## 4 Observation Simulations of GMR, GeoSTAR, and GIMS
### Simulation of the Observed Brightness Temperature TA of GMR, GeoSTAR, and GIMS
#### 4.2.3 Geostationary Interferometric Microwave Sounder (GIMS)
Table 5 lists the root mean square error (RMSE) between the upwelling brightness temperature TB and observation TA of GMR, GeoSTAR, and GIMS for all 8 channels in Figure 4. The RMSEs are smaller at opaque channels 5-8 than at more transparent channels 1-4. Among the three instruments compared, GMR has the smallest error for more transparent channels 1-4, but the difference is not significant, and for opaque channels 5-8, GeoSTAR has the smallest error. Figure 9 shows the average RMSE of 13 TA images of Typhoon _Hagibis_ from 06:00 to 12:00 UTC on 8 October 2019, used in the GEO-MW OSSE. It is clear that the RMSE results in Figure 9 are consistent with Table 5.  
denoted CTRL, DA-GMR, DA-GeoSTAR, and DA-GIMS, respectively. Besides, the typhoon best track obtained from the CMA is called REAL. All of these are summarized in Table 6.  
The control experiment, CTRL, assimilated no GEO-MW brightness temperature data, which directly predicts 72 h using initial and lateral boundary conditions provided by FNL reanalysis data, serving as a benchmark forecast compared with other GEO-MW data assimilation experiments. The assimilation experiments (DA-GMR, DA-GeoSTAR, and DA-GIMS corresponding to simulated observation TA of GMR, GeoSTAR, and GIMS, respectively) used the initial field of the CTRL as the background field and assimilated GEO-MW brightness temperature data at half-hourly time intervals within the six-hour time period to generate the analysis field. Then, the analysis field was used as a new initial for the 72 h prediction. The GEO-MW brightness temperature data assimilated in all experiments were thinned to a mesh of 60 km to reduce the impact of potential correlations among the observations. The REAL-track selected the 72 h best typhoon tracks of CMA starting from the assimilation time as the reference true value to evaluate the prediction error of the assimilation experiments. The sequence diagram of the 5 experiments is shown in Figure 10.  
The WRFDA-4 Dvar assimilation experimental parameter settings of Typhoons _Hagibis_ and _Buuloi_ are shown in Table 7. The GEO-MW brightness temperature data of _Hagibis_ were simulated by the Nature Run data at half-hour intervals from 06:00 UTC to 12:00 UTC on 8 October. In the CTRL experiment of _Hagibis_, the 6-h short-term forecast field initialized with the FNL reanalysis data at 00:00 UTC on 8 October was used as the initial field for the 72-h WRF prediction to obtain the typhoon track from 06:00 UTC on 8 October to 06:00 UTC on 11 October. In the assimilation experiments, the analysis field at 06:00 UTC on 8 October was used as the initial field for the 72-h WRF prediction to obtain the typhoon track with the same time range as the CTRL experiment. The sequence of Typhoon _Buuloi_ was similar to that of Hagibis, except the date was 22 October. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Experiment** & **DA Scheme** & **Data** & **Spatial Resolution** \\ \hline
**1** & CTRL & No DA & & \\
**2** & REAL & No DA & & \\
**3** & DA-GMR & Half-hourly 4D-Var DA & GMR & **52 km** \\
**4** & DA-GeoSTAR & Half-hourly 4D-Var DA & GeoSTAR & **74 km** \\
**5** & DA-GIMS & Half-hourly 4D-Var DA & GIMS & **70 km** \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Detailed description of the four experiments and REAL. Figure 10: Sequence diagram of CTRL, REAL, DA-GMR, DA-GeoSTAR, and DA-GIMS experiments. As future satellite data, GEO-MW brightness temperature data are not yet supported by the WRFDA. Therefore, to assimilate the GEO-MW observation data using the WRFDA-4 Dvar system, it is necessary to first convert it into a BUFR (binary universal form of the representation of meteorological data, see more details in [[https://noaa-emc.github.io/NCEPLIBS-buff/](https://noaa-emc.github.io/NCEPLIBS-buff/)]([https://noaa-emc.github.io/NCEPLIBS-buff/](https://noaa-emc.github.io/NCEPLIBS-buff/)) (accessed on 21 March 2021)) format file supported by the WRFDA.