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Inferring Cloud Fractional Cover and Optical Depths from MFRSR Measurements

Qilong Min State University of New York at Albany
Tianhe Wang State University of New York at Albany

Category: Cloud Properties

Spectral characteristics of optical property of aerosols and clouds are substantially different. A simple transmittance ratio between 415 and 860 nm from measurements of muti-filter rotating shadowband radiometer (MFRSR) provides a measure of partition between aerosols and clouds in the atmosphere. Such a ratio is insensitive to solar zenith angle and slightly modulated by particular sizes of respective species. A localized normalization procedure is applied to time series of transmittance ratio to further minimize the retrieval uncertainty. A comprehensive uncertainty analysis indicates that overall uncertainty is less than 5 percent. Validation against in situ cloud cover measurements from the total sky imager (TSI) shows that the regression slope between the two is 1.02 with correlation coefficient of 0.948. If further restricting to opaque cloud fraction for TSI retrievals, the correlation coefficient improves to 0.957.

A new method has been developed to retrieve cloud optical depths for optically thin clouds from simultaneous measurements of direct and diffuse radiation of the MFRSR. This method allows distinguishing cloud phases and thus substantially improves retrievals of cloud optical depth. Validation and evaluation from measurements at the Point Reyes site illustrate that the new retrieval algorithm provides not only accurate retrievals of cloud optical depth in terms of radiation closure but also unique mixed-ratio of cloud water and ice for optically thin clouds. A sensitive study on the effective radius of clouds illustrates that the maximum biases (relative errors) of cloud optical depth within the range of effective radius of clouds are 0.15 (4.5 percent) and 0.31 (7.2 percent) for retrievals from direct beam radiation and from total radiation, respectively.

This poster will be displayed at ARM Science Team Meeting.