Retrieving optical depths of optically thin and mixed‐phase clouds from MFRSR measurements

Abstract

A new method has been developed to retrieve cloud optical depths for optically thin clouds (τ < 10) from the Multifilter Rotating Shadowband Radiometer (MFRSR). On the basis of simultaneous measurements of direct and diffuse radiation from MFRSR, this method allows partition of water and ice clouds and thus improves cloud optical depth retrievals. The new retrieval algorithm achieves the high consistency of retrieved cloud optical depth from both direct‐beam and total radiation: the slope of 0.95 between the two with correlation coefficient of 0.90 and RMS of 1.00. A sensitivity study illustrates that the maximum biases (relative errors) of cloud optical depth within the range of effective radius of clouds are 0.16 (4.7%) and 0.36 (8.3%) for retrievals from direct‐beam radiation and from total radiation, respectively. Validation and evaluation from measurements at the Point Reyes site have been conducted, illustrating that the new retrieval algorithm provides not only accurate retrievals of cloud optical depth in terms of radiation closure but also unique mix ratio of cloud water and ice for optically thin clouds under overcast conditions. Because of the climatologic importance of thin clouds, this algorithm with unique mix ratio retrievals is important for the climate study.