# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 2 Materials and Methods  
### CYGNSS Wind Speed Data Products  
#### 2.1.1 Project v3.2 L2 Fully Developed Seas and L3 Merged Storm Wind Speed  
The NASA project science team supporting the CYGNSS mission is responsible for developing, validating, improving, and revising the project data products. These include the Level 1 (L1) calibrated normalized bistatic radar cross section (NBRCS) and slope of the leading edge of the radar return waveform (LES) of the ocean surface [8] and the Level 2 (L2) wind speeds that are retrieved from it. The wind speeds are referenced to a 10 m height above the surface. There are several project wind speed products. The L2 Fully Developed Seas (FDS) wind speed is the general-purpose product that is appropriate for use in conditions other than major organized storms, as described in [9]. In organized cyclonic storm conditions, the sea state tends not to be fully developed, which alters the mapping from wind speed to radar cross section. This is accommodated by adjusting the wind speed retrieval algorithm, as described in [10], to produce the L2 Young Seas Limited Fetch (YSLF) product. The FDS retrieval algorithm uses empirical geophysical model functions (GMFs) that relate wind speed to NBRCS and LES. The GMFs are derived from a large collection of CYGNSS measurements of NBRCS and LES matched up with near-coincident estimates of the equivalent neutral stability wind speed at 10 m height above the surface produced by the European Centre for Medium Range Weather Forecasting Reanalysis version 5 (ERA5) model. The GMFs are generated using the cumulative distribution function (CDF) matching method, which adjusts the mapping from NBRCS or LES to wind speed so that the resulting distributions of wind speed match that are derived from the population of coincident ERA5 wind speeds [11]. CDF matching is performed separately as a function of incidence angle. The retrieval algorithm inverts the GMF-given NBRCS and LES measurements to estimate the wind speed. The two individual wind speed estimates are then combined using a minimum variance optimal estimator [12]. A secondary correction is made to the retrieved wind speed using a lookup table indexed by the Significant Wave Height (SWH) of the ocean surface estimated by ERA5 at the time of the CYGNSS measurement. The correction accounts for the sensitivity of the measurements to long (swell) wave portions of the ocean surface roughness spectrum that are not directly forced by the local winds. This is in contrast to traditional ocean scatterometer radars, which are only sensitive to the small (capillary) wave portion of the spectrum forced by the local winds. A flowchart description of the retrieval algorithm that generates the FDS and YSLF products is provided in Figure 1a. The YSLF retrieval algorithm uses the same general approach as FDS with some modifications. An empirical GMF is constructed from near-coincident matchups between CYGNSS measurements of NBRCS and 10 m referenced wind speeds provided by NOAA's Hurricane Weather Research and Forecast system for hurricane prediction (HWRF) reanalysis model. LES measurements are not used because they have a much weaker sensitivity to high winds. HWRF, rather than ERA5, model winds are used as the reference because they are better able to predict the high wind speeds present in major organized storms. The need for a separate GMF in storms results from the fact that the long wave portion of the sea surface roughness spectrum tends to be underdeveloped in organized storms due to the rapidly changing wind speed and direction. As a result, the mapping from wind speed to NBRCS is significantly different. As with the FDS algorithm, a SWH correction is also made to the YSLF estimate. YSLF is considered an intermediate data product that should not be used indiscriminately due to its limited applicability only in storms. To account for this, a third product, the Level 3 (L3) Merged Storm (MRG) product is provided. MRG combines the YSLF product near storms with the FDS storm away from them.