Stepped Frequency Microwave Radiometer algorithm improvements addressing rain contamination of surface wind speed measurements in tropical cyclones

During the past decade, new developments and improvements have been made on the Stepped Frequency Microwave Radiometer (SFMR) algorithm. This instrument, now used on NOAA and U.S. Air Force Reserve aircraft, has been useful for determining real-time surface winds in tropical cyclones where other observation platforms are unavailable or unreliable. In turn, forecasters can use this data to enhance their intensity forecasts of tropical cyclones. More recently, it has become increasingly noticeable that the SFMR algorithm has issues computing accurate wind speed measurements in the presence of weakly moderate to heavy rain. The goal of this research is to address this contamination issue and to improve the accuracy of the retrieval algorithm. SFMR data are taken from two NOAA WP-3D aircraft over the 2006-2009 Atlantic hurricane seasons. These data are collocated with GPS dropwindsonde measurements during the same flights. After removing winds that are under the influence of rain (> 5 mm/hr), a new wind algorithm is found to retrieve uncontaminated wind speeds. Removing the high bias in the rain model using Doppler radar data, solving the radiative transfer equation for only rain contributions, and finding a new κ-R relationship from the new optical depth (τ) will provide an improved algorithm to reduce the effects of rain on the wind measurements and should also provide a more accurate set of wind speed measurements to forecasters.