The Comparison of Royal Rainmaking Beneficial Area Evaluation Methods Effectiveness in Thailand

In 2006 – 2016, Royal Rainmaking’s beneficial area was evaluated using Fix Target Area method (Fix). However, with weather and land use continually changed, Fix method may not be longer performed well with present operation. Consequently, Radar Composite and Dynamic Target Area methods (Dyn) have been developed and implemented since 2017.

Z-R relationship obtained from Department of Royal Rainmaking and Agricultural Aviation (DRRAA)’s research or other institutes’ research were used for radar rainfall estimation of Royal Rainmaking Radar stations. If no research conducted at that particular radar station, Z-R relationship of convective rain was automatically used. Accumulated Radar Rainfall acquired from above mentioned Z-R was statistically constructed as CountMax, AvgMax and AvgAvg composite radar data.

Single ratio between rain gauge rainfall amount (G) from automatic rain gauge stations of Hydro and Agro Informatics Institute (HAII) and radar estimated rainfall (R ) in DRRAA’s Sattahip and Pimai radar, single and composite radar measurement data were examined to identify the most appropriated statistical data sets for evaluation system. Inner and outer composite radar area’s rainfall amount were underestimated when compare to rainfall derived from rain gauges in both wet and dry season. Radar rainfall calculated from DRRAA’s research Z-R relationship was closer to rain gauges rainfall than Convective Rain Z-R relationship rainfall. Furthermore, statistical composite radar data, AvgMax can be well represent rainfall rain gauges data more than CountMax and AvgAvg data. Hence, AvgMax data will be utilized as one of variables for Royal Rainmaking’s beneficial area evaluation.

Rain Area and Rain Volume were calculated from Royal Rainmaking Huahin Center‘s daily operation over Kaeng Krachan and Pranburi Basin data during wet and dry season in 2018. They were employed to assess Fix, Dyn and Storm Tracking (ST) system performance. Fix and Dyn’s Accumulative Rain Area and Volume were higher than ST’s for all season. However, Dyn evaluation results were closer to ST’s than Fix’s. In addition, Rain Area ratio of Dyn/ST was 3.3, Fix/ST was 7.0, Rain Volume ratio of Dyn/ST was 1.7 and Fix/ST was 3.6 for all season.

Based on the current Royal Rainmaking daily operation which focuses on the designated targets, Royal Rainmaking’s beneficial area evaluation using Automatic Dynamic Target Area (Dyn) is the most suitable system owing to the evaluation data which is proximate to natural cloud properties.