# A Study on Assimilation of CYGNSS Wind Speed Data for Tropical Convection during 2018 January MJO
## 3 Results
### CYGNSS Data Assimilation
In comparison, a strong quadrant asymmetry was found in all numerical experiments with the strongest wind appearing in the front-left quadrant. In addition, all three numerical experiments produced a larger area of wind speed \(>\) 18 m s\({}^{-1}\). The best track data shows the maximum surface wind of 49 (0000 UTC) to 46 m s\({}^{-1}\) (0600 UTC) at this time. Therefore, all numerical experiments produced an underestimate of the storm intensity. With assimilation of the YSLF data, DA_cyg_lf (Figure 6d) generated a stronger TC than CTRL (Figure 6b) and DA_cyg_fd (Figure 4c). A smaller eye with stronger maximum surface wind speed (28 m s\({}^{-1}\)) was found in DA_cyg_lf when comparing with CTRL (25 m s\({}^{-1}\)) and DA_cyg_fd (22 m s\({}^{-1}\)). Also note that the center location in the three experiments  
Figure 4: CYGNSS wind speed observation (**a**) FDS and (**b**) YSLF. The analysis increments from (c) DA_cyg_fd and (**d**) DA_cyg_lf. The blue contours represent negative values and red contours represent positive values. The contour interval is 1 m s\({}^{-1}\). (**e**) wind field in background WRF simulation. All plots are at 1500 UTC on 6 January 2018.  
are quite different. At this time, the observed storm center derived from the Joint Typhoon Warning Center (JTWC) best track data was near 79.9\({}^{\circ}\)E, 17.05\({}^{\circ}\)S. The 51-h forecast in CTRL predicted the storm center at 81.15\({}^{\circ}\)E, 18.5\({}^{\circ}\)S, which is 208-km away from the observed one. After seven cycles of CYGNSS data assimilation, the predicted center location was at 80.5\({}^{\circ}\)E, 18.5\({}^{\circ}\)S and 80.5\({}^{\circ}\)E, 17.8\({}^{\circ}\)S in DA_cyg_lf and DA_cyg_fd, respectively. Both forecasts were better than CTRL, with track errors of 174-km and 105-km for DA_cyg_lf and DA_cyg_fd, respectively. Figure 5: Time series of domain averaged RMS difference of surface wind speed between data assimilation experiments and the background field from 1500 UTC on 6 January 2018 to 0000 UTC on 7 January 2018. Figure 6: Surface wind speed from (**a**) ASCAT around 0239 UTC and (**b**) CTRL, (**c**) DA_cyg_fd, and (**d**) DA_cyg_lf at 0300 UTC on 8 January 2018. The impact of CYGNSS data assimilation on the intensity and track forecasts of TC Irving was compared with the JTWC best track data in Figures 7 and 8. From 00 UTC on 6 January to 00 UTC on 8 January, TC Irving experienced an intensification, from a tropical storm to a category-2 tropical cyclone, with a peak intensity of 962 hPa in MSLP of and 49 m s\({}^{-1}\) in maximum surface wind speed, followed by a weakening stage until 0000 UTC on 9 January with 973 hPa in MSLP of and 41 m s\({}^{-1}\) in maximum surface wind speed. From Figure 7, CTRL greatly underestimated the intensity of TC Irving. The forecast in CTRL shows a slow strengthening of Irving from 999 hPa in MSLP and 17 m s\({}^{-1}\) in maximum surface wind speed at 0000 UTC on 6 to 990 hPa and 25 m s\({}^{-1}\) at 0000 UTC on 9 January 2018. Improvement in intensity forecast by the assimilation of the CYGNSS data was found from 0000 UTC on 7 January to 0000 UTC on 8 January 2018. During this time period, both DA_cyg_lf and DA_cyg_fd outperformed CTRL with the strongest storm generated by DA_cyg_lf. CTRL missed the weakening of the storm from 0000 UTC on 8 to 0000 UTC on 9 January 2018. The intensity of the storms in DA_cyg_lf and DA_cyg_fd showed some fluctuations, but no apparent weakening was predicted in either experiment. It is noteworthy that the intensity forecast in data assimilation experiments was not always better than CTRL (e.g., from 1200 UTC on 8 January to 0000 UTC on 9 January 2018 in DA_cyg_fd). The track forecasts from CTRL and data assimilation experiments were compared with the JTWC best track in Figure 8. As shown in Figure 8a, all experiments captured the southwestward path of TC Irving. CTRL produced the track to the south of the observed storm with a track error ranging from 35 km to 265 km. The assimilation of CYGNSS FDS data provided help with the tracking forecast.