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Irma (September 2017)

Hurricane Irma over the Bahamas and Cuba on Sept 8, 2017. Images by the LANCE/EOSDIS Rapid Response team.

Copolarized and Cross‐Polarized SAR Measurements for High‐Resolution Description of Major Hurricane Wind Structures: Application to Irma Category 5 Hurricane

As part of the journal article published in the “Journal of Geophysical Research: Oceans”, Mouche et al 2019 confirms the potential to quantitatively interpret, at very high resolution, the sea surface imprints captured by spaceborne SAR measurements under extreme weather conditions. When coanalyzed with Stepped Frequency Microwave Radiometer 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 copolarized 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 7 September 2017, Sentinel‐1 measurements intercepted Hurricane Irma when it was at category 5 intensity. When compared to Stepped Frequency Microwave Radiometer, SAR‐derived wind speeds yield bias and root‐mean‐square of about 1.5 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 and the radius of maximum wind. Near the radius of maximum wind 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 (1,011 hPa), SAR‐derived wind speeds correspond to a central surface pressure of 918 hPa (921 hPa from the Best‐Track) in Irma’s eye.

Ocean surface wind speed for Irma category 5 hurricane on 7 September 2017. (a) Map of SAR‐derived wind speed. Gray solid line stands for collocated SFMR track. Colored circles along this track indicate SFMR measurements with time difference lower than 2 hr and 30 min. Color code is the same for SAR and SFMR wind speeds. Concentric circles indicate radius from the cyclone center, and dotted black line stands for the Best‐Track trajectory, black circle being hurricane position every 6 hr. (b) Scatter plot between SAR‐derived and SFMR‐derived ocean surface wind speed. (c) Same as Figure 1 but for 3‐km resolution ocean surface wind speed from SFMR and Sentinel‐1. SAR = synthetic aperture radar; SFMR = Stepped Frequency Microwave Radiometer.
Ocean surface wind speed for Irma category 5 hurricane on 7 September 2017. (a) Map of SAR‐derived wind speed. Gray solid line stands for collocated SFMR track. Colored circles along this track indicate SFMR measurements with time difference lower than 2 hr and 30 min. Color code is the same for SAR and SFMR wind speeds. Concentric circles indicate radius from the cyclone center, and dotted black line stands for the Best‐Track trajectory, black circle being hurricane position every 6 hr. (b) Scatter plot between SAR‐derived and SFMR‐derived ocean surface wind speed. (c) Same as Figure 1 but for 3‐km resolution ocean surface wind speed from SFMR and Sentinel‐1. SAR = synthetic aperture radar; SFMR = Stepped Frequency Microwave Radiometer.

The full reference of this study can be also found in Documentation section.