Harmonic Analysis of SFMR Off-Nadir Brightness Temperature Measurements in Tropical Cyclones

A wind direction signal has been detected in Stepped-Frequency Microwave Radiometer (SFMR) off-nadir brightness temperature measurements collected in tropical cyclones. Surface wind speed, which dynamically couples the atmosphere and ocean, can be inferred from SFMR ocean surface brightness temperature measurements using a radiative transfer model and inversion algorithm. In order to obtain accurate off-nadir SFMR wind speed estimates from the ocean surface brightness temperature measurements, it is necessary to understand the relationships between wind speed, wind direction, and incidence angle at the frequencies used by the SFMR. SFMR measurements have been collected at a variety of incidence angles in several different tropical cyclones since 2008 as part of NOAA/AOML/HRD’s Hurricane Field Program. This study will show results from harmonic analyses of the SFMR off-nadir measurements that will be used to develop corrections to the SFMR wind speed algorithm, so that wind speeds can be obtained while the aircraft is turning. In addition, Wide-Swath Radar Altimeter (WSRA) wave spectra data will be used to investigate additional wave impacts on the off-nadir asymmetry to further understand the interaction between the wind and waves at the ocean’s surface. The results from this study will improve the understanding of the relationships between brightness temperature and surface wind vectors at various incidence angles, which can be extended for use in the development of geophysical model functions for instruments like the Hurricane Imaging Radiometer (HIRad).