## 6 Summary  
At present, GMR with a 5 m real aperture antenna, GeoSTAR with a stationary Y-shape synthetic aperture array, and GIMS with a rotating circular synthetic aperture array are three promising solutions for GEO-MW sounders. The objective of this study was to provide preliminary results on the potential impact of assimilation of the GEO-MW observations of GMR, GeoSTAR, and GIMS at 50-60 GHz on typhoon prediction. The GEO-MW OSSEs were performed using the WRF model and WRFDA-4 DVar assimilation system for Typhoons _Hagibis_ and _Bualoi_.  
New 50-60 GHz observed brightness temperature data from GMR, GeoSTAR, and GIMS onboard a geostationary satellite were simulated from the Nature Run obtained from the ERA-5 reanalysis data and WRF forecast. The brightness temperature observation capabilities of GMR, GeoSTAR, and GIMS for 8 channels at 50-60 GHz were analyzed. Data assimilation produced analyses and forecasts for GMR, GeoSTAR, and GIMS, using the WRFDA-4 DVar assimilation system. The forecast errors in the typhoon track were computed against the best typhoon track data from the tropical cyclone data center of CMA.  
Verification against the control experiment showed that the experiment with the assimilated GEO-MW data gave a consistently more accurate prediction of the track for Typhoons _Hagibis_ and _Bualoi_. The assimilation of the three GEO-MW instruments with channels 4-6 and 8 at 50-60 GHz could have led to the general positive impacts in this study. However, these positive impacts seemed to depend on the observation error of the three potential instruments. For both Typhoons _Hagibis_ and _Bualoi_, GeoSTAR obtained the best assimilation effect, followed by GMR, and GIMS had the largest track error. For opaque channels 4-6 and 8 used in assimilation experiments, the observed brightness temperature error was mainly due to the random system noise. GeoSTAR, as a stationary SAIR, had the longest integration time in the simulation configuration, resulting in the smallest observation error in channels 4-6 and 8. GIMS, as a rotating SAIR, was set for a shorter integration time, and the interpolation of the UV plane visibility may have also slightly increased the reconstructed brightness temperature error, which resulted in the largest observation error.  
This is the first study to compare the three geostationary microwave sounders, GMR, GeoSTAR, and GIMS, in the GEO-MW OSSEs. Only preliminary results for two cases have been provided. To further consolidate the conclusions drawn here, more experiments concerning different cases are needed. In addition, the relationship between the brightness temperature observation capability and the impact of assimilation of different GEO-MW observation system configurations will be explored further in future studies.  
Writing--original draft: K.C. and G.W. All authors have read and agreed to the published version of the manuscript. This research was supported by Shanghai Aerospace Science and Technology Foundation of Shanghai Academy of Spaceflight Technology (SAST2019-102). Not applicable.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Track Error (km)** & **CTRL** & **DA-GMR** & **DA-GeoSTAR** & **DA-GIMS** \\ \hline
**Hagibis** & 340 & 170 & 110 & 270 \\
**Bualoi** & 370 & 270 & 270 & 360 \\ \hline \end{tabular}
\end{table}
Table 10: The 72-h error of the predicted track.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Track Error (km)** & **CTRL** & **DA-GMR** & **DA-GeoSTAR** & **DA-GIMS** \\ \hline
**Hagibis** & 140 & 80 & 70 & 100 \\
**Bualoi** & 210 & 160 & 140 & 190 \\ \hline \end{tabular}
\end{table}
Table 9: The average error of the predicted track.