## 4 Results
### Assessment of Global Forecast Impact
In the Northern Hemisphere (30\({}^{\circ}\) N-90\({}^{\circ}\) N), the positive/negative impacts are spatially blended, leading to overall neutral impact. However, in the Tropic (30\({}^{\circ}\)N-30\({}^{\circ}\) S), the departures of RMSEs between two trial runs are almost zero at all three pressure levels at T+72 and T+96 forecast hours (Figure 9). Figure 8: The (**a**) mean and (**b**) normalized difference of RMSE of 250 hPa, 500 hPa, and 700 hPa geopotential height forecast up to 120–h forecast hours between the latency_20m and control runs. Grey, green, red, and magenta boxes indicate the results over the Global, Northern Hemisphere, Tropic, and Southern Hemispheres, respectively. Vertical bars representing 95% confidence level of the uncertainty are overlaid. For temperature forecast verification, the ACs tend to increase slightly in the forecast range of T+48 to T+96 for the latency_20m run at three different pressure levels (Figure 10). Similar to the results of geopotential heights, the maximum forecast impact is exhibited in the Southern Hemisphere, with a maximum improvement of 0.5% at 500 hPa for the T+96 forecast hour. However, in the Tropic, there is strong negative impact at 250 hPa for the forecast range from T+96 to T+120. For the RMSE analysis, it is found that reducing the data latency to 20 min has contributed to improving the temperature forecast at three pressure levels, especially in the Southern Hemisphere, where the error reduction of about 1.0% is present in the range of T+24 to T+96 forecast hour (Figure 11). The positive impact is mainly exhibited in the latitudinal range from 30\({}^{\circ}\) S to 70\({}^{\circ}\) S at all three pressure levels at T+72 and T+96 forecast hours (Figure 12). In the Northern Hemisphere (30\({}^{\circ}\) N-90\({}^{\circ}\) N), the positive impacts conflict with other negative impacts in the spatial distribution, making the impact overall neutral. In contrast, in most of the regions in the Tropics (30\({}^{\circ}\) N-30\({}^{\circ}\) S), the difference of RMSEs between the latency_20m run and the control run is close to zero. The neutral impact shown in the Tropics can be related to many potential factors but seems to because many IR and MW sounding data were filtered out over the cloud that frequently occurs in the Tropics, leading to a small number of satellite data to be assimilated. Considering that the difference between two trial runs is only an amount of LEO IR and MW sounding data assimilated in the GFS due to different data latency, the positive forecast impact of reduced data latency is attributed to newly added sounding measurements for the latency_20m, which provide unique information on the vertical structure of primary atmospheric variables (i.e., temperature and moisture). Based on the forecast verification results, it can be concluded that WSP-recommended data latency of 20 min on the LEO IR and MW sounders is beneficial for improving the global weather forecasting. Figure 9: Global map of the mean difference of RMSE of (**a,b**) 250 hPa, (**c,d**) 500 hPa, and (**e,f**) 700 hPa geopotential height at (**a,c,e**) T+72 and (**b,d,f**) T+96 forecast hours between the latency_20m run and the control run. Figure 11: The (**a**) mean and (**b**) normalized difference of RMSE of 250 hPa, 500 hPa, and 850 hPa temperature forecast up to 120-h forecast hours between the latency_20m and control runs. Grey, green, red, and magenta boxes indicate the results over the Global, Northern Hemisphere, Tropic, and Southern Hemispheres, respectively. Vertical bars representing 95% confidence level of the uncertainty are overlaid. Figure 10: The (**a**) mean and (**b**) normalized difference of AC of 250 hPa, 500 hPa, and 850 hPa temperature forecast up to 120-h forecast hours between the latency_20m and control runs. Grey, green, red, and magenta boxes indicate the results over the Global, Northern Hemisphere, Tropic, and Southern Hemispheres, respectively. Vertical bars representing 95% confidence level of the uncertainty are overlaid.