Simulation of atmospheric flow field over the complex terrain of Kaiga using WRF: sensitivity to model resolution and PBL physics

In this study, atmospheric flow field over the complex terrain of Kaiga is simulated in different seasons using the Weather Research and Forecasting (WRF) model. The sensitivity of simulation of the surface parameters to model horizontal and vertical resolution and Planetary Boundary Layer (PBL) physics was examined. The results indicate that the simulated surface parameters are more sensitive to horizontal resolution than vertical resolution. Simulations with higher horizontal resolution simulated fine scale features such as flow channeling in the valley portions, stronger winds in the valley mouth, blocking effect by hills in the narrow valley, flow divergence in the windward and flow convergence in the lee side. Among the different PBL schemes studied, second-order closure scheme MYNN3 showed the lowest bias as compared to first-order closure schemes YSU, ACM2 and 1.5 closure schemes MYJ and BouLac. However, the YSU scheme with the incorporation of subgrid scale drag parameterization produced the wind more realistically with substantial reductions in bias (10–40% at different observation locations). Although the model simulated the calm wind over the Kaiga region, it overestimated the wind relative to observations. The results suggest that the high occurrences of calm wind during the summer season are due to the blockage of the southwesterly wind by the surrounding hills which may results in high pollutant concentrations. In winter, northeasterly flow is noted most of the time with weak sea breeze circulation which would carry the pollutant to sea most of the time. Strong southwesterly flow in the monsoon season leads to the transportation of pollutant plumes to northeastern regions with rapid dispersion.