This study investigates the mesoscale interaction of typhoon circulation and southwesterly flow during the passage of Typhoon Morakot (2009) over Taiwan using radar observations. Single Doppler radar analysis characterized typhoon features with remarkably distant rainbands, and identified the strong southwesterly monsoonal flow with a maximum speed of more than 45 m s -1 in the southern flank of distant rainbands. Dual-Doppler synthesized winds elucidated a confluent mechanism on the northern side of the rainbands with a maximum convergence over 1.5 × 10 -3 s -1 . Velocity azimuth display (VAD) winds showed the intensification of southwesterly flow at low levels. The southwesterly monsoonal flow initiated about 6 hrs before typhoon landfall, and then became prominently involved with typhoon circulation. Also, the radial component with respect to the typhoon center was enhanced and became comparable with the tangential one (~30 m s -1 ) about 7.5 hrs after landfall. The variation in intensity of radial components can be regarded as a unique precursor for the extension of influence of the southwesterly monsoonal flow onto the typhoon circulation. Furthermore, the strong convergence showed that the interaction between the southwesterly flow and typhoon circulation might contribute to the development of rainbands, as well as the intensification of the inward radial flow embedded within typhoon.

Through the analysis of Doppler radar data, this study focuses on the characteristics and evolution of convection embedded within the principal band in Typhoon Morakot (2009) under the impingement of the intense southwesterly (SW) monsoonal flow. The intensity of the SW flow is comparable with the typhoon circulation at the third quadrant. The kinematic analysis shows that the northward component of the SW flow decelerates while approaching the rainband, creating significant convergence zones which results in the initiation and development of convective cells within the principal band.The vertical kinematic characteristics of the rainband reveal two types of downdrafts namely inner-edge and low-level downdrafts. The inner-edge downdrafts coupled by the radially inward tilting convection were initiated by the precipitation drag. Dynamically, the existence of the perturbed high at 1.5 km altitude in the inner-edge downdrafts supported the finding. Furthermore, it is evident that the distribution of two perturbation highs in the vicinity of the rainband could lead to SW flow deformation locally and fortify the mechanism of convergence, resulting in the merging of convective cells into the rainband. The maximum vertical vorticity coupled with the horizontal wind maximum at the middle levels of the rainband was also observed.

The Global Satellite Mapping of Precipitation (GSMaP) was used to estimate the accumulated rainfall in May from the Mei-Yu front in Taiwan. Rainfall estimation from GSMaP during 2002-2017 were evaluated using more than 400 local gauge observations, collected from the Taiwan Central Weather Bureau (CWB). Studies have demonstrated that the GSMaP rainfall estimation estimates can be biased, depending on the target region, elevation, and season. In this experiment, we have evaluated GSMaP over three elevation ranges. The GSMaP systemic errors for each elevation range were identified and corrected using regression analysis. The results indicated that GSMaP estimation can be improved significantly through adjustment over three elevation ranges (elevation less than 50 m, elevation of 50-100 m, and elevation higher than 100 m). For these three elevation ranges, the correlation coefficient between the GSMaP estimations and CWB rainfall data was 0.76, 0.78, and 0.59, respectively. This indicated that the GSMaP estimation was more accurate for low-elevation regions than high-elevation regions. After the proposed approaches were employed to correct the errors, the bias errors were respectively improved by 5.64(13.7%), 7.33(38.4%) and 10.52(31.2%) mm for low-, mid-and high-elevation regions. This study demonstrated that the local correction approaches can be used to improve GSMaP estimation of Mei-Yu rainfall in Taiwan.

In Taiwan, the frequency of afternoon convection increases in summer (July and August), and the peak hour of afternoon convection occurs at 1500–1600 local solar time (LST). Afternoon convection events are forecasted based on the atmospheric stability index, as computed from the 0800 LST radiosonde data. However, the temporal and spatial resolution and forecast precision are not satisfactory. This study used the observation data of Aqua satellite overpass near Taiwan around 1–3 h before the occurrence of afternoon convection. Its advantages are that it improves the prediction accuracy and increases the data coverage area, which means that more airports can use results of this research, especially those without radiosondes. In order to determine the availability of Atmospheric Infrared Sounder (AIRS) in Taiwan, 2010–2016 AIRS and radiosonde-sounding data were used to determine the accuracy of AIRS. This study also used 2017–2018 AIRS data to establish K index (KI) and total precipitable water (TPW) thresholds for the occurrence of afternoon convection of four airports in Taiwan. Finally, the KI and TPW were calculated using the independent AIRS atmospheric sounding (2019–2020) to forecast the occurrence of afternoon convection at each airport. The average predictive accuracy rate of the four airports is 84%. Case studies at Hualien Airport show the average predictive accuracy rate of this study is 81.8%, which is 9.1% higher than that of the traditional sounding forecast (72.7%) during the same period. Research results show that using AIRS data to predict afternoon convection in this study could not only increase data coverage area but also improve the accuracy of the prediction effectively.