# Assessment of CYGNSS Wind Speed Retrievals in Tropical Cyclones
## 4 Assessment at High Winds in the Tropical Belt  
There are currently only three publicly available CYGNSS datasets suitable for high wind speeds: the NOAA V1.1 (suitable at all winds and shown in Figure 2e for a sample day) and the YSLF wind products from the CDR V1.0 and the SDR V3.0. The images for these two YSLF wind products on a selected sample day are shown in Figure 5a,b. When compared to Figure 2e-g for the same sample day, but representative of any other day, these YSLF maps display the significant limitations of the two products: the SDR V3.0  
Figure 4: Top panels: Joint PDF of CYGNSS colocated within 60 min with the AMSR2 (global winds, rain-free, for three CYGNSS datasets); (**a**) NOAA V1.1; (**b**) CDR V1.1 FDS winds; and (**c**) SDR V3 FDS winds. The statistics refers to August 2018–December 2020. The joint PDFs are normalized to the total number of observations, which were binned in 0.2 m/s intervals. The pink dashed line displays the average bias as a function of wind speed between each pair of wind datasets. The thin black dotted line long the diagonal is a reference for unbiased data. Bottom panels (**d–f**): Wind speed PDFs for colocated CYGNSS (pink) and AMSR2 (blue) for the CYGNSS datasets displayed in panels (**a–c**), respectively.  
YSLF are biased high in storm areas and have significant, spurious, high-biased winds in large regions over each basin, making the product unreliable at high winds; on the other hand, the CDR V1.0 winds display a more realistic distribution of the wind field, even in storm areas, but suffer from a dramatic loss of data coverage due to extremely strict quality control, which makes them practically unusable for research purposes.  
Similar to Figure 3, the average biases for CYGNSS high-wind products above 10 m/s colocated within 1 h with SMAP are displayed in Figure 6. In this figure, the NOAA V1.1 is almost unbiased, the CDR V1.0 displays a moderate positive bias of 1-2 m/s, and the SDR V3.0 YSLF is consistently biased high by about 6-8 m/s. The fine details of the wind-speed distributions for the three products suitable at high winds compared with colocated SMAP observations are displayed in Figure 7. We used a logarithmic scale to better visualize the tail of the PDFs at high winds. The PDFs in this figure refer to the full tropical region and are not limited to TCs, i.e., all wind observations are used, including those in rainy regions. Again, the NOAA V1.1 wind displays a good consistency with colocated SMAP winds, with the tails of the PDFs closely matching each other. This is not the case for the two YSLF products, for which the mismatch is severe.  
Figure 5: Similar to Figure 2, for the same sample day, but for the Young Sea Limited Fetch (YSLF) wind speed products developed for high winds: (**a**) SDR V3.0 YSLF, and (**b**) CDR V1.0 YSLF. Notice Super Typhoon Mangkhut in the Western Pacific and Hurricanes Florence and Helene in the Atlantic. Areas of missing data are depicted in white.  
Figure 6: Bias maps for CYGNSS datasets suitable for high winds compared to the SMAP radiometer for wind speeds above 10 m/s: (**a**) NOAA V1.1; (**b**) CDR V1.0 YSLF; and (**c**) SDR V3.0 YSLF. The bias maps were determined from CYGNSS colocations with SMAP (which includes wind measurements in rain) within 60 min, for the period August 2018–December 2020. The color bars in (**a,b**) extend from \(-\)3 to 3 m/s; For CYGNSS V3.0 versus SMAP (panel (**c**)), the color bar has been extended to cover from \(-\)8 to 8 m/s due to the large biases between the two products. Light grey areas indicate no CYGNSS/SMAP observations for the selected wind range (w \(>\) 10 m/s).  
Figure 7: Wind-speed PDFs for the CYGNSS wind products suitable for high winds colocated within 60 min with SMAP (all-weather): (**a**) NOAA V1.1; (**b**) CDR V1.0 YSLF; and (**c**) CYGNSS SDR V3 YSLF winds (2018–2020). In order to emphasize the tail at high winds, the PDFs are displayed on a logarithmic scale and for wind speeds between 10 and 50 m/s.