Coverage of tropical cyclones revealed by CYGNSS data

The CYGNSS constellation of eight (now seven) small satellite receivers has been collecting ocean surface wind speed data for eight years. In contrast to scatterometers, CYGNSS passively relies on reflections of GPS signals off the ocean surface. This relatively new technology has presented challenges and opportunities. The main challenge has been to derive a useful signal at higher wind speeds. At the 19-cm wavelength at which GPS satellites transmit, the sea state can be a combination of freshly-generated waves ("young seas with limited fetch" or YSLF) and more mature long-distance swells ("fully-developed seas" or FDS). Depending on the age of the sea state, different wind speeds could be present, and this ambiguity begins to appear above ~20 m/s. Two opportunities presented by CYGNSS are its ability to retrieve wind speeds through precipitation (because of the longer wavelength) and its more frequent revisit times over a given area (because of its lower-inclination orbit).

In this study, we examine the coverage provided by CYGNSS compared with scatterometers and other available ocean surface wind data in tropical cyclones. We use the version 3.2 CYGNSS Level 3 science data record merged storm (MRG) wind speed product. This product combines storm-centric gridded wind speeds derived from the Level 2 YSLF winds for a region surrounding a given tropical cyclone, with Level 3 FDS winds away from the tropical cyclone center. In addition to illustrating the ocean surface wind data coverage for selected tropical cyclones, we will also present a “data coverage index”, which quantifies the availability of information within the tropical cyclone. We will also present CYGNSS estimates of critical wind radii (34 kt, 50 kt, 64 kt) versus estimates provided by the National Hurricane Center and in operational model analyses.