Impact of topography on black carbon transport to the southern Tibetan Plateau during the pre-monsoon season and its climatic implication
Authors: Zhang Meixin , Zhao Chun , Cong Zhiyuan , Du Qiuyan , Xu Mingyue , Chen Yu , Chen Ming , Li Rui , Fu Yunfei , Zhong Lei , Kang Shichang , Zhao Delong , Yang Yan 
Published: May 2020

Abstract. Most previous modeling studies about black carbon (BC) transport and its
impact over the Tibetan Plateau (TP) conducted simulations with horizontal
resolutions coarser than 20 km that may not be able to resolve the
complex topography of the Himalayas well. In this study, the two experiments
covering all of the Himalayas with the Weather Research and Forecasting model
coupled with Chemistry (WRF-Chem) 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 topography on modeling the transport and
distribution of BC over the TP. Both experiments show the 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 station near Mt. Everest due to heavy
biomass burning near the southern Himalayas is well captured by the
simulations. The simulations indicate that the prevailing upflow across the
Himalayas driven by the large-scale westerly and small-scale southerly
circulations during the daytime is the dominant transport mechanism of southern
Asian 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 mountain
ridges and shows that the BC transport across the Himalayas can overcome the
majority of mountain ridges, but the valley transport is more efficient. The
complex topography results in stronger overall cross-Himalayan transport
during the simulation period primarily due to the strengthened efficiency of
near-surface meridional transport towards the TP, enhanced wind speed at
some valleys 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 the 4 km complex topography
than that with the 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
resolutions coarser than 20 km may introduce significant negative biases in
estimating light-absorbing aerosol radiative forcing over the TP during the
pre-monsoon season.
Highlights.
The black carbon (BC) transport across the Himalayas can overcome the
majority of mountain ridges, but the valley transport is much more efficient
during the pre-monsoon season. The complex topography results in stronger overall cross-Himalayan
transport during the study period primarily due to the strengthened
efficiency of near-surface meridional transport towards the TP, enhanced
wind speed at some valleys and deeper valley channels associated with
larger transported BC mass volume. The complex topography generates 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 Tibetan Plateau (TP) than the
smoother topography, which implies that the smooth topography used by the
models with relatively coarse resolution may introduce significant negative
biases in estimating BC radiative forcing over the TP during the pre-monsoon
season. The different topography also leads to different distributions of snow
cover and BC forcing in snow over the TP.