Impact Of Complex Topography On Black Carbon Transport To The Southern Tibetan Plateau During The Pre-monsoon Season

Most of previous modeling studies about black carbon(BC)transport and impact over the Tibetan Plateau(TP)conducted simulations with horizontal resolutions coarser than 20 km and corresponding smooth topography that may not be able to resolve well the complex topography of the Himalayas.In this study,the two experiments covering entire Himalayas with the Weather Research and Forecasting Model coupled with chemistry(Weather Research and Forecasting Model coupled with chemistry)at the horizontal resolution of 4 km but with two different topography datasets(4-km complex topography and 20-km smooth topography)are conducted for pre-monsoon season(April,2016)to investigate the impacts of complex topography on modeling the transport and distribution of BC over the TP.Both modelling experiments 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 concentration at the Mt.Everest station due to heavy biomass burning near the southern Himalayas is well captured by the simulations.The simulations indicate that the simulation with 4-km topography resolves more valleys and mountain ridges,and shows that the BC transport across the Himalayas can overcome a majority of mountain ridges but the valley transport is more efficient.The prevailing up-flow across the Himalayas driven by the large-scale westerly and small-scale southerly circulations caused by heating of solar radiation 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 complex topography results in stronger overall crossing-Himalayas transport during the simulation period primarily due to the strengthened efficiency of near-surface meridional transport towards the TP,stronger vertical mixing on slopes caused by increased topography complexity,and more small-scale and deeper valley channels associated with larger transported BC mass volume.This results in 50%higher transport flux of BC across the Himalayas and 30%-50%stronger BC radiative heating in the atmosphere up to 10 km over the TP from the simulation with 4-km complex topography than that with 20-km smoother topography.The different topography also leads to different distributions of snow cover and BC forcing in snow.This study implies that the relatively smooth topography used by the models with coarser resolutions than 20 km may introduce significant negative biases in estimating light absorbing aerosol radiative forcing over the TP during the pre-monsoon season.