## 4 Observation Simulations of GMR, GeoSTAR, and GIMS
### Experimental Results  
The scatter plots of model-calculated brightness temperatures versus the assimilated GEO-MW observations of Typhoon Hagibis valid at 06:00 UTC on 8 October, 2019 for GMR, GeoSTAR, and GIMS are shown in Figures 11-13, respectively. The top panels of each figure show the background-calculated brightness temperature versus the GEO-MW observations. The bottom panels show the analysis-calculated brightness temperature data versus the observations. After assimilating the GEO-MW brightness temperature data, the analysis results had closer agreements with the observations than the background, i.e., the RMSEs of channels 4-6 and 8 for GMR were reduced by approximately 6%, 10%, 7%, and 26%, respectively (Figure 11), the RMSEs of channels 4-6 and 8 for GeoSTAR were reduced by approximately 14%, 28%, 20%, and 2%, respectively (Figure 12), and the RMSEs of channels 4-6 and 8 for GIMS were reduced by approximately 9%, 7%, 7%, and 23%, respectively (Figure 13), indicating that the WRFDA-4 Dvar was effective in directly assimilating the simulated GEO-MW brightness temperature data. Table 8 show lists the RMSEs (K) of brightness temperature calculated from the background field and analysis field versus the relative to observation. there is no clear difference between the CTRL and the three DA experiments. The predicted tracks began to diverge from the best track after 30 h, and from 30 h onward, the track forecasts given by the three DA experiments were consistently better than those given by CTRL. Of the three DA experiments, from 30 h onward, the predicted track of DA-GeoSTAR was consistently the closest to the best track, followed by DA-GMR, and the track forecast given by DA-GMS had the largest deviation. Figure 11: Scatter plots of model-calculated brightness temperature versus the observations (K) of GMR for (**a–d**) background and (**e–h**) analyses valid at 06:00 UTC on 8 October 2019. (**a,e**) represents channel-4 53.596 GHz, (**b,f**) represents channel-5 54.4 GHz, (**c,g**) represents channel-6 54.94 GHz, and (**d,h**) represents channel-8 57.29 GHz. Figure 12: Scatter plots of model-calculated brightness temperature versus the observations (K) of GeoSTAR for (**a–d**) background and (**e–h**) analyses valid at 06:00 UTC on 8 October 2019. (**a,e**) represents channel-4 53.596 GHz, (**b,f**) represents channel-5 54.4 GHz, (**c,g**) represents channel-6 54.94 GHz, and (**d,h**) represents channel-8 57.29 GHz. Figure 14: The predicted track of Hagibis initialized from 06:00 UTC on 8 October 2019 (**a**) and track forecast errors (**b**). The red curve in (**a**) represents the best typhoon track data. Figure 13: Scatter plots of model-calculated brightness temperature versus the observations (K) of GIMS for (**a–d**) background and (**e–h**) analyses valid at 06:00 UTC on 8 October 2019. (**a,e**) represents channel-4 53.596 GHz, (**b,f**) represents channel-5 54.4 GHz, (**c,g**) represents channel-6 54.94 GHz, and (**d,h**) represents channel-8 57.29 GHz. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **RMSE** & **53.596 GHz** & **54.4 GHz** & **54.94 GHz** & **57.29 GHz** \\ \hline \multirow{2}{*}{GMR} & background field & 0.276 & 0.259 & 0.242 & 0.345 \\  & analysis field & 0.259 & 0.232 & 0.226 & 0.255 \\ \hline \multirow{2}{*}{GeoSTAR} & background field & 0.222 & 0.154 & 0.118 & 0.491 \\  & analysis field & 0.191 & 0.111 & 0.094 & 0.484 \\ \hline \multirow{2}{*}{GIMS} & background field & 0.282 & 0.246 & 0.24 & 0.331 \\  & analysis field & 0.256 & 0.229 & 0.223 & 0.256 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: RMSE (K) of brightness temperature calculated from background field and analysis field relative to observation. Figure 15 shows the 72-h best track, and the predicted tracks of Typhoon _Bualoi_ initialized from 06:00 UTC on 22 October and the forecasting track error. For _Bualoi_, in the first 18 h, the locations of the typhoon's center in CTRL were close to those in the three DA experiments.