Apparent Sea Surface Wind Sudden Decreases in the Presence of Heavy Rain or Graupel in or Near Tropical Cyclone Eyewalls Observed By the Airborne Stepped Frequency Microwave Radiometer (SFMR) and Sentinel-1 Synthetic Aperture Radar (SAR)

In past years, the instances of sudden decrease of SFMR-measured sea surface wind in presence of significant rainbands in the proximity of the hurricane eye wall have been observed. This presentation will provide an in-depth analysis of one such instance during mission 20230908I in TC Lee supported with a second case in Hurricane Ian, 2022. When a significant drop of SFMR sea surface wind is observed, the first question is usually whether or not this is a result of imperfections in various models for sea roughness and rainfall attenuation used in the SFMR wind retrieval algorithm, or if it is a consequence of complex real atmospheric structures and drastic changes in atmospheric and sea surface structures in close proximity to the eye wall.

This paper presents supporting observations from other instrument systems, including satellite-based observations from SAR, SSMI and airborne observations from WSRA, TDR and DMT airborne disdrometer coincidental to the SFMR measurements. In the TC Lee case, the supplemental observations suggest that SFMR wind reduction in the presence of rain, for the case that occurred during mission 20230908I1 in TC Lee, is mostly due to the sudden drastic change in rainfall attenuation due to a change in raindrop size distribution, possibly including the presence of graupel, a not-uncommon occurrence within intense convection in hurricane rainbands and eyewalls.

We will also show additional cases of SFMR wind reduction when it is predominantly due to the change in atmospheric attenuation, and also show how measurement signatures from SAR, SSMI, and other instruments differ from those in cases where SFMR-estimated wind reduction is mostly due to the effect of the large rain droplets impacting the sea surface, causing enhanced surface roughness.