Determination of the Dates of the Southwest Monsoon in Northeastern Thailand from the Data on Precipitable Water Vapor Obtained by GPS

Prawit Uang-aree, Sununtha Kingpaiboon, and Kulyakorn Khuanmar

This paper outlines a new method for determining the onset and withdrawal of the southwest monsoon using Precipitable Water Vapor (PWV) estimated from GPS data: GPS-PWV. In this study, PWV is computed from GPS data collected from four ground-based GPS receivers in north- eastern Thailand. Initially, we study the temporal and geographical distributions of the GPS-PWV data. Their statistical characteristics show that GPS-PWV can be used as a tool for determining the onset and withdrawal of the southwest monsoon in northeastern Thailand. The GPS-PWV data were processed using a decision tree analysis to obtain a new set of criteria for determining the onset and withdrawal of the monsoon. The new criterion specifies the onset as a period when the daily averaged GPS-PWV exceeds 52.1 mm for eight consecutive days. The withdrawal is a period when the daily averaged GPS-PWV is 52.1 mm or less for six consecutive days. The dates of onset and withdrawal of monsoons calculated from this method was compared with those obtained with other methods. The difference equaled 1–2 day except years 2010 and 2012 when the errors were more than two days.