Copolarized and Cross‐Polarized SAR Measurements for High‐Resolution Description of Major Hurricane Wind Structures: Application to Irma Category 5 Hurricane
|Author(s)||Mouche Alexis1, Chapron Bertrand1, Knaff John2, Zhao Yili3, Zhang Biao4, Combot Clement1|
|Affiliation(s)||1 : IFREMER, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie Physique et Spatiale (LOPS) Brest, France
2 : NOAA/NESDIS Regional and Mesoscale Meteorological Branch Fort Collins Colorado ,USA
3 : National Ocean Technology CenterState Ocean Administration Tianjin 300112 ,China
4 : School of Marine SciencesNanjing University of Information Science and Technology Nanjing 210044, China
|Source||Journal Of Geophysical Research-oceans (2169-9275) (American Geophysical Union (AGU)), 2019-06 , Vol. 124 , N. 6 , P. 3905-3922|
|WOS© Times Cited||48|
|Keyword(s)||SAR, copolarization and cross-polarization, high resolution, category 5 Hurricane, Hurricane Irma|
C‐Band high resolution radar (SAR) is the only space‐borne instrument able to probe at very high resolution and over all ocean basins the sea surface under extreme weather conditions. When co‐analyzed with Stepped Frequency Microwave Radiometer (SFMR) wind estimates, the radar backscatter signals acquired in major Hurricanes from Sentinel‐1 and Radarsat‐2 SAR, reveal high sensitivity in the cross‐polarized channel for wind speeds up to 75 m/s. The combination of the two co‐ and cross‐ polarized channels can then be used to derive high resolution surface wind estimates. The retrieval methods and impacts of intense rainfall are discussed in the context of a Hurricane Irma (2017) case study.
On September 7, 2017, Sentinel‐1 measurements intercepted Hurricane Irma when it was at category 5 intensity. When compared to SFMR, SAR‐derived wind speeds yield bias and rms, of about 1.5 m/s and 5.0 m/s, respectively. The retrieved wind structure parameters for the outer core are found to be in agreement with the Best‐Track and combined satellite and aircraft based analyses. SAR measurements uniquely describe the inner core and provide independent measurements of the maximum wind speed, the radius of maximum wind (RMW). Near the RMW a 65 m/s increase in wind speed in less than 10 km is detected, corresponding to an instantaneous absolute vorticity of order 210 times the Coriolis parameter. Using a parametric Holland model and the environmental surface pressure (1011 hPa), SAR‐derived wind speeds correspond to a central surface pressure of 918 hPa (921 hPa from the Best‐Track) in Irma's eye.