Abstract
Abstract. The StratoClim stratospheric aircraft campaign took place in summer 2017 in Nepal (27 July–10 August) and provided for the first time a wide dataset of observations of air composition inside the Asian monsoon anticyclone (AMA). 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 trajectory 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 trajectory 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 ERA5) at 3 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 ERA5-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 ERA5 runs. We therefore adopt this setting to analyse the convective influence in the UTLS starting from the StratoClim observations. A large variety of transport conditions have been individuated during the eight 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, which, unexpectedly, was not particularly active during the period of the campaign. In addition, a mass of clean air injected from a typhoon has also been detected at around 18 km. 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 with a predominant South China origin. The analysis revealed a case of direct injection of highly polluted air close to the level of the tropopause (anomalies of around 80 ppbv injected at 16 km) that then kept rising inside the anticyclonic circulation. Due to the location of the campaign, air from continental India, in contrast, has been only observed to be linked to air masses that recirculated within the anticyclone for 10 to 20 d, resulting in a lower concentration of the trace gas. The analysis of a flight overpassing an intense convective system close to the southern Nepalese border revealed the injection of very young air (few hours of age) directly in the tropopause region (∼18 km), visible in the trace gases as an enhancement in CO and a depletion in the O3 one. From the whole campaign, a vertical stratification in the age of air is observed: up to 15 km, the age is less than 3 d, and these fresh air masses constitute almost the totality of the air composition. A transition layer is then individuated between 15 and 17 km, where the convective contribution is still dominant, and the ages vary between 1 and 2 weeks. Above this level, the mean age of the air sampled by the aircraft is estimated to be 20 d. There, the convective contribution rapidly decreases with height and finally becomes negligible around 20 km.
Highlights
The upper troposphere–lower stratosphere (UTLS) dynamics of the Northern Hemisphere (NH) during the summer season (June to August) is dominated by the summer Asian monsoon anticyclone (AMA) system (Randel and Park, 2006)
In the ERA-Interim version (Fig. 5f) the relative enhancement between outside and inside the plume is damped, and the timing is not consistent with the observations. This is visible in the kinematic computation, while the diabatic one looks closer to the COLD2 measurements
We provide a detailed discussion of the transport properties for two noteworthy cases of convective influence observed during the campaign: F6, which provides a clear case of deep-convective injection of fresh pollution in the upper troposphere, and F8, in which the aircraft first flew over an extended continental convective system, sampling air from both fresh and old convection, and later captured an older plume of clean oceanic air injected into the UTLS by a typhoon system
Summary
The upper troposphere–lower stratosphere (UTLS) dynamics of the Northern Hemisphere (NH) during the summer season (June to August) is dominated by the summer Asian monsoon anticyclone (AMA) system (Randel and Park, 2006). Tissier and Legras (2016), with a study based on backward and forward trajectories on ERA-Interim winds between the top of convective clouds to the tropopause, found that, for the boreal summers of years 2005–2008, the Asian mainland was representing the main source for the AMA composition (∼ 50 %), followed by the Tibetan Plateau and the North Asian Pacific Ocean to a similar extent (∼ 20 %) These studies mostly rely on simulation results, potentially depending on the choice of the driving model setting, while little in situ observational evidence has been collected over this region (mostly from balloon soundings; Bian et al, 2012). We present a statistical analysis, based on all of the campaign flights, of the vertical distribution of convective influence and age of convective air inside the AMA
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