Increasing trend of extreme rain events over Bangkok Metropolitan area

Sangchan Limjirakan¹, Atsamon Limsakul², Thavivongse Sriburi¹

¹ Environmental Research Institute, Chulalongkorn University Pathumwan, Bangkok 10330 Thailand

² Environmental Research and Training Center, Technopolis Klong 5 Klong Luang Pathumthani 12120 Thailand

Warming of the climate system is now unequivocal. Thailand is already experienced on its adverse impacts. The biggest threats are arguably the increasingly frequent and more intense extreme climate events such as tropical cyclones, floods, droughts and heavy rainfall. The urban areas especially heavily populated mega-cities located on or near the coastal and river flood plains are highly vulnerable to such extreme events. To better understand changes in extreme rainfall characteristics and associated consequences in Bangkok Metropolis which is situated on the mouth of the Chao Praya River, high-quality daily rainfall data between 1965 and 2006 were analyzed. A set of core extreme indices recommended by the WMO-CCL/CLIVAR Expert Team for Climate Change Detection Monitoring and Indices (ETCCDMI) were calculated by measuring different aspects of extreme events such as wetness and dryness conditions, frequency and intensity rainfall events. The results reveal that Bangkok Metropolis has experienced notable changes in extreme rainfall events. Total annual rainfall increased significantly by about 4% (relative to long-term mean) per decade, whereas the annual number of rainy days decreased slightly by about 1% per decade. Consequently, intensity of rainfall measured by the Simple Daily Intensity Index (SDII) showed significant increasing trend over the 1964-2006 periods. It is also observed that more intense rainfall occurred during May to October when the southeast monsoon, originally from the Indian Ocean, prevails over central Thailand. Other rainfall indices such as R10, R95T and CWD showed discernable and coherent changes towards wetter condition and increases in magnitude and frequency of more intense rainfall events. Early findings can be concluded that the risk of severe and flash floods and associated disasters in Bangkok Metropolitan area will increase and affect on millions of people, socio-economic and biophysical environment as well. Therefore, detailed study on vulnerability and risk assessment is urgently needed to provide vulnerable insight of adaptation strategy, disaster preparedness and management plan for Bangkok Metropolis, and to move forward as climate resilient sustainable development in a mega-city.       

Keywords: Increasing trend, extreme rain events, Bangkok Metropolitan area