# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 2 Materials and Methods
### Reference Wind Speed Products
#### 2.2.4 Storm Winds  
The CYGNSS MRG and NOAA wind speed products over most major hurricanes during 2018-2022 were matched up with two reference ocean surface wind products, with measurements being made by the Stepped Frequency Microwave Radiometer (SFMR) and HWRF reanalysis model winds. The MRG 34-knot wind radius (R34) product was validated against R34 estimates derived from the ASCAT-B scatterometer described above, the Soil Moisture Active Passive (SMAP) passive microwave radiometer, and Best Track reanalysis storm winds.  
SFMR instruments are installed on all U.S. Air Force Reserve Command (AFRC) and NOAA \"hurricane hunter\" aircraft. SFMR measures the brightness temperature of the ocean surface and atmosphere at six C-band microwave channels, and an overconstrained inversion of the radiative transfer forward model retrieves near surface wind speed and column averaged precipitation. SFMR wind speed uncertainty of 3.9 m/s was estimated from comparisons to coincident dropsonde observations [32]. The version 3 SFMR data product was used in this analysis. SFMR winds were resampled to CYGNSS resolution and were co-located to CYGNSS wind estimates with a maximum temporal separation of 180 min and maximum spatial separation of 0.25 deg lat and lon. The v1.0 SMAP data product was used in this analysis [33]. These data products are L3 daily gridded products with a grid spacing of 0.25 deg. HWRF is an operational model developed by the National Centers for Environmental Prediction. HWRF provides three domains (one parent and two nested) and is based on the initial position of the storm and on the U.S. National Hurricane Center (NHC) forecast of the 72 h storm position. The two nested domains move with the storm with a coverage of 24 deg \(\times\) 24 deg and 7 deg \(\times\) 7 deg for the middle and the inner nest, respectively [34]. For our purposes, we used the inner nest gridding, which offers the finest resolution of \(\sim\)0.015 deg (approx. 2 km). The CYGNSS wind speed estimates are matched to the HWRF inner nest grid, which has a spacing of 2 km. The version 4.0a HWRF winds are re-sampled to CYGNSS resolution and are colocated to CYGNSS wind estimates with a maximum temporal separation of 180 min and a maximum spatial separation of 0.25 deg lat and lon. The Best Track reference wind data come from the NHC's HURDAT2 database [35]. Best Track data were used to assess the performance of the MRG storm product, which is generated at the same 6-hourly cadence as Best Track and uses Best Track to define its storm center location for purposes of storm centric re-gridding.  
Two matchup datasets were assembled, one for the MRG winds and another for the NOAA winds, by constraining the different reference dataset measurements to lie within 0.25 deg lat/lon of each other as well as with CYGNSS and within a time window of 180 min. The CYGNSS winds are filtered using several quality measures. Only observations with the overall quality flag set to best quality were used. This resulted in a total dataset consisting of \(\sim\)1.87 million samples in the MRG-NOAA-SFMR-HWRF dataset. Figure 4 shows the distribution of wind speeds for the two reference and two CYGNSS wind speed products in the matchup dataset. In the figure, the distributions for HWRF and MRG havemore similar high wind tails, which show a lower probability of occurrence than those for SFMR and NOAA. The greater likelihood of high winds with SFMR is probably due to its higher spatial resolution than either HWRF or CYGNSS, which is better able to resolve and detect localized high wind regions of the wind field. The greater occurrence of high wind speeds with NOAA may be due to the hybrid way its GMF was constructed, which tends to overestimate high winds speeds away from the inner core of storms (see Section 3.5 for details).