Estimation of the Radius of Maximum Wind Using Passive Microwave Satellite Data

Estimating the radius of maximum wind (RMW) with high accuracy is important for tropical cyclone (TC) wind-field monitoring. Recent advances in spaceborne synthetic aperture radar (SAR) have made it possible to observe detailed inner-core wind speeds of TCs. To leverage this opportunity, two methods are developed to estimate the RMW from 89-GHz passive microwave satellite data, with SAR observations used as ground truth data. The first method, for TCs with an eyewall ring, is based on the relationship between the RMW and the radius of the azimuthally averaged 89-GHz brightness temperature (89TB) at −40°C. After objective exclusion of cases with concentric eyewalls or no ring structure, this method achieved a mean absolute error (MAE) of less than 5 km. When the first method cannot be used, the second method estimates the RMW as the radius where the azimuthally averaged 89TB radial gradient is at its most negative (Rgrad). After exclusion of cases with a small 89TB radial gradient or a large Rgrad and high-latitude cases, the second method yielded an MAE on the order of 20 km. By using a combination of the two methods, a 72% chance of estimation among the available microwave satellite data samples was achieved.