## 3 Results
### A Rainstorm Forecast Result Analysis
#### 3.2.3 Effects of Assimilation on Initial Field  
In order to figure out the contributions to the improved precipitation forecasts on how the assimilation changes the initial conditions, the analysis increments (analysis minus background) at 0000 UTC 30 June 2017 are compared from Figures 12-14. Figure 10: The SAL scores of 24-h accumulated precipitation from 0000 UTC 30 June to 0000 UTC 01 July 2017 at thresholds of (**a**) \(>\)0 mm, (**b**) \(>\)50 mm, and (**c**) \(>\)100 mm. Figure 12a shows the overlaid of water vapor mixing ratio increments and analysis wind field. Obvious southwesterly flow is found on the south of the precipitation area, advecting warm and moist air from the south. In the main precipitation area (Hunan Province), POD produces positive increments at 700 hPa, improving the water vapor mixing ratio. To better understand the vertical structure of water vapor changes, the cross-section is made along the red line in Figure 12a, which is shown in Figure 12b. Large positive increments appear under 500 hPa, revealing that assimilating satellite data increase water vapor dramatically in the mid-lower level. Similarly, the temperature increments are shown in Figure 12c-d. Negative increments are found mainly at 700 hPa, indicating that assimilation has a cooling effect in the low level of precipitation area. However, the temperature increase is found in the upper level when cross-section is made (Figure 12d). It can be inferred that strong convection develops in the heavy rainfall event, with convective condensation latent heat in the upper level of precipitation area. Figure 11: The observed radar reflectivity at **(a)** 0100 UTC, **(b)** 0200 UTC, **(c)** 0300 UTC 30 June 2017, and corresponding simulated reflectivity at the first model level from **(d–f)** CON, and **(g–i)** POD. Figure 13 shows the increments of vertical velocity at 500 hPa and associated cross-sections of the wind field. From Figure 13a, assimilation strengthens the updraft in the precipitation area. Clearly, southern wind increments appear in the same area, promoting the transportation of warm and wet air from the south of the rainfall area. Figure 13b displays the wind increments and relative humidity analysis, in which a large area of near-saturation humidity covering the main precipitation area, with the top at almost 300 hPa. In addition, assimilation clearly exhibits a strong and deep updraft within the high humidity area. The above results suggest that the combined assimilation of satellite and radar data changes the dynamical (wind) and thermodynamical (temperature and humidity) features which are important for convection development. Positive increments are clearly found at 850 hPa (Figure 14a). Same results appear in the cross-section along the red line in Figure 14a, in which rainwater mixing ratio is mainly enhanced in the precipitation area from surface to almost 500 hPa. As the hydrometeor, the improvement of rainwater mixing ratio directly strengthens the intensity of heavy rainfall in POD. Since the initial rainwater has been improved in POD, the vertical structure of hydrometeors as the forecast time leads is showed to exam the continued effect of assimilation. The pressure-time cross-sections are presented for rainwater mixing ratio and cloud water mixing ratio in the different experiments. For rainwater (Figure 15a,b), it is mainly located below 600 hPa. Rainwater appears clearly at about 4 h in CON (Figure 15a), and reach its first maximum magnitude at almost 7 h. In comparison, POD (Figure 15b) generates rainwater at the beginning of forecast because rainwater  
Figure 14: (**a**) The POD analysis increment of rainwater mixing ratio (shaded, g/kg) and analysis of horizontal wind (vectors) at 850 hPa at 0000 UTC 30 June. (**b**) The east-west cross-section of rainwater mixing ratio (shaded, g/kg) increments along the red line in (**a**). Figure 13: (**a**) The increment of vertical velocity (shaded, m/s) and horizontal wind field (vector) at 500 hPa.