Correcting For Precipitation Effects In Satellite Based Passive Microwave Tropical Cyclone Intensity Estimates

Accurate tropical cyclone (TC) intensity estimates are best achieved from satellite observations. The Advanced Microwave Sounding Unit (AMSU) has operated since 1998 on polar-orbiting environmental satellites and is able to measure the warm temperature anomaly in the upper troposphere above a TC's center. Through hydrostatic equilibrium, this warm anomaly is roughly proportional to the TC's sea-level pressure anomaly. Based on this principle, the Cooperative Institute for Meteorological Satellite Studies (CIMSS) provides near real-time AMSU-based estimates of TC minimum sea-level pressure (MSLP) to forecast centers worldwide. These estimates are as accurate as the benchmark Dvorak technique, but are subject to error caused by precipitation effects (primarily brightness temperature reduction by scattering) on the AMSU 55 GHz channels sensitive to upper-tropospheric temperature. Simulated AMSU brightness temperatures (TB's) are produced by a polarized reverse Monte Carlo radiative transfer model using representative TC precipitation profiles. Results suggest that precipitation depression of high-frequency window channel TB's is correlated with depression of sounding channel TB's and can be used to correct for scattering effects on the AMSU channels used in TC intensity estimates. Analysis of AMSU data over the tropical oceans confirms this, and forms the basis for an empirical scattering correction using AMSU 31 and 89 GHz TB's. This scattering correction reduces CIMSS TC MSLP algorithm RMS error by 10% in a 7-year, 497 observation sample.