The afternoon thunderstorm system is associated with short rainfall time period, confined rainfall area and strong rainfall intensity. The objective of this study is to understand the synoptic weather patterns that are prone for the development of the afternoon thunderstorm system through cluster analysis method and at the same time, to investigate the meteorological characteristic including the local circulations and thermal structures of the afternoon thunderstorm system .
The observation data from weather stations of Central Weather Bureau (CWB) is used for cluster analysis. The data period covers the summer season (July, August and September) from 2013 to 2015. The selected meteorological variables are temperature, humidity, wind speed, wind direction, pressure, rainfall, and specific humidity. The cluster analysis using these surface meteorological variables are tested in different combinations to find out the best weather classification that can identify the characteristics of the afternoon thunderstorm system.
In first experiment, the cluster analysis was applied using U- and V- component wind to classify the weather pattern. The classification results exhibit three types of prevailing winds in northern Taiwan including the northeasterly flow, southwesterly flow, and southeasterly flow.
In second experiment, the cluster analysis was applied using pressure, temperature, and relative humidity. Five clusters are found using the above method. Cluster one and cluster five exhibit similar synoptic weather pattern for which Taiwan is under influence of sub-tropical high-pressure system. Cluster one is associated with southeasterly wind in northern Taiwan and with less rainfall. Cluster five is associated with southwesterly wind and exhibits the afternoon thunderstorm rainfall pattern. The occurrence of cluster two is less and is associated with the northeasterly wind due to the influence of the continental high-pressure system. Cluster two has less rainfall. In Cluster three, the prevailing wind is from northeast and the strength of the Pacific high-pressure system weakens. Taiwan is influence by the low-pressure system. Cluster four is associated with the highest rainfall due to the typhoons. Cluster six doesn’t have apparent characteristics.
Lastly, only cases associated with the afternoon thunderstorm are selected from three-year summertime observation data and the cluster analyzed was applied using U- and V-component wind field. Three clusters are found using the above method. The cluster one is with less occurrence and due to the influence of the sub-tropical high pressure system the dominant wind is southeasterly. The second cluster has the highest rainfall amounts and the precipitation occurs in wider area among three clusters. The dominant wind is northeasterly in northern Taiwan when Pacific high-pressure system weakens. The cluster three has similar weather pattern as the cluster one. The synoptic weather pattern is under the influence of sub-tropical high-pressure system but the dominant wind is southwesterly. The occurrence of cluster three is more than 50 percent of the days examined.
Furthermore, CWB observation data is employed to analyze an afternoon thunderstorm case (19 August 2014) and the Team-R observation data from National Central University (NCU) and sounding data from Private Chinese Culture University (PCCU) are utilized to analyze the thermal and dynamic structure of the system.