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The Indian summer monsoon rainfall shows intraseasonal oscillations manifested as active and break phases, and depending on the phase, the water vapor concentration in the tropical tropopause layer (TTL) varies. Using dedicated Lagrangian backward trajectory analysis and circulation dynamics within the Asian monsoon anticyclone, this study investigates in detail the factors governing water vapor in the TTL during the active and break phases. Backward trajectory simulations have been performed using wind field and heating rate from ERA5 re-analysis data and the convective sources in the TTL are estimated from high resolution satellite measurements. A water vapor maximum in the TTL is found from Iran to the Tibetan Plateau during the active phase, whereas it is concentrated mostly over the Tibetan Plateau during the break phase. This study shows that the differences in the water vapor concentration in the TTL between these phases are primarily caused by the variability in the strength of Asian monsoon anticyclone and convective sources. At TTL, the influence of convective sources from the ocean on water vapor is small and continental regions dominate, mainly from the Indian subcontinent and the Tibetan plateau. The time taken (age) for the vertical transport of air parcels from major convective sources to the 380 K isentropic level, that is close to the thermal tropopause, is estimated to be around 3 weeks. [ABSTRACT FROM AUTHOR]

The Asian monsoon anticyclone (AMA) represents one of the wettest regions in the lower stratosphere (LS) and is a key contributor to the global annual maximum in LS water vapour. While the AMA wet pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, there remain ambiguities regarding the role of tropopause-overshooting convection in maintaining the regional LS water vapour maximum. This study tackles this issue using a unique set of observations from aboard the high-altitude M55-Geophysica aircraft deployed in Nepal in summer 2017 within the EU StratoClim project. We use a combination of airborne measurements (water vapour, ice water, water isotopes, cloud backscatter) together with ensemble trajectory modelling coupled with satellite observations to characterize the processes controlling water vapour and clouds in the confined lower stratosphere (CLS) of the AMA. Our analysis puts in evidence the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures in the AMA. We show that all of the observed CLS water vapour enhancements are traceable to convective events within the AMA and furthermore bear an isotopic signature of the overshooting process. A surprising result is that the plumes of moist air with mixing ratios nearly twice the background level can persist for weeks whilst recirculating within the anticyclone, without being subject to irreversible dehydration through ice settling. Our findings highlight the importance of convection and recirculation within the AMA for the transport of water into the stratosphere. [ABSTRACT FROM AUTHOR]

During the monsoon season of the year 2017 the airborne StratoClim mission took place in Kathmandu, Nepal, with eight mission flights of the M-55 Geophysica in the upper troposphere–lower stratosphere (UTLS) of the Asian monsoon anticyclone (AMA) over northern India, Nepal, and Bangladesh. More than 100 events of new particle formation (NPF) were observed. In total, more than 2 h of flight time was spent under NPF conditions as indicated by the abundant presence of nucleation-mode aerosols, i.e. with particle diameters dp smaller than 15 nm , which were detected in situ by means of condensation nuclei counting techniques. Mixing ratios of nucleation-mode particles (nnm) of up to ∼ 50 000 mg-1 were measured at heights of 15–16 km (θ ≈ 370 K). NPF was most frequently observed at ∼ 12–16 km altitude (θ ≈ 355–380 K) and mainly below the tropopause. Resulting nnm remained elevated (∼ 300–2000 mg-1) up to altitudes of ∼ 17.5 km (θ ≈ 400 K), while under NPF conditions the fraction (f) of sub-micrometre-sized non-volatile residues (dp > 10 nm) remained below 50 %. At ∼ 12–14 km (θ ≈ 355–365 K) the minimum of f (< 15 %) was found, and underneath, the median f generally remains below 25 %. The persistence of particles at nucleation-mode sizes is limited to a few hours, mainly due to coagulation, as demonstrated by a numerical simulation. The frequency of NPF events observed during StratoClim 2017 underlines the importance of the AMA as a source region for UTLS aerosols and for the formation and maintenance of the Asian tropopause aerosol layer (ATAL). The observed abundance of NPF-produced nucleation-mode particles within the AMA is not unambiguously attributable to (a) specific source regions in the boundary layer (according to backward trajectory analyses), or (b) the direct supply with precursor material by convective updraught (from correlations of NPF with carbon monoxide), or (c) the recent release of NPF-capable material from the convective outflow (according to air mass transport times in the tropical tropopause layer, TTL). Temperature anomalies with ΔT of 2 K (peak-to-peak amplitude), as observed at a horizontal wavelength of ∼ 70–100 km during a level flight of several hours, match with NPF detections and represent an additional mechanism for local increases in supersaturation of the NPF precursors. Effective precursor supply and widely distributed temperature anomalies within the AMA can explain the higher frequency of intense NPF observed during StratoClim 2017 than all previous NPF detections with COPAS (COndensation PArticle counting System) at TTL levels over Brazil, northern Australia, or West Africa. [ABSTRACT FROM AUTHOR]

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 µ m 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of - 0.05 W m -2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources. [ABSTRACT FROM AUTHOR]

The StratoClim aircraft field campaign took place from Kathmandu, Nepal, in thesummer 2017 in order to better clarify the atmospheric composition, chemistry, anddynamics in the Asian Summer Monsoon Anticyclone (ASMA) which is known todistribute surface emissions to the mid-latitude lower stratosphere and the stratosphericoverworld.We here report on in-situ measurements of gaseous HNO3 and HCN in the UpperTroposphere and Lower Stratosphere (UTLS) employing the Chemical-IonizationTime-of-Flight Mass Spectrometer FUNMASS (CI-ToF-MS) on board the high-altituderesearch aircraft M55-Geophysica. While HCN is primarily emitted from biomassburning, HNO3 is mainly a product of NOx oxidation. The time series of thesespecies from flights F6 and F7 on 6th and 8th of August 2017 show interestingsignatures of convective activity and lightning NOx production at the tropopauselevel (F6) as well as vertical profiles under much more quiet conditions (F7). Herewe present comparisons of the in-situ data with two sets of backward trajectorycalculations by the lagrangian models TRACZILLA and CLaMS. The influence ofconvective sources and other physical-chemical processes on the observed time series isdiscussed. [ABSTRACT FROM AUTHOR]

We show that a fire plume originating at high northern latitudes during the Canadian wild fireevent in July/August 2017 reached the tropics, and subsequently the stratosphere via theascending branch of the Brewer-Dobson-Circulation (BDC). For this, we use a combinationof aerosol extinction data from the Stratospheric Aerosol and Gas Experiment III (SAGEIII)and the Ozone Mapping Profiler Suite (OMPS), carbon dioxide measurements from theInfrared Atmospheric Sounding Interferometer (IASI), FLEXPART-TRACZILLAback-trajectories and information for the position and strength of the Asian MonsoonAnticyclone (AMA) transport barrier from the Chemical Lagrangian Model of theStratosphere (CLaMS). The transport from high to low latitudes in the upper troposphere andlowermost stratosphere was mediated by the anticyclonic flow of the Asian monsooncirculation. The Canadian fire plume reached the Asian monsoon area in late August/earlySeptember, when the AMA was still in place. While there is no evidence of systematicmixing into the center of the AMA, we show that a substantial part of the Canadian fire plumeis entrained into the circulation at the AMA edge, and is transported into the tropicalUTLS, and possibly the Southern Hemisphere particularly following the North-Southflow on the eastern side. In the tropics the fire plume is lifted by about 1.5 km permonth. Inside the AMA we find evidence of the Asian Tropopause Aerosol Layer (ATAL) inAugust, doubling background aerosol conditions with a top of the atmosphere shortwaveradiative forcing of -0.05 W/m2. This is estimated using the UVSPEC radiative transfermodel and the LibRadtran package. The regional climate impact of the fire signal in the widerAsian monsoon area in September exceeds the impact of the ATAL by a factorof ∼3 (-0.13W/m2). Once in the stratosphere, the climate impact of such kind oftrans-continental transported plumes can be hemispheric and long-lasting, pointing atthe importance of this long-range dynamical interconnection of pollution sources. [ABSTRACT FROM AUTHOR]