# Comparing Winds near Tropical Oceanic Precipitation Systems with and without Lightning
## 3 Results  
### Global comparison of CYGNSS and ASCAT  
Before delving into the analysis of lightning, it is useful to understand how CYGNSS FDS winds and ASCAT winds compare in a general sense. This will provide valuable context for interpreting the lightning-wind results. For the following analysis, matched tracks for the time period 1 May 2017 through 31 December 2019 were used. Table 1 shows basic statistics for the 32-month period. The comparisons were very similar whether ASCAT-A or -B was used. CYGNSS  
Figure 2: **(a)** Example of a CYGNSS track (arc of colored wind speeds) on a latitude/longitude map, along with ASCAT-A wind speeds (dual swaths) and wind barbs (black; circles \(<\) 5 m s\({}^{-1}\) speed) for the matching orbit. IMERG precipitation rate is also shown, as are WWLLN flashes during the track (magenta dots). **(b)** Along-track series of CYGNSS FDS wind speeds (raw and filtered using a 5-point boxcar mean), matched ASCAT-A wind speeds, matched IMERG precipitation rate, and CYGNSS RCG. Shown also are flagged raining features with WWLLN lightning (magenta bars). FDS winds--the retrieval approach appropriate for non-tropical cyclone wind strengths--had a root mean square difference (RMSD) with ASCAT-A and -B of less than 2 m s\({}^{-1}\) when matched IMERG rain rates were zero, which was within the expected uncertainty for CYGNSS [20; 37]. CYGNSS wind speeds averaged approximately 0.5 m s\({}^{-1}\) stronger than ASCAT in non-raining circumstances. However, when IMERG rain rates were nonzero, the RMSD increased by \(\sim\)0.5 m s\({}^{-1}\), while the mean offset decreased slightly. There was approximately a factor of 10 reduction in the available number of matchups in rain, compared to non-raining matchups. The magnitudes of the offset and RMSD changes in rain increased as rain rate increased (Figure 3). To develop Figure 3, the raining statistics were reevaluated at increasing thresholds for minimum rain rate. RMSD increased quasi-asymptotically toward \(\sim\)4 m s\({}^{-1}\) as rain rate increased toward 30 mm h\({}^{-1}\) (Figure 3a). Meanwhile, the slight positive offset for CYGNSS wind speeds rapidly became negative (ASCAT wind speeds stronger), reaching approximately \(-\)2 m s\({}^{-1}\) once rain rate reached 30 mm h\({}^{-1}\) (Figure 3b). Approximately 95% of raining matchups occurred in rain rates below 5 mm h\({}^{-1}\), so results above this threshold were increasingly more uncertain as rain rate increased. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **ASCAT-A** & **ASCAT-B** \\ \hline No Rain RMSD (m s\({}^{-1}\)) & 1.94 & 1.93 \\ No Rain Offset (m s\({}^{-1}\)) & +0.52 & +0.53 \\ No Rain Matchups & 39,763,736 & 40,099,155 \\ Rain RMSD (m s\({}^{-1}\)) & 2.47 & 2.46 \\ Rain Offset (m s\({}^{-1}\)) & +0.45 & +0.47 \\ Rain Matchups & 4,238,318 & 4,275,117 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: RMSD, mean offset, and number of matchups for winds outside of rain and winds in rain. Convention is CYGNSS FDS wind speeds minus ASCAT wind speeds (positive offset: CYGNSS winds were stronger). Figure 3: (a) Mean offsets between CYGNSS FDS and ASCAT-A and -B wind speeds as a function of IMERG rain rate, conditioned on the rain rate being nonzero. (b) RMSD between CYGNSS FDS and ASCAT-A and -B wind speeds as a function of IMERG rain rate, conditioned on the rain rate being nonzero. All data were from May 2017 through December 2019. Figure 4 shows that RMSD and mean offset have, in general, been increasing with time against both ASCAT-A (Figure 4a) and -B (Figure 4b). This was likely due to increasing uncertainty in GPS power calibration, which affects the CYGNSS v2.1 wind retrieval. Newer versions of CYGNSS data are expected to have improved performance, and should remove these trends. In summary, we expect reduced agreement between CYGNSS and ASCAT in rain, and the 2019 matchups should show worse agreement than 2018.