Improved SFMR surface wind measurements in intense rain conditions

With the recent installation of stepped-frequency microwave radiometers (SFMR) on Air Force Reserve WC-130J aircraft, forecasters are increasingly relying on SFMR surface wind measurements to assign a TC's intensity. It has become apparent that in heavy rain, SFMR wind estimates are consistently biased high relative to in situ surface winds measured by GPS dropwindsondes, especially for wind speeds below hurricane force. This artifact is particularly troublesome for forecasters when trying to determine intensity at the depression or tropical storm stage. The bias is hypothesized to be a result of an inaccurate rain/microwave-absorption model, which is required for retrieving surface winds in TCs. To address this problem, a two-pronged approach is proposed. First, a new wind/emissivity geophysical model function is developed from SFMR measurements in rain-free conditions, which is the subject of a companion talk. Second, and presented here, are data from a NOAA WP-3D flight in which SFMR measurements were obtained in convective tropical rain with weak surface winds. In addition, rain rates derived from a Droplet Measurement Technologies Precipitation Imaging Probe (PIP) measurements are used to compute a new absorption/rain rate (k-R) relationship applicable to SFMR C-band channels. Combining the results of these two approaches are shown to improve SFMR wind speed retrieval accuracy under such conditions, without compromising accuracy at extreme wind speeds.