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

Abstract

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.

Impacts of atmospheric transport and biomass burning on the inter-annual variation in black carbon aerosols over the Tibetan Plateau

“…Due to weak anthropogenic activities, the contribution of local emissions to atmospheric BC in the TP is low (Zhang et al, 2015). Concentrations of atmospheric BC in the TP are greatly influenced by the long-range transport of BC from non-local regions (Kopacz et al, 2011;Lu et al, 2012;Kang et al, 2019).…”

“…South Asia and East Asia are suggested to be two main source regions of atmospheric BC in the TP (Lu et al, 2012). Zhang et al (2015) estimated that, in 2001, the local contribution to BC column burden in the TP was only around 10 %, while the contributions from South Asia and East Asia are respectively about 50 % and 20 %. Some studies also investigated the pathways of BC transport to the TP and derived their characteristics (Cao et al, 2011).…”

“…Some studies also investigated the pathways of BC transport to the TP and derived their characteristics (Cao et al, 2011). The Asian summer monsoon system was identified as an important influencing factor for transport of atmospheric species from South Asia to the TP (Chen et al, 2013;Han et al, 2014;Xu et al, 2014;Zhang et al, 2015). In summer, BC from northern India can be transported to the middle and upper troposphere and then cross the Himalayas to the TP via south-westerly winds (Yang et al, 2018).…”

“…In summer, BC from northern India can be transported to the middle and upper troposphere and then cross the Himalayas to the TP via south-westerly winds (Yang et al, 2018). BC emitted in East Asia can be uplifted to upper layers by the summer monsoon circulation and then transported to the northeastern TP (Zhang et al, 2015). The mid-latitude westerlies are favourable to BC transport from central Asia and northern India to the western TP (Chen et al, 2018) but unfavourable to BC transport from eastern China to the TP (Cao et al, 2011).…”

“…Previous observations and simulations showed that biomass burning is a major source of atmospheric BC in the TP (Lu et al, 2012;Zhang et al, 2015). Zhang et al (2015) estimated that biomass burning together with biofuel emissions can contribute to around half of the annual mean BC column burden over the TP.…”