# Assessment of CYGNSS Wind Speed Retrievals in Tropical Cyclones
## 3 Assessment of Global CYGNSS Retrievals: Low-to-Moderate Wind Regimes  
This section focuses on an overview assessment of the different CYGNSS datasets at low-to-moderate wind regimes, before shifting the focus to extreme winds in storms. While the individual datasets have already been validated by the investigators who developed them, here, we perform an intercomparison that uses consistent methodology and external datasets for the assessment. Our aim is to help the users navigate all the available datasets and choose the ones more suitable for their specific research. Figure 2 displays the CYGNSS wind field for a sample day for each dataset used in this study (panels a-e; datasets listed in Table 1a), for the wind products that are most suitable for the wind regime between 0 to 20 m/s. Figure 2 only shows data for which the composite quality flag labeled as \"fatal\" in the SOC L2 files is off (as defined in [[https://podaac-tools.jpl.nasa.gov/drive/files/allData/cygnss/L2/docs/148-0348-10_L2_v3.0_netCDF_Data_Dictionary.xlsx](https://podaac-tools.jpl.nasa.gov/drive/files/allData/cygnss/L2/docs/148-0348-10_L2_v3.0_netCDF_Data_Dictionary.xlsx)]([https://podaac-tools.jpl.nasa.gov/drive/files/allData/cygnss/L2/docs/148-0348-10_L2_v3.0_netCDF_Data_Dictionary.xlsx](https://podaac-tools.jpl.nasa.gov/drive/files/allData/cygnss/L2/docs/148-0348-10_L2_v3.0_netCDF_Data_Dictionary.xlsx)), accessed on 25 October 2021) or data for which the \"sample_flag\" in the NOAA files is off. The first feature that emerges is that the data coverage is quite different among the datasets, due to the different quality control applied, with the preliminary NOAA V1 (beta version) and the CDR V1.1 displaying some noticeable gaps in the equatorial band (15S-15N). The NOAA V1.1 (public version) successfully improved the data coverage in this region by relaxing the quality control without compromising the quality itself. Several TCs are evident in the sample day, 13 September 2018: Super Typhoon Mangkhut in the Western Pacific, and Hurricanes Florence, Isaac, and Helene in the Northern Atlantic. The FDS wind products have weaker signal over these TCs, as they are developed for low-to-moderate winds. The wind fields for ascending and descending SMAP passes are displayed in Figure 2f-g, respectively. It can be seen that SMAP captured a full view of Mangkhut and Florence, but Isaac and Helene are not visible because they were located in the satellite gaps. Overall, Figure 2 shows the advantage of having a frequent revisit time as in the CYGNSS constellation mission over a single polar orbiter as SMAP, which might completely miss a storm over a 24-h observational period. Figure 3 presents the average bias between CYGNSS colocated within 1 h of AMSR2 global rain-free winds, for the most recent CYGNSS datasets suitable in the low-to-moderate wind regimes and their common period of availability, 1 August 2018 to 31 December 2020. Note that all wind speeds are included in the statistics. From the figure, it can be seen that the NOAA V1.1 wind fields are highly consistent with AMSR2. The small regional biases are less than 0.5 m/s in amplitude. These regional biases might be due to a difference in the definition of surface winds between CYGNSS and the radiometers: CYGNSS winds are based on corrections developed from a numerical prediction model, which uses winds at 10 m (U10) as a reference ([PERSON], Science Team meeting presentation, 27 July 2021). Radiometer winds instead are defined as 10 m equivalent neutral (U10 EN) winds [49; 50]. The regional bias pattern in Figure 3a resembles the pattern between U10 and U10 EN; residual differences are likely due to atmospheric effects in the radiometer observations [46]. The CDR V1.1 FDS, which, similar to the NOAA dataset, includes a track-wise bias correction, presents some residual negative biases larger than 0.5 m/s. On the contrary, the SDR V3.0 FDS displays negative biases larger than 1 m/s, with the Atlantic, Central Pacific, and Southern Indian Ocean basins being affected the most.