Tropical Cyclone Winds and Inflow Angle Asymmetry From SAR Imagery

This study developed a morphological model for Tropical Cyclone (TC) wind and inflow angle asymmetry based on sea surface wind fields derived from spaceborne synthetic aperture radar (SAR) images. The model extracts the standard TC morphological information (center, intensity, and radius of the maximum wind) and decomposes the SAR-derived winds into vortex rotation winds and motion vector, making the reconstruction of the entire TC structure reliable, even in areas not mapped by SAR. The derived wind speeds and inflow angles are verified with aircraft measurements by stepped-frequency microwave radiometer and dropsondes, obtaining root-mean-square errors of 4.32 m/s and 16.04 degrees, respectively. A systematic analysis of 130 SAR TCs images, collected by RADARSAT-2 and SENTINEL-1, reveals that the model can capture the main asymmetrical TC structure. Both TC asymmetry and eye size decrease as TC intensity increases. Plain Language Summary Tropical Cyclone (TC) movement is essential in creating asymmetry in its surface winds. TCs include hurricanes and typhoons. However, the relations between asymmetrical TC surface wind field and TC movement are limited because of the lack of adequate observations. This study proposes an idealized model to investigate TC inflow angle asymmetry in the surface winds. The model has applied to 130 spaceborne synthetic aperture radar (SAR) images over a large variety of different TCs. This model can detect the asymmetrical surface wind structure, TC surface motion vectors and the associated inflow angles, and other standard morphological information such as center locations, symmetrical intensities, and radius of maximum winds. The model results are validated by comparing the wind speeds and inflow angles to measurements from the airborne stepped-frequency microwave radiometer and dropsondes, obtaining a root-mean-square error of 4.32 m/s and 16.04 degrees, respectively. We compose TC surface wind speed distributions mapped by SAR and reconstructed by the model. They are shown to be similar in any TC category. The asymmetrical features are related to the storm intensity both in SAR winds and model results. In either the SAR observations or the model results, the maximum winds occur in the northeast quadrant. Key Points A Tropical Cyclone (TC) inflow angle asymmetry model is developed to decompose synthetic aperture radar (SAR)-derived TC winds into rotation speeds and motion vector Validation against aircraft data shows the model-reconstructed wind speeds, and inflow angles are accurate even in areas not covered by SAR A systematic analysis of 130 SAR TC images using the model reveals that TC asymmetry and eye size decrease as TC intensity increases