Weather Research and Forecasting Model with Chemistry (WRF-CHEM) over South Asia

Abstract

The Weather Research and Forecasting model with Chemistry (WRF-Chem) has been set-up and used for the first time to simulate the meteorology and air quality of South Asia during the year 2008. The model performance is thoroughly evaluated by comparing model results with ground-, balloon- and satellite-based observations of meteorological and chemical fields. The spatial and temporal variability of meteorological parameters is reproduced well by the model with index of agreement greater than 0.6. The mean bias (MB) and root mean square error (RMSE) values are within ±2 K and 1-4 K for temperature, 30% and 20-65% for water vapor, 1.6 ms-1 and 5.1 ms-1 for wind components and within 25 hPa for tropopause pressure. The model overestimates precipitation in summer and underestimates during other seasons. The modeled meteorology is found to be of sufficient quality for use in chemistry simulations with meteorological biases affecting chemistry simulations within ± (10-25%). The model shows good ability in simulating ozone and CO variability but shows differences for NOx. The model underestimates TES ozone, OMI tropospheric column NO2 and MOPITT total column CO retrievals during all the months except MOPITT retrievals during August- January and OMI retrievals during winter. The evaluation results indicate large uncertainties in anthropogenic and biomass burning emission estimates, especially for NOx. The model results indicate clear regional differences in surface ozone seasonality over South Asia with estimated daytime (1130-1530 hours) net ozone production of 0-5 ppbv hr-1 over inland regions and 0-2 ppbv hr-1 over marine regions during outflow periods. The model results indicate that ozone production in this region is mostly NOx-limited. Eleven CO tracers are included in the model to study wintertime CO budget over South Asia. CO mixing ratios at the surface (318±290 ppbv) and in the planetary boundary layer (277±207 ppbV) are mainly due to pollution inflow (surface: 60±30%; PBL: 63±29%) and regional anthropogenic emissions (surface: 34±27%, PBL: 30±25%), while those in free troposphere (124±27 ppbV) are mainly due to pollution inflow (89±13%). Biogenic, biomass burning and photochemical sources contribute less than 10%. Regional emissions are generally constrained within lowest 3 km of the atmosphere but biomass burning emissions reach up to altitudes as high as 5 km over Burma. Intra-regional transport is estimated to enhance surface CO by 20-100 ppbV. Anthropogenic surface CO over the Arabian Sea mainly comes from northern (36%), western (27%) and southern India (22%), while eastern (62%) and northern India (14%) are the main contributors to that over the Bay of Bengal. This study shows that WRF-Chem model captures many important features of the observations over South Asia and gives confidence to using the model for future studies in this region.