CYGNSS Tropical Cyclone Wind Speed Performance Update

The CYGNSS constellation of eight satellites was launched in December 2016. Seven of the eight continue to operate nominally. The CYGNSS science payload is a bistatic radar receiver tuned to navigation signals at 1575 MHz transmitted by the constellation of GPS satellites and reflected by the Earth surface. Near surface wind speed over the ocean is estimated from direct measurements of the bistatic radar scattering cross section of the ocean surface.

The CYGNSS engineering team continues to refine and improve upon the calibration of scattering cross section measurements. Similarly, the science algorithm team has been continuously refining and improving the retrieval algorithm that estimates wind speed. An update will be presented of current calibration and retrieval algorithm performance, with an emphasis on: improved long term stability of the calibration, improved retrieval performance at moderate to high wind speeds, and the development of a new wind speed data product that can be used in all sea state conditions, eliminating the need to choose between parallel fully developed sea (FDS) and young seas/limited fetch (YSLF) products.

Level 1 engineering calibration improves upon the correction for coarse quantization effects in the previous version to include a correction to the noise floor portion of the DDM. This update is found to improve the sensitivity to soil moisture over land and to have a minimal effect on the sensitivity to wind speed over ocean. An update is also made to the correction for the temperature dependence of the receiver electronics. This update reduces slow variations in calibration bias associated with a ~60 day oscillation in the mean temperature of the satellites. Level 2 FDS and YSLF ocean surface wind speeds are retrieved from the improved Level 1 measurements.

A new Level 3 merged storm gridded wind speed product (L3 MRG) has been developed which combines CYGNSS Storm-Centric Gridded (SCG) wind speeds, derived from the Level 2 YSLF winds near the center of tropical cyclones (TCs), with Level 3 FDS winds away from the TC center. A tapered weighted averaging scheme, centered on the 34-knot wind radius (R34) of the storm, is used. The R34 value in each storm quadrant is also reported. The algorithm produces global (+/- 40 deg latitude) wind speeds reported on a 0.1x0.1 deg grid every 6 hours for each tropical cyclone, although some 6-hourly increments may be missing if there are an insufficient number of satellite overpasses of the storm during that time interval.

Descriptions of the new Level 1, 2 and 3 algorithms will be presented, together with validation results assessing the performance of the new L3 MRG product. The estimated R34 values range from ~50 to 500 km for 6-hourly predictions throughout the life cycles of the 179 storms considered over 2018-2023, with an RMS difference relative to NHC Best Track values of ~75 km.