## 3 Results
### Verification of the ATs  
In this section, the 6-h accumulated precipitation from forecasts launched between 0500 and 1100 UTC, the major impact period of ATs, was verified. We first compared groups C1, C2 and C3 in predicting the location, intensity, and wind field of the ATs. Subsequently, verification methods were used to evaluate the forecast performance of each group. Compared with the observations (Figure 8l), the simulated accumulated precipitation of group C1 without data assimilation (NDA01-04) revealed incorrect positions and underestimated precipitations of the ATs (Figure 8a-d). In contrast to group C1, there was  
Figure 7: The hourly surface pressure for (**a**) observations (solid lines), DA01 (dot lines) and NDA01 (dashed lines) simulations, and for (**b**) observations (solid lines), DA02 (dot lines) and NDA02 (dashed lines) simulations of Pingtung Airport from 0500 to 1000 UTC (1300 to 1800 LST) on 5 August 2018. (**c**,**d**) are similar to (**a**,**b**), but for the hourly surface temperature. Note that Figure 7a,c are similar to Figure 6a,b, respectively, but different in the time span. significant improvement with respect to the distribution of precipitations of ATs in the 3 DVAR experiments of data assimilation for groups C2 (Figure 8e-h) and C3 (Figure 8i-k). In group C3, CDA01 (Figure 8i), CDA02 (Figure 8j) and CDA03 (Figure 8k) experiments used the same initial time as that of DA04 (Figure 8h). However, CDA02, assimilated during 0200-0400 UTC, and CDA03, assimilated during 0300-0400 UTC, had stronger intensity of precipitation than that of DA04. The overall forecasting abilities of precipitations between observations and CDA02, CDA03 and DA04 simulations were pretty close. The quantitative comparison between them will be further examined in Section 3.3. The precipitation in CDA01 assimilated during 0100-0400 UTC periods was underestimated on the west coast close to Tainan due to significant northeasterly at 0100 UTC, causing downdraft and a stable atmosphere in the leeward side of the mountains. The observed and simulated 10-m wind field and lowest model-level radar reflectivity for the C1 and C2 AT forecasts are illustrated in Figures 9 and 10, respectively. Panels in the top row in both figures represent the observed ATs, and panels in the other rows illustrate the forecasted ATs. Compared with the observed data (Figure 9a-c), the C1 simulations without data assimilation revealed incorrect tracks of the AT movement toward the west coast of southern Taiwan during 0600 UTC, 0700 UTC, and 0800 UTC, and the simulated onshore wind was clearly underestimated for the experiments of NDA01 (Figure 9d-f), NDA02 (Figure 9g-i), NDA03 (Figure 9j-l), and NDA04 (Figure 9m-o). A substantial improvement with respect to the distribution of wind fields and the intensity and locations of ATs was evident in the C2 simulations with data assimilation in the 3 DVAR experiments of DA01, DA02, DA03, and DA04. Figure 8: The simulated accumulated precipitation (mm) during 0500-1100 UTC (1300-1900 LST) on 5 August 2018, (**a–d**) NDA01-NDA04, (**e–h**) DA01-DA04, (**i–k**) CDA01-CDA03 simulations, and (**l**) observations. Figure 9: The observed and simulated 10-m wind (in arrow; intervals of 5 kt) and lowest model-level radar reflectivity (in color; dBZ) from 0600 to 0800 UTC (1400 to 1600 LST) on 5 August 2018, (**a–c**) observations, same as Figure 4, (**d–f**) NDA01, (**g–i**) NDA02, (**j–l**) NDA03, and (**m–o**) NDA04 simulations. Additionally shown are the approximate location of Pingtung Air Force Base (black star) and vertical cross-section planes (black line) for subsequent figures. The lines between x and x’ are plotted for further comparison of vertical distribution of convective cells near Pingtung Airport.