TC observations from Synthetic Aperture Radar: short term perspectives
Authors: Husson Romain , Mouche Alexis , Longepe Nicolas , Berger Henrick , Archer Olivier , Soulat François 
Published: March 2020

&lt;p&gt;&lt;span&gt;More than &lt;/span&gt;&lt;span&gt;2&lt;/span&gt;&lt;span&gt;00 Sentinel-1 acquisitions over Tropical Cyclones (TC) eyes have been accumulated since 2016 thanks to the SHOC scheme (Satellite Hurricane Observation Campaign) operated in collaboration with ESA ground segment. These high-resolution observations have shown the great potential offered by S1 constellation in dual-polarization to monitor TC along their lifetime and to provide numerous observable parameters such as maximum sustained wind speed (up to 80 m/s) and TC structure (e.g. wind radii, eye geometry and position). Co-locations with the Stepped Frequency Microwave Radiometer (SFMR) confirm that, even for extreme cases, S1-derived ocean wind speeds are found in agreement and able to provide consistent measurements in the eyewall. Similarly, co-locations between SMOS and S1-wind degraded at a similar medium resolution are in good agreement. Also, Hurricane experts listed in their recommendations at the 40&lt;/span&gt;&lt;sup&gt;&lt;span&gt;th&lt;/span&gt;&lt;/sup&gt;&lt;span&gt; WMO Hurricane Committee for USA/Caribbean region that &amp;#8220;Special acquisitions plans during Irma, Jose and Maria having demonstrated the high value of kilometric-scale information provided by Sentinel-1 SAR data, HC40 recommends that these data are made available to help monitor critical aspects of the TC structure&amp;#8221;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Based on this demonstration, a new ESA-funded project called CYMS (CYclone Monitoring service with S-1) starts in February 2020, with the objective of scaling up the SHOC initiative for its potential integration as part of a Copernicus Service. One objective is the operational delivery of tailored S1-derived TC observations to tropical cyclone forecasters of all tropical cyclone Regional Specialized Meteorological Centres (RSMCs) and Tropical Cyclone Warning Centres (TCWCs). Besides, S1 TC observations will contribute to a new database for science applications. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In order to continuously keep improving the S1-derived TC observations, current limitations in the wind field retrieval are recalled and perspectives to overcome them are proposed. First, the presence of rain signatures over SAR images requires a fine pre-processing filtering of these non-wind related features in order not to interpret them as wind speed. Second, the current inversion using the co- and cross-polarized NRCS channels via a noise-dependent mixing can show some limitations for wind speed around 30m/s. Alternative schemes are proposed to mitigate this issue. Third, S1 wind directions are mostly influenced by the co-located atmospheric model which can show some significant shifts with respect to the actual situation. Pre-processing methods based on the exploitation of wind rolls signatures, ubiquitous under intense TC, are presented to improve the wind direction retrieval. Finally, improvements of the current cross-polarized Geophysical Model Function (GMF), MS1A, are proposed taking advantage of a more complete dataset of S1 TC observations since its first estimation in 2017. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Overall, the current and future developments for S1 wind field retrieval aim at integrating all valuable observations as inputs. Additional candidate parameters of interest are the Doppler Centro&amp;#239;d anomaly, which is related to the radial wind, and the Co-Cross Polarization Coherence (CCPC), which is related to both the wind speed and direction.&lt;/span&gt;&lt;/p&gt;