Analysis of Air Force Weather Agency cloud data for assessing single and multiple scattering through clouds at optical wavelengths

Abstract

The Air Force Weather Agency (AFWA) has a long history of providing global cloud analyses and forecasts. Until recently, their focus has been on determining the cloud amount and cloud type. Satellite-based World-Wide Merged Cloud Analysis (WWMCA) data provided by the AFWA are analyzed to understand and assess their capability to characterize cloud single scattering parameters at optical wavelengths. WWMCA represents the most refined version of AFWA’s cloud depiction and forecast system and includes up to four cloud layers and 38 cloud parameters per file at each hemispheric grid point. Findings on WWMCA’s determination of cloud optical depth (COD), consistency with synoptic-scale cloud fields, and its ability to support radiative transfer calculations are as follows: (1) the WWMCA optical depth is strongly correlated with the theoretical optical depth at 550 nm computed using WWMCA’s cloud microphysical parameters. (2) WWMCA captures the large-scale spatial variation of COD as represented by the Mei-yu/Baiu onset and progression of synoptic cloud fields as well as the time-dependent character of mesoscale features. (3) WWMCA does not provide single scattering albedo and the cloud phase function which are needed to solve the radiative transfer equation. Because WWMCA is based on passive sensor processing, obscured cloud layers are not accounted for in the calculation of COD, which may lead to an underestimation of total COD.