Convective uplift of pollution from the Sichuan Basin into the Asian monsoon anticyclone during the StratoClim aircraft campaign

Abstract

Abstract. The StratoClim airborne campaign took place in Nepal from 27 July to 10 August 2017 to document the physical and chemical properties of the South Asian upper troposphere–lower stratosphere (UTLS) during the Asian summer monsoon (ASM). In the present paper, simulations with the Meso-NH cloud-chemistry model at a horizontal resolution of 15 km are performed over the Asian region to characterize the impact of monsoon deep convection on the composition of Asian monsoon anticyclone (AMA) and on the formation of the Asian tropopause aerosol layer (ATAL) during the StratoClim campaign. StratoClim took place during a break phase of the monsoon with intense convective activity over South China and Sichuan. Comparisons between brightness temperatures (BTs) at 10.8 µm observed by satellite sensors and simulated by Meso-NH highlight the ability of the model to correctly reproduce the life cycle of deep convective clouds. A comparison between CO and O3 concentrations from Meso-NH and airborne observations (StratoClim and IAGOS (In-service Aircraft for a Global Observing System)) demonstrates that the model captures most of the observed variabilities. Nevertheless, for both gases, the model tends to overestimate the concentrations and misses some thin CO plumes related to local convective events probably because the resolution is too coarse, but the convective uplift of pollution is very well captured by the model. We have therefore focused on the impact of Sichuan convection on the AMA composition. A dedicated sensitivity simulation showed that the 7 August convective event brought large amounts of CO deep into the AMA and even across the 380 K isentropic level located at 17.8 km. This Sichuan contribution enhanced the CO concentration by ∼15 % to reach more than 180 ppbv over a large area around 15 km height. It is noteworthy that Meso-NH captures the impact of the diluted Sichuan plume on the CO concentration during a StratoClim flight south of Kathmandu, highlighting its ability to reproduce the transport pathway of Sichuan pollution. According to the model, primary organic aerosol and black carbon particles originating from Sichuan are transported following the same pathway as CO. The large particles are heavily scavenged within the precipitating part of the convective clouds but remain the most important contributor to the particle mass in the AMA. Over the whole AMA region, the 7 August convective event resulted in a 0.5 % increase in CO concentration over the 10–20 km range that lasted about 2 d. The impact of pollution uplift from three regions (India, China, and Sichuan) averaged over the first 10 d of August has also been evaluated with sensitivity simulations. Even during this monsoon break phase, the results confirm the predominant role of India relative to China with respective contributions of 11 % and 7 % to CO concentration in the 10–15 km layer. Moreover, during this period a large part (35 %) of the Chinese contribution comes from the Sichuan Basin alone.

A Multimodel Investigation of Asian Summer Monsoon UTLS Transport Over the Western Pacific

“…transport (e.g., Bergman et al, 2013;Lau et al, 2018;Lee et al, 2021;Vogel et al, 2015). The "mushroom" structure clearly supports the previous analysis that, although enhanced UT CO is found to span a large longitudinal range from the Mediterranean to the Western Pacific, the BL source region is dominated by the 70°-120°E segment along the monsoon trough.…”

“…The results from the set of state-of-the-art models also provide new perspectives on the seasonal-average structure of the ASM transport, chemical perturbations to the global lower stratosphere on seasonal time scales, and the leading ASM convective transport source regions. This new information complements the increasing number of ASM transport modeling studies (e.g., Bossolasco et al, 2021;Gottschaldt et al, 2017;Lee et al, 2021;Pan et al, 2016;Park et al, 2009;Ploeger et al, 2017;Vogel et al, 2014Vogel et al, , 2015Vogel et al, , 2019Yu et al, 2017) and provides additional insight into data analysis from recent airborne studies (e.g., Adcock et al, 2021;Bucci et al, 2020;Höpfner et al, 2019;von Hobe et al, 2021) and the analyses of satellite observations in the last decade (e.g., Park et al, 2007Park et al, , 2008Randel et al, 2010;Santee et al, 2017).…”

“…This “two‐stem” structure is also shown in previous work using MERRA reanalysis data (Lau et al., 2018), where the two “stems” are associated with the persistent convective transport over northern India and the southern edge of the Tibetan Plateau, and over southwestern China including the Sichuan Basin, respectively. These locations along the monsoon trough (Figure 2) are known as “hot spots” of deep convective transport (e.g., Bergman et al., 2013; Lau et al., 2018; Lee et al., 2021; Vogel et al., 2015). The “mushroom” structure clearly supports the previous analysis that, although enhanced UT CO is found to span a large longitudinal range from the Mediterranean to the Western Pacific, the BL source region is dominated by the 70°–120°E segment along the monsoon trough.…”

Aerosol Effects on Clear‐Sky Shortwave Heating in the Asian Monsoon Tropopause Layer

“…This peak could indicate persistent southeastward transport from the Sichuan Basin; however, the mean upper‐level flow from the Sichuan Basin is southwestward (see also K.‐O. Lee et al., 2021). We note further that W. K. M. Lau et al.…”

Distribution of cross-tropopause convection within the Asian monsoon region from May through October 2017

“…The research produced by the StratoClim project, which examined deep convection within the Asian monsoon region during the same time period as the present study (2017), provides important context. Aircraft in situ measurements of water vapor (Khaykin et al, 2022;Lee et al, 2019), CO Lee et al, 2021;, ozone (Johansson et al, 2020;, and N 2 O (von Hobe et al, 2021), as well as HNO3, peroxyacetyl nitrate, ethylene, and formic acid (Johansson et al, 2020), show clear evidence of convective influence on the UTLS comprising up to 100 % of the air . While some studies indicate that transport of convectively influenced air across the tropopause primarily occurs via slow ascent , others demonstrate that cross-tropopause convection directly impacts the LS (Khaykin et al, 2022;Lee et al, 2019Lee et al, , 2021.…”