30 April 2003 QuikSCAT geophysical model function and winds for tropical cyclones
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A revised geophysical model function for applications of QuikSCAT data to tropical cyclones is described. An analysis of QuikSCAT σ0s from the fore- and aft-beams indicates a directional dependence of about 0.5-1 dB for above 40 m/s wind speeds. The differences between QuikSCAT fore- and aft-beam σ0s were used to estimate the second harmonics coefficients, characterizing the upwind and crosswind asymmetry. The results show that the QuikSCAT σ0s have a peak-to-peak wind direction modulation of ~1 dB at 35 m/s wind speed, and the amplitude of modulation decreases with wind speed. The trend agrees well with the QSCAT1 model function at near 20 m/s. A simple analytic correction of the QSCAT1 model function is presented. We explored two microwave radiative transfer models to account for the attenuation and scattering effects of rain. One is derived from the collocated QuikSCAT and SSM/I data set, and the other one is a published parametric model developed for precipitation radars. The comparison of these two radiative transfer models indicates the relative significance of volume scattering, scattering from rain-roughened surfaces and rain attenuation. The models suggest that the σ0s of wind-induced surfaces at 40-50 m/s are comparable to the contributions of rain for up to 10-20 mm/h. The radiative transfer models have been used to retrieve the ocean wind vectors from the collocated QuikSCAT and SSM/I rain rate data for several tropical cyclones. The resulting wind speed estimates of these tropical cyclones show improved agreement with the expected wind fields derived from the best track analysis and Holland's model for up to about 15 mm/h rain rate.
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Simon H. Yueh, Bryan W. Stiles, W. Timothy Liu, "QuikSCAT geophysical model function and winds for tropical cyclones", Proc. SPIE 4894, Microwave Remote Sensing of the Atmosphere and Environment III, (30 April 2003); doi: 10.1117/12.466298; https://doi.org/10.1117/12.466298

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