Downscaling of monsoon onset using pseudo global warming method

Climate change of monsoon onset was simulated by WRF regional model using pseudo global warming (PGW) method. The pseudo global warming method is a dynamical downscaling method that allows the projection of regional climate change using a regional climate model. In the method, initial and lateral boundary conditions are given by the sum of 6-hourly reanalysis data and the components of climate change, which are the monthly averaged differences between the present and the future climate.

However, in the case of that component of synoptic-scale systems change drastically in a shorter interval than month (for example, onset of monsoon), it is not suitable way to use the monthly averaged difference. In preset study, we focused on climate change of monsoon onset, and investigated about time intervals of climate difference that indicate effective components of climate change during the monsoon onset.

The reanalysis data of NCEP1 was used for the analysis. Distinguished climate difference in zonal wind at 850 hPa was appeared around Asian-monsoon region in a pair of decade between 1960-1960 and 1985-1994. The climate difference was calculated in 5, 10, and 14-daily. Moreover, T-test was performed to confirm a significance of horizontal distribution in the zonal wind difference. For downscaling by pseudo global warming method, the WRF ARW model ver. 3.3.1 was used.

The most significant difference in zonal wind during the monsoon onset was appeared in 10-daily difference. The difference is significant around the southern Indochina and the Bay of Bengal from middle of May to early June. On the other hand, in the 5-daily difference, the number of significant grids was lower, because the standard deviation of the 5-daily temporal variation is larger than 10-daily difference. These results have influence on the downscaled results by PGW method, especially, the transportation of water vapor by the zonal wind make fluctuation of precipitation during monsoon onset.