Estimation of water cloud properties from satellite microwave, infrared and visible measurements in oceanic environments: 1. Microwave brightness temperature simulations

Theoretical calculations are used to examine the spectral characteristics of SSM/I (special sensor microwave/imager) brightness temperature (Tb) values for non-precipitating clouds over oceans. It was found that liquid water path (LWP) and the cloud water temperature (Tw) could be derived simultaneously with a technique using the SSM/I 37-GHz and 85-GHz brightness temperatures. Uncertainties in column water vapor (CWV) are the most important error sources in the estimation of LWP and Tw, while ice particles smaller than 100 μm in nonprecipitating clouds have a very weak effect ( 0.4 mm. For most marine stratocumulus clouds (LWP ∼0.1 to 0.2 mm) the Tw bias and standard deviation are about 2 and 4 K, respectively, resulting in cloud height errors of ∼1 to 2 km. The method should yield an improvement in the accuracy of retrieved LWP because it more closely approximates cloud temperature than previous techniques. To use the radiative transfer results, it is necessary to normalize or calibrate them to the observations. This relative calibration using 22-GHz brightness temperatures reveals differences of 2.86 K and −1.93 K for the 37-GHz horizontal and 85-GHz vertical channels, respectively, between the SSM/I observations and the model simulations. In multilayered cloud conditions, this new microwave analysis method, when combined with infrared data, should make it possible to determine cloud temperature for an upperlevel ice cloud from the infrared brightness temperatures while simultaneously deriving Tw and LWP for the lower liquid water cloud with the microwave data.

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