Impact of topography on black carbon transport to the southern Tibetan Plateau during pre-monsoon season and its climatic implication

Abstract. Most of previous modeling studies about black carbon (BC) transport and impact over the Tibetan Plateau conducted simulations with horizontal resolutions coarser than 10 km that may not be able to resolve well the complex topography of the Himalayas. In this study, the experiments with WRF-Chem at two horizontal resolutions (20 km and 4 km) are conducted for pre-monsoon season (April, 2016) to investigate the impacts of topography on modeling the transport and distribution of BC over the TP. The simulations at both resolutions show evident accumulation of aerosols near the southern Himalayas during the pre-monsoon season, consistent with the satellite retrievals. The observed episode of high surface BC concentrations at the station near the Mt. Everest due to heavy biomass burning near the TP is well captured by the simulations. The simulations at both resolutions indicate that the prevailing up-flow across the Himalayas driven by the large-scale circulation during the daytime is the dominant transport mechanism of South Asian BC into the TP, and is much stronger than that during the nighttime. The valley wind can strengthen the prevailing up-flow transport. The simulations at coarse resolution (20 km) and fine resolution (4 km) show large differences in representing the distributions of topography of the Himalayas. The simulation at 4 km resolution resolves more valleys and thus produces much stronger transport fluxes, which indicates that although the transport of South Asian BC across the Himalayas can overcome the mountain ridges, the valley transport is more efficient and cannot be ignored. This results in 50 % higher transport flux of BC across the Himalayas and 30–40 % stronger BC radiative heating in the atmosphere over the TP from the simulation at 4 km than that at 20 km resolution. The different topography also leads to different distributions of snow cover and BC forcing in snow. This study implies that global climate models generally with even coarser resolutions than 20 km may introduce significant negative biases in estimating light absorbing aerosol radiative forcing over the TP.