PNNL

Abstract

A cloud optical depth retrieval algorithm that uses solar irradiance and zenith downwelling radiance data collected at a fixed surface site is assessed using model generated cloud fields and simulated radiation measurements. The retrieval algorithm utilizes time series of radiation data, but to date has only been assessed using instantaneous cloud fields in which time series were mimicked via the frozen turbulence assumption. In this study, time series of radiation data are generated from a series of snapshots of an evolving cloud field and used in the retrieval algorithm, with values of optical depth retrieved for clouds occurring at the midpoint of the time series. This approach resembles conditions encountered in the field much better that those arising from the convenient frozen turbulence assumption. Values of optical depth are also retrieved for the same cloud field by employing the frozen turbulence approach. For the field of broken, shallow cumulus considered here, differences between the two sets of retrievals are small. This suggests that the encouraging results obtained thus far for this retrieval algorithm have not been secured falsely by the frozen turbulence assumption.