Rainfall estimation from radar in different seasons over Northern Thailand

Thailand was located on the path of the tropical cyclones which were formed in the South China Sea and the Pacific ocean. The storms in Thailand are often downgraded to tropical depression. Northern Thailand has a tropical climate which can be divided into three seasons: rainy (May–October), winter (October–February) and summer (March-May). The averages rainfall is around 1,200–1,600 mm/year influenced by the two types of the monsoon winds. Northeast monsoon brings the cold and dry air mass from China to cover the major part of Thailand. Southwest monsoon brings the warm and moist air mass from the Indian Ocean toward Thailand causing abundant rain over the country. The objective of the research is to evaluate Z-R relationship (Z = AR^b) for rainfall estimation in different seasons. This study uses reflectivity data from Omkoi radar station in Chiang Mai Thailand and rainfall data in the radius of 240 kilometers from the Omkoi weather radar station from the Thai Meteorological Department. The method for matching reflectivity data (Z) and rainfall rate (R) relationship is PMM (Probability Matching Method). The result show that the Z-R relationship in the rainy season is Z = 103.83R¹⁵¹ and the Z-R relationship in summer and winter is Z = 102.18R¹⁴⁶. For the same rainfall intensity, reflectivity value in summer and winter season is higher than in the rainy season. This research shows that each type of monsoon wind causes different characteristics of rainfall. Thus rainfall estimation for different seasons should be based on the different Z-R relationship for more accuracy.