Deep convective influence on the UTLS composition in the Asian Monsoon Anticyclone region: 2017 StratoClim campaign results

Abstract

Abstract. The StratoClim stratospheric aircraft campaign took place in summer 2017 in Nepal (the 27th of July–10th of August) and provided a wide dataset of observations of air composition inside the Asian Monsoon Anticyclone. In the framework of this project, with the purpose of modelling the injection of pollutants and natural compounds into the stratosphere, we performed a series of diffusive back-trajectories runs along the flights' tracks. The availability of in-situ measurements of trace gases has been exploited to evaluate the capability of the trajectory system to reproduce the transport in the Upper Troposphere–Lower Stratosphere (UTLS) region. The diagnostics of the convective sources and mixing in the air parcel samples have been derived by integrating the trajectories output with high-resolution observations of cloud tops from the Meteosat Second Generation (MSG1) and Himawari geostationary satellites. Back-trajectories have been calculated using meteorological fields from European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-Interim and ERA-5) at 3 h and 1 h resolution, using both kinematic and diabatic vertical motion. The comparison among the different trajectory runs shows, in general, a higher consistency with observed data, as well as a better agreement between the diabatic and kinematic version, when using ERA-5 based runs with respect to ERA-Interim. Overall, a better capacity in reproducing the pollution features is finally found in the diabatic version of the ERA-5 runs. Adopting this setting for the analysis, a large variety of transport conditions have been individuated during the 8 flights of the campaign. The larger influence by convective injections is found from the continental sources of China and India. Only a small contribution appears to be originated from maritime regions, in particular the South Pacific and the Bay of Bengal that, unexpectedly, was not particularly active during the period of the campaign. Thin filamentary structures of polluted air, characterized by peaks in CO, are observed, mostly associated with young convective air (age less than a few days) and a predominant South-China origin. Observed air from continental India, on the contrary, is often linked to a lower concentration of the trace gas and to air masses that recirculated within the anticyclone for 10 to 20 days. Vertical stratification in the age of air is observed: up to 15 km, the age is less than 3 days and these fresh air masses constitute almost the totality of the air composition. A transition layer is then individuated between 15 km and 17 km, where the convective contribution is still dominant and the ages vary between one and two weeks. Above this level, the mean age of the air sampled by the aircraft is estimated to be 20 days. There, the convective contribution rapidly decreases with height, and finally became negligible around 20 km.

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

“…For the four analysed flights, the local Indian contribution is generally larger than the Sichuan and Chinese remote contributions. This is in agreement with Bucci et al (2020) who quantified the convective contribution of different source regions to the CO concentration along the StratoClim flight tracks using a Lagrangian dispersion model. The Indian larger impact is especially clear at take-off and landing at altitudes where Chinese air masses are not efficiently transported by the UTLS easterlies.…”

“…It is noteworthy that O 3 concentrations measured around Kathmandu (about 22500 s, 19.3 km) are one third lower during flight #7 than during flight #5. This is probably linked to the transport of relatively fresh BL air masses from the Tibetan Plateau (Bucci et al, 2020). Observed-CO concentrations corroborate this hypothesis with lower CO during flight #5 ( Fig.…”

“…4a) took place over the Nepalese region and sampled the 16−20 km altitude range before the strong convective events in China. Therefore, as documented in Bucci et al (2020), there is no Chinese contribution during this flight. The local (Indian subcontinent) BL sources contribution since the start of the simulation (1 August 00:00 UTC) to the CO sampled up to 18 km is 10−15 ppbv (≥ 10 %).…”

“…During the whole flight at 17 km the Chinese contribution is larger than the Indian one, which remains weak or null. According to Bucci et al (2020), the two largest CO peaks at ~32500 and ~35500 s are resulting from Sichuan BL air masses convectively uplifted 2 days before. As mentioned in section 3.2, the Meso-NH model is not able to capture these 2 peaks but rather smooth CO enhancements.…”

“…Nevertheless, deep convection is characterised by an important inter-annual variability. The active phase of the StratoClim (Stratospheric and upper tropospheric processes for better Climate predictions, www.stratoclim.org) airborne campaign took place during a monsoon break phase with a convective activity particularly strong over the Sichuan Basin compared to Nepal and northern India (Lee et al, 2019;Bucci et al, 2020).…”