# A Study on Assimilation of CYGNSS Wind Speed Data for Tropical Convection during 2018 January MJO
## 3 Results  
### CYGNSS Data Assimilation  
To examine the impact of the CYGNSS wind speed data, the analysis fields and short-term forecasts from experiments with and without the assimilation of CYGNSS data are compared and analyzed in this section. Figure 4 plots the CYGNSS wind, background wind field, and data assimilation analysis increment ('analysis - background', or the difference between data assimilation analysis and the background) in the surface wind field at 1500 UTC on 6 January 2018. At this time, CYGNSS FDS (Figure 4a) observed a maximum wind speed of 18 m s\({}^{-1}\), with most of the observed wind being slower than 15 m s\({}^{-1}\), while CYGNSS YSLF (Figure 4b) detected wind as strong as 47 m s\({}^{-1}\) near TC Irving and many points with wind speed above 15 m s\({}^{-1}\). In the background field (Figure 4e), an area of wind above 15 m s\({}^{-1}\) was predicted over the eyewall region of TC Irving with the maximum wind speed of 20 m s\({}^{-1}\). Therefore, the wind speed observed by CYGNSS FDS is generally lower than the background. When the FDS data were assimilated, the analysis increments were mostly negative near TC Irving (Figure 4c). With the assimilation of high winds from CYGNSS YSLF data, the surface wind increased by up to 6 m s\({}^{-1}\) near TC Irving and 2 m s\({}^{-1}\) for the WWB event (Figure 4d). However, the limitation of assimilating the CYGNSS data was also apparent. Due to the track-based nature of CYGNSS observation, the analysis increment in DA_cyg_lf shows elongated shapes at several places. This very likely does not represent the correct characteristics of the wind field, and therefore, may cause inaccurate asymmetry in wind and, hence, mass fields. The RMS difference in surface wind speed is calculated between the data assimilation experiments (DA_cyg_lf and DA_cyg_fd) and the background field, and averaged over the model domain. Figure 5 shows the domain averaged RMS difference of surface wind speed within 9 h after the assimilation of CYGNSS data was complete at 1500 UTC on 6 January 2018. At the analysis time, the averaged RMS was 1.2 m s\({}^{-1}\) for DA_cyg_lf and 0.86 m s\({}^{-1}\) for DA_cyg_fd. It rapidly declined for both experiments within the first hour after data assimilation. After 4 h, the averaged RMS were only 0.41 m s\({}^{-1}\) and 0.32 m s\({}^{-1}\) for DA_cyg_lf and DA_cyg_fd, which were 30% and 37% of the original values, respectively. This indicates that the strongest impact of the CYGNSS data will be within the first \(\sim\)4 h of the forecast. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Experiment** & **Data** & **Data Assimilation Analysis Times** \\ \hline
**CTRL** & N/A & N/A \\ \hline
**DA\_cyg\_fd** & CYGNSS FDS wind speed & 0000, 1200, 1500, and 2100 UTC on 6–8 January 2018 \\ \hline
**DA\_cyg\_lf** & CYGNSS YSLF wind speed & 0000, 1200, 1500, and 2100 UTC on 6–8 January 2018 \\ \hline
**DA\_com** & Combined IMERG rainfall, & 0000, 0230, 1100, 1500, 2000, and 2300 UTC on 6 January; \\
**DA\_com** & ASCAT vector wind, and CYGNSS & 0300, 1100, 1530, 2000, and 2300 UTC on 7 January; 0300, \\  & wind speed & 1100, 1500, and 2000 UTC on 8 January 2018 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Numerical Experiments setup. Figure 3: Data assimilation process for the experiment DA_com. Orange boxes show the time windows for CYGNSS data assimilation, and the blue boxes show the time windows when the ASCAT and IMERG data were assimilated. In the time window near 1500 UTC of each day, all three types of data were assimilated. With the assimilation of CYGNSS data in the initial conditions, positive impacts have been made in the simulations of TC Irving and the WWB event. Figure 6 displays the surface wind field from DA_cyg_fd, DA_cyg_lf, and CTRL compared with ASCAT observation around 0239 UTC on 8 January 2018. ASCAT (Figure 6a) observed an area of wind speed over 18 m s\({}^{-1}\) around the vortex with a maximum surface wind speed over 30 m s\({}^{-1}\). There is no apparent quadrant asymmetry in ASCAT data.