# Assessment of CYGNSS Wind Speed Retrievals in Tropical Cyclones
## 3 Assessment of Global CYGNSS Retrievals: Low-to-Moderate Wind Regimes
The regional biases are the result of residual calibration biases in the GPS blocks, particularly in block IIF, which was discarded in the previous version SDR L2 V2.1 and is now reintroduced in V3.0. These regional biases are discussed in great detail in [13; 51], and their reduction was the main motivation for the development of the track-wise bias-adjusted products whose latencies are about 6-30 days. The unadjusted FDS V3.0 has a shorter latency, currently of about 1-2 days, and it is recommended for users needing more recent observations. The observed biases do not significantly depend on which radiometer is chosen for the assessment, as shown in Supplementary Figure S1. The overall statistics of the 1-h colocations (2018-2020) between the CYGNSS datasets and AMSR2 are illustrated in Figure 4, as joint probability distribution functions (PDFs) of the colocated winds (top panels). While all the datasets have small overall biases in the 0-20 m/s range, the NOAA V1.1 winds stand out as almost unbiased and with smaller uncertainty (~0.14 m/s and 1.0 m/s, respectively). The bias-adjusted CDR V1.1 has a larger negative bias of ~0.62 m/s, and standard deviation of 1.4 m/s, while the V3.0 FDS displays the largest overall bias and standard deviation (~1.04 and 1.6 m/s, respectively). Both datasets present a spurious dispersion at a wind regime of about 10 m/s, for which CYGNSS wind retrievals are above 20-35 m/s. This is regardless of which radiometer is used for the assessment, which do not display any spurious biases at 10 m/s [21], and seems consistent with a similar pattern for two-dimensional histograms of CYGNSS versus radiometers or ECMWF in [13]. The origin of this spurious dispersion should be investigated in more detail. Recent analyses seem to display a much-reduced dispersion around 10 m/s when SWH is included in the wind algorithm, as in the new CYGNSS SDR V3.1 algorithm still under development at the SOC [11]. Figure 2: CYGNSS wind speeds as reported in different datasets for a sample day, 13 September 2018: (**a**) Science data record (SDR) V3.0 Fully Developed Sea (FDS) winds; (**b**) climate data record (CDR) V1.0 FDS; (**c**) CDR V1.1 FDS winds; (**d**) NOAA V1.0; (**e**) NOAA V1.1; and, for comparison, in (**f**,**g**), SMAP winds for the ascending and descending passes, respectively, on the same sample day. Areas of missing data or locations that are not sampled are depicted in white. It is important to note that the reduction in the number of observations in the NOAA V1.1 wind product versus the other two datasets is due to the fact that the original L1 observations are averaged over 0.25\({}^{\circ}\) boxes to reduce the noise before the bias adjustment is applied, and the final NOAA L2 product is reported at this resolution rather the original one, unlike the CYGNSS CDR and SDR L2 products. The finer details of the colocated wind speed distributions are illustrated in the bottom panels of Figure 4, where, again, the NOAA V1.1 distribution seems to better match the one from AMSR2, while both others are shifted by 1 m/s or more. Figure 3: **(a,b)** Wind speed bias for CYGNSS datasets that have been track-wise corrected for calibration biases when compared to the AMSR2 rain-free radiometer winds colocated within 60 min: NOAA V1.1 winds and CDR V1.1 FDS winds, respectively. **(c)** Wind speed bias for the CYGNSS dataset not corrected for track-wise biases: the SDR V3.0 FDS versus AMSR2. These statistics refer to all daily data starting from August 2018 until the end of 2020. Areas in light grey correspond to missing CYGNSS or radiometer data.