Simulation of the temporal and spatial characteristics of diurnal rainfall cycle over Borneo

Analysis of TRMM satellite data shows that the diurnal variation in rainfall over the islands is a striking feature of the Maritime Continent (MC). It has been noticed that, in the western part of this region, covering Borneo, the rainfall peaks occur over land during the midnight hours and in the leeward side of the mountains. Especially in the winter season, there exists a high variability in the peak time, magnitude, and propagation speed of the diurnal rainfall cycle. This variability is primarily associated to the synoptic-scale Borneo vortex, the northeast cold surge, and the intra-seasonal Madden–Julian oscillation (MJO).

The focus of this work is – (a) to improve our understanding about the atmospheric processes leading to diurnal rainfall variability over Borneo and surrounding ocean during the winter season, and (b) to investigate the time and space characteristics of the diurnal cycle of rainfall over Borneo under the influence of the atmospheric processes. We have done a systematic analysis of TRMM rainfall data for three years (2008 – 2010) to categorize the winter season days as representative of the aforementioned three main large-scale processes. Specifically, we classify the days into multiple categories based on the occurrence of various combinations of Borneo vortex, the northeast cold surge, and the intra-seasonal MJO. Note that there can be at most 8 combinations of these three large scale processes and the data analysis for the rainfall characteristic is done for all possible classifications. The WRF model has been configured with two nested domains to simulate the winter season of these three years. The outer domain covers MC at 30 km resolution and inner domain covers Borneo and surrounding ocean at 10 km resolution. These high resolution regional climate simulation uses time varying SST data for lower boundary condition and NCEP/NCAR reanalysis data for initial and nudged boundary conditions. A systematic comparison has been made between the simulation results and observation data.

Our analysis confirms that the presence of Borneo vortex enhances deep convection over southern South China Sea (SCS) and Borneo. We also observe that in certain scenarios, this deep convection is further strengthened by the presence of the northeast cold surge. Diurnal rainfall characteristics show a systematic departure from the normal pattern during the vortex and surge days. Depending on the MJO phase, the impact of surge and vortex is modulated, which eventually causes further deviations to the normal pattern of the diurnal cycle of rainfall over Borneo. Detailed analysis for three years 2008-2010 using the TRMM-3B42 precipitation data and NCEP-NCAR reanalysis wind data will be presented. These three years had varied strengths of surge, vortex and the MJO and hence capture an exhaustive view of the rainfall variability in this region. The WRF model gives satisfactory results in simulating the variations in the diurnal cycle, but the magnitude is generally overestimated and the variability in the propagation speed is not well captured. Comparative results between observed satellite data and the model output will be presented.