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. Most previous modeling studies about black carbon (BC) transport and itsimpact over the Tibetan Plateau (TP) conducted simulations with horizontalresolutions coarser than 20 km that may not be able to resolve thecomplex topography of the Himalayas well. In this study, the two experimentscovering all of the Himalayas with the Weather Research and Forecasting modelcoupled with Chemistry (WRF-Chem) at the horizontal resolution of 4 km butwith two different topography datasets (4 km complex topography and 20 kmsmooth topography) are conducted for pre-monsoon season (April 2016) toinvestigate the impacts of topography on modeling the transport anddistribution of BC over the TP. Both experiments show the evident accumulationof aerosols near the southern Himalayas during the pre-monsoon season,consistent with the satellite retrievals. The observed episode of highsurface BC concentration at the station near Mt. Everest due to heavybiomass burning near the southern Himalayas is well captured by thesimulations. The simulations indicate that the prevailing upflow across theHimalayas driven by the large-scale westerly and small-scale southerlycirculations during the daytime is the dominant transport mechanism of southernAsian BC into the TP, and it is much stronger than that during the nighttime.The simulation with the 4 km topography resolves more valleys and mountainridges and shows that the BC transport across the Himalayas can overcome themajority of mountain ridges, but the valley transport is more efficient. Thecomplex topography results in stronger overall cross-Himalayan transportduring the simulation period primarily due to the strengthened efficiency ofnear-surface meridional transport towards the TP, enhanced wind speed atsome valleys and deeper valley channels associated with larger transportedBC mass volume. This results in 50 % higher transport flux of BC acrossthe Himalayas and 30 %–50 % stronger BC radiative heating in the atmosphereup to 10 km over the TP from the simulation with the 4 km complex topographythan that with the 20 km smoother topography. The different topography alsoleads to different distributions of snow cover and BC forcing in snow. Thisstudy implies that the relatively smooth topography used by the models withresolutions coarser than 20 km may introduce significant negative biases inestimating light-absorbing aerosol radiative forcing over the TP during thepre-monsoon season. Highlights. The black carbon (BC) transport across the Himalayas can overcome themajority of mountain ridges, but the valley transport is much more efficientduring the pre-monsoon season. The complex topography results in stronger overall cross-Himalayantransport during the study period primarily due to the strengthenedefficiency of near-surface meridional transport towards the TP, enhancedwind speed at some valleys and deeper valley channels associated withlarger transported BC mass volume. The complex topography generates 50 % higher transport flux of BCacross the Himalayas and 30 %–50 % stronger BC radiative heating in theatmosphere up to 10 km over the Tibetan Plateau (TP) than thesmoother topography, which implies that the smooth topography used by themodels with relatively coarse resolution may introduce significant negativebiases in estimating BC radiative forcing over the TP during the pre-monsoonseason. The different topography also leads to different distributions of snowcover and BC forcing in snow over the TP.

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