The atmospheric circulation patterns that were responsible for the heavy flooding that occurred in Thailand in 2011 are examined. This paper also investigates the interannual variation in precipitation over Indochina over a 33-yr period from 1979-2011, focusing on the role of westward-propagating tropical cyclones (TCs) over the Asian monsoon region. Cyclonic anomalies and more westward-propagating TCs than expected from the climatology of the area were observed in 2011 along the monsoon trough from the northern Indian subcontinent, the Bay of Bengal, Indochina, and the western North Pacific, which contributed significantly to the 2011 Thai flood. The strength of monsoon westerlies was normal, which implies that the monsoon westerly was not responsible for the seasonal heavy rainfall in 2011. Similar results were also obtained from the 33-yr statistical analysis. The 5-month total precipitation over Indochina covaried interannually with that along the monsoon trough. In addition, above-normal precipitation over Indochina was observed when enhanced cyclonic circulation with more westward-propagating TCs along the monsoon trough was observed. Notably, the above-normal precipitation was not due to the enhanced monsoon westerly over Indochina. Therefore, the 2011 Thai flood was caused by the typical atmospheric circulation pattern for an above-normal precipitation year. It is noteworthy that the effect of sea surface temperature (SST) forcing over the western North Pacific and the Niño-3.4 region on total precipitation during the summer rainy season over Indochina was unclear over the 33-yr period.

Abstract:Statistical and dynamic methods were used in the downscaling process from Global Climate Model (GCM) to Regional Climate Model (RCM). We selected the European Centre for Medium-Range Weather Forecasts model, Hamburg version 4 (ECHAM4) with 300 × 300 km resolution for A2 scenario. We focused on SE Asia domain located between 20°S to 30°N and 80°E to 135°E for 1960-2099 with wind components, temperature, geo-potential height, and specific humidity as data input in Providing Regional Climates for Impacts Studies (PRECIS) RCM analysis. The downscaling process output was 50 km resolution for 1971-2010 and precipitation, temperature, wind, relative humidity, radiation from 8 meteorological stations in Chao Phaya River Basin; Lampang, Suphanburi, Nan, Sisamrong, Takfa, Chainat, Uthong and Bangna selected and used for bias correction. Three methods, namely 1) adjusting the mean based on RCM, 2) adjusting the mean based on observation, and 3) quantile-based mapping were used. Methods were compared using observed climatic data, RCM outputs of calibration period, and RCM outputs from the validation period. RSME was found to be lower for method 2 compared to other methods implying a relatively superior technique for improving the model. As such method 2 was used to correct the PRECIS products during [2001][2002][2003][2004][2005][2006][2007][2008][2009]. These products are useful in the studies of impact of climate change and for early warning systems in Thailand.

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Abstract:Rainfall patterns during summer monsoon in 2009 and 2010 over the middle of the Indochina Peninsula (ICP) are investigated using calibrated daily accumulated radar rainfall (CDARR). Empirical orthogonal function (EOF) analysis applied to CDARR shows that the first three modes explain 40% of the total rainfall variance. The pattern of the first EOF mode is only positive over the radar observation area with a large value near the foot of the Annam range in the eastern region of the radar site. The second EOF mode is a dipole pattern that has positive and negative regions in the eastern and western regions of the radar observation area, respectively. The third EOF mode also shows a dipole pattern with positive and negative areas in the southern and northern regions of the observation area, respectively. Composite analysis results suggest that the first EOF mode is possibly produced by a difference in positive vorticity, in which the difference in the southerly wind component likely causes orographic rainfall in the eastern region of the radar site. In addition, the second and third EOF modes are possibly produced by differences in westerly and southwesterly wind components, respectively.

A classification system for rain clouds was developed using ground-based radar reflectivity and infrared brightness temperature (TBB) data from multifunctional transport satellites (MTSAT) and applied to the Phimai radar station, Thailand. The proposed method can classify cloud types into convective rain, stratiform rain and non-rain for areas covered with cumulus and/or cirrus clouds by applying a statistical integration analysis of rain gauges, ground-based radar, and MTSAT data. The classified precipitation areas were used to estimate quantitative precipitation amounts over Phimai. To merge different rainfall data sets derived from these three sources, the bias among the data must be removed. A combined correction method was developed to estimate the spatially varying multiplicative biases in hourly rainfall obtained from the radar and MTSAT using the rain gauges. This consecutive analysis was applied to the rainy season (July to September) in 2009 to obtain the multiplicative bias correction and to combine the data sets. The correlation coefficient, root mean square error, and mean bias were used as indicators to evaluate the performance of our bias-correction method. The combined method is simple and useful. The combined rainfall data were more useful than the data of TRMM 3B42 V7 and ground-based radar estimates.(Citation: Wetchayont, P., T. Hayasaka, T. Satomura, S. Katagiri, and S. Baimoung, 2013: Retrieval of rainfall by combining rain gauge, ground-based radar and satellite measurements over Phimai, Thailand. SOLA, 9, 166−169,