Remote Sensing for Convective System Tracking and Associated Sea Surface Wind Pattern Detection

Convective Systems (CS) are dangerous weather events since they are associated with intense precipitation (up to 50 mm/hr) and strong surface winds (exceeding 20 m/s), for instance over the sea surface. Furthermore, they happen suddenly and evolve quickly, and thereby their effects on the sea surface are difficult to track and predict. Thanks to the geostationary meteorological satellites of METEOSAT (Europe), GOES (USA), and Himawari (Japan), the CS detection and tracking can be performed in most of the world with a 5-15-minute observation time sampling and about 2.8-km spatial resolution (up to about 1-km for the new–generation satellites). Indeed, the instruments onboard these satellites perform the CS detection based on the identification of deep convective clouds. The deeper the convective clouds, the lower the brightness temperature is. The highest (coldest) clouds have the lowest brightness temperature (200 K–205 K).

While the CS detection has been significantly improved for recent years thanks to the infrared images, the investigation of strong winds (or wind gusts) produced by the CS downdrafts hitting the sea surface did not progress a lot. It is mainly due to the lack of in-situ data and (especially) high-resolution remote sensing images. Some studies proposed the use of ASCAT scatterometers for the detection of surface wind patterns associated with the CS. However, the ASCAT only identified the mesoscale patterns (100–300 km) and failed to detect the convective-scale gust fronts (5–20 km), due to their large spatial resolution (12.5–25 km wind grid). To be able to observe both small- and large-scale surface wind patterns, Synthetic Aperture Radar (SAR) images are used in this study thanks to their high spatial resolution, wide swath, and availability in most weather conditions. Indeed, the obtained results in (La et al., 2018, 2020) illustrate that Sentinel-1 (C-band SAR) may detect surface wind patterns in shapes of a mesoscale squall line and sub-mesoscale convection cells. The associated wind intensity with the patterns exceeds 10–25 m/s.

To strengthen the assumption that the detected wind patterns on SAR images are produced by the CS downdrafts hitting the sea surface, we use the corresponding METEOSAT images for the detection of deep convective clouds (200 K–205 K brightness temperature). The comparisons between Sentinel-1 and METEOSAT images illustrate that surface wind patterns and deep convective clouds have a matching in spatial location (and sometimes in shape). In particular, the coldest spots of deep convective clouds correspond to the one with high wind intensity (15–25 m/s) of the patterns. This result thus permits to highlight a strong relationship between the detected wind patterns on the sea surface and the CS aloft.