The Effect of Grid Spacing and Domain Size on the Quality of Ensemble Regional Climate Downscaling over South Asia during the Northeasterly Monsoon

Abstract

The performance of an ensemble-based dynamical regional climate downscaling system is evaluated over southern Asia in a northeasterly monsoon season for different choices in grid spacing and domain size. A seven-member ensemble of the ECHAM4.5 global climate model at a resolution of about 300-km grid size was used to drive the RegCM3 regional climate model with grid sizes of 100, 50, 25, and 20 km, respectively. The performance is reported in detail over Sri Lanka. Two sets of regional model runs were undertaken to assess the effect of grid spacing and model domain size on the downscaling performance. The RegCM3 simulation with 100-km grid size significantly underestimates the height of the central mountain range in Sri Lanka, in a manner that is too coarse to capture orographic influences on the rainfall. However, the RegCM3 simulations with grid sizes from 20 to 50 km capture mesoscale features that arise from uplift condensation on the windward side of the monsoon winds due to the topography. These simulations also capture the orographic influences on the month-to-month rainfall over Sri Lanka that were absent in the ECHAM4.5. While the ''small domain'' runs [where only the forcings for the region immediately around Sri Lanka (48-118N, 768-858E) are used] are computationally more efficient, the results are overly controlled by the lateral boundary driving of the ECHAM4.5 so they inherit large uncertainty from the seven ECHAM4.5 realizations used for the RegCM3 ensemble runs. The ''large domain'' simulation used a domain comprising both land and ocean (approximately 48S-228N, 658-968E). The large-domain group of simulations produced reasonable spatial distribution of precipitation over the region. Moreover, the ensemble spread was considerably reduced in the large-domain high-resolution runs. Therefore, fine enough grid resolution (25 km or less) and sufficiently large domain size are both needed to simulate the essential features of precipitation in this tropical and monsoonal region.

Development of New Ensemble Methods Based on the Performance Skills of Regional Climate Models over South Korea

“…The RCM simulation with 50-km grid size also significantly underestimates the height of the topography in South Korea; the resolution is too coarse to capture orographic influences on the rainfall. The impacts of grid and domain sizes on the RCM's rainfall simulation have been well documented in many works (e.g., Leduc and Laprise 2009;Qian and Zubair 2010). Figure 3 shows the 20-yr biases of monthly-mean temperature and daily precipitation simulated by the eight ensemble members for four selected months over South Korea.…”

A review on regional dynamical downscaling in intraseasonal to seasonal simulation/prediction and major factors that affect downscaling ability

“…In general, the high resolution RCM produces better results than the GCM-produced LBCs (e.g., Qian and Zubair, 2010;De Sales and Xue, 2011;Caron et al, 2011). However, such comparison may be misleading because the improvement could be due to the better dynamic or physical treatments in the RCMs.…”

Evaluating global climate models for the Pacific island region

“…This characteristic has been very difficult to correctly simulate; thus, many models underestimate factors such as the interannual variability in monsoon precipitation (Kim et al 2008). There is evidence that some of this deficiency may originate from the relatively low resolution and therefore deficient topography of the CMIP3 models (Qian & Zubair 2010).…”