Rains influenced by tropical cyclones over Thailand

Abstract

One natural hazard that Thailand has encountered annually is heavy rains, which can negatively affect the public in several ways, e.g., flooding, transportation/traffic hampering, and property damage. This paper focuses on one contributor to heavy rains in Thailand, which is tropical cyclone (TCs). Due to its location in the center of the Indochinese Peninsula, Thailand is generally and overall much less impacted by TCs in comparison with neighboring countries such as Vietnam, Laos, Cambodia, and Myanmar. Nevertheless, they have played an significant roles in causing heavy rains in many parts of Thailand in some particular years. Here, a set of results from an ongoing study are presented, which is still considered preliminary and subject to further analysis. Here, two main datasets used are 1) historical daily rainfall data from about 250 stations combined across the country and spanning 41 years (1970-2009), supported by the Thailand Meteorological Department, the Royal Irrigation Department, and the Department of Water Resources, and 2) TC track data of the International Best Track Archive for Climate Stewardship (IBTrACS). The daily rainfall data of each station was quality- checked and passed the homogeneity test, with >80% being non-missing values. A distance-intensity relationship was explored to see how the rainfall at a station statistically varies with the distance between a TC center and a rainfall station, and a cutoff distance was chosen as an aggregate threshold for TC-influenced rains. There are a total of 345 daily rainfalls affected by TCs (shortly, TC- rain days or events), most of which (68%) are of heavy rain (>35 mm). Most of the influencing TCs were found to be tropical depressions, and about one-third is typhoons or cyclones. In terms of number of TC-rain events and the annual cycle, Sep.-Oct. appears to be the peak months, while no TC-rain events in Thailand was observed in Jan.-Mar. during the period considered. As for TC- impacted areas, the Northeast and the South are the sub-regions impacted.