3D Aerosol-Cloud Radiative Interaction in a Cumulus Field in Brazil

This paper presents examples of cloud-aerosol 3D radiative interaction in a biomass burning region of Brazil. To quantify this impact, we use MODIS retrieved cloud optical properties to produce a realistic cloud field with the cloud top height estimated from MODIS brightness temperature at 11 microns. With realistic cloud properties and aerosol optical depth from MODIS, the 3D radiation field is computed using a Monte Carlo radiation code. The reflectance from 1D approximation that ignores the presence of clouds can be easily obtained for the same aerosol optical thickness. Since both 3D and 1D radiation codes are very accurate, the difference between the true 3D radiation field from MC simulation and that from 1D approximation in clear regions of the cumulus cloud field determines the 3D impact due to aerosol-cloud radiative interactions. Using ASTER to estimate cloud optical properties, we further examine 3D aerosol-cloud interactions at scales not resolved by MODIS. A series of studies are performed to examine the dependence of 3D cloud effects on wavelength, aerosol amounts, and surface types. An example of 3D cloud effects on clear region reflectance is demonstrated in Fig. 1. Assuming the 3D bias in 1D estimated aerosol optical thickness is linear as a function of aerosol optical thickness, the true aerosol optical thickness in clear regions will be estimated.

Figure 1. 3D cloud effects for an open area of a cumulus cloud field in Brazil. The cloud optical depth is retrieved from ASTER image at 90 m resolution (masked as white) with aerosol optical thickness of 0.05 and surface reflectance of 0.35 at wavelength of 0.86 microns. The incoming solar beam is from southeast with solar zenith angle of 31 degrees, and solar azimuth angle of 129 degrees.