# Applications of GNSS-RO to Numerical Weather Prediction and Tropical Cyclone Forecast
## 2 Research and Applications of RO in NWP
### Impact of the Amount of RO Data on NWP and Future Trends  
As a small amount of RO observation data has an impressive impact on the NWP, the authors of [46] analyzed the upper limit of the amount of RO data that benefits NWP. The authors of [46] simulated the impact of the number of assimilated RO observations varying from 2000 to 128,000 on NWP using the ensemble of data assimilation (EDA) approach, namely the OSSE (Observing System Simulation Experiment). The operational ECMWF NWP analysis is used as the proxy of the truth to simulate the bending angle. Figure 7 shows that increasing the amount of assimilated RO data leads to a continuous reduction in the EDA spread, a parameter that estimates the forecast error. Specifically, Figure 7a-c indicate that, at heights of 500 and 100 hPa, the simulated temperature and geopotential height show better results in the southern hemisphere than in the tropics and the northern hemisphere. Furthermore, the higher the geopotential height, the greater the error reduction. For example, the statistical errors, shown in Figure 7a,b, at 500 hPa decrease by approximately 20%, whereas they decrease by approximately 50% at 100 hPa. Notably, the maximum error reduction is 80% when using 128,000 observations, and impact saturation has not been reached, i.e., if the amount of RO data keeps increasing, the forecast error will keep decreasing. However, 128,000 is far from achievable, as the total amount of RO observations in 2020 is only approximately 10,000. Therefore, the greater the amount of RO data, the better the accuracy of the NWP. Another notable result in Figure 7 is that half of the maximum impact can be attained using only 16,000 RO observations, a more reachable number. However, the current RO data in the assimilation system are far from reaching 128,000 and 16,000. So far, the ECMWF could assimilate approximately 5000 soundings per day from the seven missions. As listed in Table 1, these missions include COSMIC-2, MetOp-A/B/C, FY-3C/D, TanDEM-X, TerraSAR-X, KOMPSAT-5, and PAZ. The total amount of RO data received by these missions is approximately 9000; however, the actual received number is only approximately 6000. After the quality control, there are only 5000 soundings left in the assimilation system. As for the NCEP Global Data Assimilation System (GDAS), there were 2000-3000 RO soundings per day until 2019, according to [47], as shown in Figure 8.  
Figure 6: Evaluation of various observations assimilated by the ECMWF. (**a**) Data Percentage; (**b**) Frequency of Error Reduction. While accounting for only 7% of the total observations in the assimilation system, RO has the fourth-largest impact on NWP. Adapted from [44]. Figure 8: Daily total RO profiles in the National Centers for Environmental Prediction (NCEP) Global Data Assimilation System (GDAS) from 01/2017 to 09/2019. Taken from [47]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Mission** & **Soundings/Day** \\ \hline COSMIC-2 & 5000 \\ MetOp-A/B/C & 1905 \\ FY-3C/D & 1000 \\ TanDEM-X & 100 \\ TerraSAR-X & 200 \\ KOMPSAT-5 & 500 \\ PAZ & 300 \\ Total data & 9005 \\ Actual received data & \(\sim\)6000 \\ Data used & \(\sim\)5000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Assimilated radio occultation (RO) data in the European Center Medium-Range Weather Forecast (ECMWF) operational numerical weather prediction (NWP) system (adapted from ECMWF) [48]. Figure 7: Normalized ensemble of data assimilation (EDA) spread (%) as a function of the assimilated number of simulated RO profiles [46]. The EDA spread is an error statistic of the ECMWF NWP theoretical forecast and analysis. (**a**) Temperature at 100 hPa; (**b**) temperature at 500 hPa; (**c**) geopotential height at 500 Pa; (**d**) relative humidity at 850 hPa. The more the RO observations, the better the NWP, and there is no saturation even with 128,000 soundings per day. The amount of RO observations will thrive in the future. According to the OSCAR website, there were approximately 10,000 soundings per day in 2020.