Aerosol Algorithm, Products, and Validation , 2005 .

[5]  Robert F. Cahalan,et al.  Cloud characterization and clear-sky correction from Landsat-7 , 2001 .

[6]  K. Masuda,et al.  Monte Carlo simulations of enhanced visible radiance in clear-air satellite fields of view near clouds , 2000 .

[7]  D. Deirmendjian.  Electromagnetic scattering on spherical polydispersions , 1969 .

[8]  Robert F. Cahalan,et al.  Impact of cumulus cloud spacing on Landsat atmospheric , 2001 .

[9]  I. A. Podgorny.  Three‐dimensional radiative interactions in a polluted broken cloud system , 2003 .

[10]  Robert F. Cahalan,et al.  Impact of 3-D clouds on clear-sky reflectance and aerosol retrieval in a biomass burning region of Brazil , 2006, IEEE Geoscience and Remote Sensing Letters.

[11]  David R. Doelling,et al.  Angular Distribution Models for Top-of-Atmosphere Radiative Flux Estimation from the Clouds and the Earth’s Radiant Energy System Instrument on the Terra Satellite. Part II: Validation , 2005 .

[12]  Georgi A. Titov,et al.  Statistical Description of Radiation Transfer in Clouds. , 1990 .

[13]  Natividad Manalo-Smith,et al.  Top-of-Atmosphere Direct Radiative Effect of Aerosols over Global Oceans from Merged CERES and MODIS Observations , 2005 .

[14]  Ilan Koren,et al.  The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. , 2005, Proceedings of the National Academy of Sciences of the United States of America.

[15]  Paul Ginoux,et al.  A Long-Term Record of Aerosol Optical Depth from TOMS Observations and Comparison to AERONET Measurements , 2002 .

[16]  J. Coakley,et al.  Aerosol and cloud property relationships for summertime stratiform clouds in the northeastern Atlantic from Advanced Very High Resolution Radiometer observations , 2005 .

[17]  Sonoyo Mukai,et al.  A study of the direct and indirect effects of aerosols using global satellite data sets of aerosol and cloud parameters , 2003 .

[18]  Steven Platnick,et al.  Impact of three‐dimensional radiative effects on satellite retrievals of cloud droplet sizes , 2006 .

[19]  Oleg Dubovik,et al.  Combined use of satellite and surface observations to infer the imaginary part of refractive index of Saharan dust , 2002 .

[20]  Robert F. Cahalan,et al.  3‐D aerosol‐cloud radiative interaction observed in collocated MODIS and ASTER images of cumulus cloud fields , 2007 .

[21]  Yoram J. Kaufman,et al.  On the twilight zone between clouds and aerosols , 2007 .

[22]  Ilan Koren,et al.  Smoke and Pollution Aerosol Effect on Cloud Cover , 2006, Science.

[23]  Bernhard Mayer,et al.  The influence of neighbouring clouds on the clear sky reflectance studied with the 3-D transport code RADUGA , 2005 .

[24]  Lorraine Remer,et al.  A critical examination of the residual cloud contamination and diurnal sampling effects on MODIS estimates of aerosol over ocean , 2005, IEEE Transactions on Geoscience and Remote Sensing.

[25]  C. Bohren,et al.  An introduction to atmospheric radiation , 1981 .

[26]  Patrick Minnis,et al.  Two MODIS Aerosol Products over Ocean on the Terra and Aqua CERES SSF Datasets , 2004 .