Satellite observations have revealed an enhanced aerosol layer near the tropopause over Asia during the summer monsoon, called the Asian Tropopause Aerosol Layer (ATAL). The chemical composition of the ATAL is investigated here using offline ionic analysis of aerosols collected with a balloon-borne impactor near the tropopause region over India onboard extended duration balloon flights in the summer of 2017 and winter 2018. We found NO₃^- and NO₂^- dominant among other ions with values ranging between 87-343 ng/m³ during the summer campaign. In contrast, SO₄ levels were found above detection limit (>10 ng/m³) only in winter. In addition, we determined the origin of the air masses sampled during the flights through back trajectory analysis combined with convection. The results obtained therein were put into a context of large-scale transport and aerosol distribution with GEOS-Chem chemical transport model simulations. The first flight of summer 2017 sampled air mass within the Asian monsoon anticyclone (AMA), associated with smaller particle size found on stage 2 (particle size cut off > 0.15 microns) of the impactor, while the second flight sampled air mass at the edge of the AMA associated with larger particle size on stage 1 (particle size cut off between 2 and 0.5 microns). The sampled air masses in winter 2018 were affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months prior to our campaign. Concentrations of SO₄^2-, NH4⁺, and Ca²⁺ were enhanced. Overall, our results suggest that nitrogen- containing particles represent a large fraction of aerosols populating the ATAL in agreement with aircraft measurements during the StratoClim campaign. Furthermore, GEOS-chem model simulations suggest that lightning NOx emissions had a minimal impact on the production of nitrate aerosols sampled during the two flights. 

How to cite: Vernier, H., Rastogi, N., Liu, H., Fairlie, D., Pandit, A., Bedka, K. M., Patel, A., Ratnam, M. V., Kumar, B. S., Gadhavi, H., Weinhold, F. G., Berthet, G., and Vernier, J.-P.: Exploring the ionic composition of the Asian Tropopause Aerosol Layer using medium duration balloon flights, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2323, https://doi.org/10.5194/egusphere-egu21-2323, 2021.

The Asian Tropopause Aerosol Layer (ATAL) has emerged over recent decades to play an increasingly prominent role in the upper troposphere and lower stratosphere above the Asian monsoon region. Although the effects of the ATAL on the surface and top-of-atmosphere radiation budget have been examined by several studies, the processes and effects by which the ATAL alters radiative transfer within the tropopause layer have been much less discussed. We have used a conditional composite approach to investigate aerosol mixing ratios and their impacts on radiative heating rates in the Asian monsoon tropopause layer in MERRA-2. We have then subsampled in time based on known volcanic eruptions and the evolution of emission and data assimilation inputs to the MERRA-2 aerosol analysis to isolate the ATAL contribution and compare it to radiative heating signatures in the monsoon anticyclone region after volcanic eruptions. The results indicate that the ATAL impact on radiative heating rates in this region is on the order of 0.1 K/day, similar to that associated with ozone variability in MERRA-2 but weaker than cloud radiative effects at these altitudes. We have validated these results and tested their sensitivity to variations in the vertical structure and composition of ATAL aerosols using offline radiative transfer simulations. The idealized simulations produce similar but slightly stronger responses of radiative heating rates to the ATAL and are in good agreement with previous estimates of the top-of-atmosphere radiative forcing. Although the ATAL perturbations inferred from MERRA-2 are only about 10% of mean heating rates at these levels, their spatial distribution suggests potential implications for both isentropic and diabatic transport within the monsoon anticyclone, which should be examined in future work. Our results are limited by uncertainties in the composition and spatiotemporal variability of the ATAL, and reflect only the conditions in this layer as represented by MERRA-2. Targeted observations and model simulations are needed to adequately constrain the uncertainties, particularly with respect to the relative proportions and contributions of nitrate aerosols, which are not included in the MERRA-2 aerosol analysis.

How to cite: Gao, J. and Wright, J.: Aerosol influences on radiative heating rates in the Asian tropopause aerosol layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5145, https://doi.org/10.5194/egusphere-egu21-5145, 2021.

Deployment of the high-altitude M55-Geophysica aircraft in Kathmandu during Summer 2017 within StratoClim campaign has yielded a wealth of unique high-resolution measurements in the Asian Monsoon Anticyclone (AMA). In a particular flight (F8, 10 August 2017) the aircraft flew at the cold-point tropopause level through active overshoots and their outflows minutes to hours old. The measurements reveal up to 2500 ppmv of ice water above 17 km in large aggregated ice crystals up to 700 µm in diameter. Smaller crystals were observed as high as 18.8 km (410 K). Tracer and thermodynamical measurements show manifestations of vigorous vertical motions and provide evidence for ongoing mixing of tropospheric and stratospheric air around the tropopause. We use an ensemble of airborne and satellite measurements inside and downwind of convective overshoots together with trajectory modeling to characterize the impact of overshooting convection on the thermodynamical structure and chemical composition of the Asian tropopause layer. The effect of cross-tropopause convective transport on the Asian lower stratospheric water vapour is discussed.

How to cite: Khaykin, S., Krämer, M., Moyer, E., Bucci, S., Afchine, A., Borrmann, S., Cairo, F., Clouser, B., D’Amato, F., Legras, B., Lykov, A., Mitev, V., Matthey, R., Rolf, C., Singer, C., Ulanovsky, A., Viciani, S., Volk, M., Yushkov, V., and Stroh, F.: Evolution of tracer and ice crystal distribution in the young plumes of overshooting turrets from the StratoClim golden flight , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9756, https://doi.org/10.5194/egusphere-egu21-9756, 2021.

Motivated by previous measurements of very low tropospheric ozone concentrations in the Tropical West Pacific (TWP) and the implied low oxidizing capacity of this key region for transport into the stratosphere in boreal winter (e.g. Rex et al. 2014), we set up an atmospheric research station in Palau (7°N 134°E) as part of the StratoClim campaign. Our analysis of regular balloon-borne tropospheric ozone observations at Palau from 01/2016-12/2019 gives unprecedented insights into transport processes and air mass origin in the TWP. We confirm the year-round dominance of a low ozone background in the mid-troposphere. Layers of enhanced ozone are often anti-correlated with water vapor and occur frequently. Moreover, the occurrence of respective layers shows a strong seasonality. Dry and ozone-rich air masses between 5 and 10 km altitude were observed in 71 % of the profiles from February until April compared to 25 % from August until October. By defining monthly atmospheric background profiles for ozone and relative humidity based on observed statistics, we found that the deviations from this background reveal a bimodal distribution of RH anomalies. A previously proposed universal bimodal structure of free tropospheric ozone in the TWP could not be verified (Pan et al. 2015).

Back trajectory calculations (ATLAS) confirm that throughout the year the mid-tropospheric background is controlled by local convective processes and the origin of air masses is thus close to or East of Palau in the Pacific Ocean. Dry and ozone-rich air originates in tropical Asia and reaches Palau in anticyclonic conditions over an area stretching from India to the Philippines. This supports the controversial hypothesis of several studies which attribute ozone enhancement against the ozone-poor background to remote pollution events on the ground such as biomass burning (e.g. Andersen et al. 2016). A potential vorticity analysis revealed no stratospheric influence and we thus propose large-scale descent within the tropical troposphere as responsible for dehydration of air masses on their way to Palau.

How to cite: Müller, K., Wohltmann, I., von der Gathen, P., Lehmann, R., and Rex, M.: Origin of Tropospheric Air Masses in the Tropical West Pacific and related transport processes inferred from balloon-borne Ozone and Water Vapour observations from Palau, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10199, https://doi.org/10.5194/egusphere-egu21-10199, 2021.

The Asian monsoon anticyclone (AMA), which is primarily driven by the latent heat released by monsoon precipitation, is one of the dominant features of the Northern Hemisphere summer circulation in the upper troposphere and lower stratosphere. Due to variations in the diabatic heating, interactions with Rossby waves propagating along the subtropical jet, and internal dynamics within the anticyclone, the circulation of the AMA is unsteady. Here we use the ERA-Interim dataset and trajectories computed with ERA-Interim winds to show that the AMA contains two or three distinct synoptic-scale subvortices 69% of the time, while a single circulation center is present only 23% of the time. More than three simultaneous subvortices are uncommon. Observed behaviors of the subvortices include 1) splitting of a single vortex into two vortices; 2) merger of two vortices into a single vortex; 3) vortex shedding in the eastward direction; 4) vortex shedding in the westward direction; and 5) formation, movement, and dissipation of a vortex. The evolution of the subvortices is closely tied to stirring and transport.

How to cite: Siu, L. W. and Bowman, K.: Unsteady vortex behavior in the Asian monsoon anticyclone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10490, https://doi.org/10.5194/egusphere-egu21-10490, 2021.

Vertical distribution of aerosols and their composition in the lower troposphere is critically important for assessing the Earth’s radiation budget and their impact on monsoon circulation. We combine the extinction coefficient, particulate depolarization ratio obtained from CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) for period of 11 years (2008-2018) over the Indian region to provide an unprecedented climatological overview of the physical and optical characteristics of quasi-aerosol layers and their source and formation mechanism throughout its annual life cycle in the free troposphere. The key findings includes: i)The quasi aerosol layer over the Indian region are found to be persistent between 4-6 km during all seasons and occasionally reach above 6 km and exhibited strong seasonal and regional dependency, ii) Layer thickness varies between 2.0 -3.0 km corresponds to primary peak are more frequent of about 80-90 % of cases over all six regions and while  secondary layer occasionally forms (10-20 %), iii) The aerosol layer thickness increases by about 36.7 and 25% during summer and fall season compared to that of spring, and winter, iv) Layer-AOT showed year-to-year variations of up to a factor of two with a relative variability of about 15-23% (1σ), v) Trend in layer AOT is not very conspicuous and showed oscillatory pattern, vi) Depolarization ratios generally increase with height suggesting that the irregularity of aerosol shape increases with altitude, vii) The polluted dust and smoke are the major aerosol components of the observed quasi aerosol layer  between 4 to 6 km for spring and fall season while these are the polluted dust during winter and summer.

How to cite: Kumar, G., Vernier, J. P., Madhavan, B. L., Ansari, K., and Sinha, P. R.: Ubiquity of quasi-aerosol layers in the free troposphere over the Indian region: Results from multiyear satellite observations , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11003, https://doi.org/10.5194/egusphere-egu21-11003, 2021.

In-situ measurements of the HDO/H₂O isotopic ratio from the Chicago Water Isotope Spectrometer (ChiWIS) during the 2017 StratoClim campaign help diagnose convective processes in the Asian Monsoon. Isotopic measurements show enormous diversity in isotopic composition, likely reflecting degree of recent convective influence. Eight flights in July—August sampled a wide range of convective influence at near-tropopause altitudes, with timescales of minutes to weeks, and mean isotopic compositions from -700 per mil in recent convective outflow to -350 per mil in more aged air that is at least several days from last convective influence. Above the tropopause, we use isotopic composition to understand the fate of convective remnants. Isotopic measurements suggest much in-situ cirrus measured during  StratoClim campaign is actually secondary cirrus which has reformed in an area of prior convective moistening. These flights allow detailed comparison between North American and Asian monsoons, and we compare StratoClim results to both satellite and in-situ measurements in other monsoon and tropical locations. Finally, we discuss prospects for detection and interpretation of convective remnants during the in the 2021/2022 ACCLIP campaign.

How to cite: Clouser, B., Singer, C., Khaykin, S., Krämer, M., Lykov, A., Bucci, S., Legras, B., Borrmann, S., Cairo, F., Mitev, V., Matthey, R., Ravegnani, F., Rolf, C., Ulanovsky, A., Viciani, S., D'Amato, F., Volk, C. M., Yushkov, V., Stroh, F., and Moyer, E.: In-situ measurements of the HDO/H2O Isotopic ratio in the Asian Summer Monsoon trace strong convective activity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14171, https://doi.org/10.5194/egusphere-egu21-14171, 2021.

Predicting the Indian summer monsoon (ISM) is a challenging task due to the complexity of the climate system. Any improvement in the prediction skill of ISM in general circulation models would highly benefit the country as a whole due to its close linkage with the economy. In this study, we have adopted a new strategy to improve the ISM rainfall (ISMR) bias and prediction using the National Centers for Environmental Prediction-Climate Forecast System version 2 (NCEP-CFSv2). This model is currently used for the seasonal prediction in many countries including India but is known to have persistent dry bias over the Indian landmass. Three sets of hindcast experiments are carried out for 9 months each, for the period 2005-2019. The experiments differ from each other in the way they are initialized. Significant reduction in dry bias over the Indian landmass in the summer season with improved representation of tropical Indo Pacific sea surface temperature is reported from the new initialization  strategy. It is found that enhanced moisture transport to Indian landmass from the Arabian Sea,  improved representation of mean cyclonic circulation over north India, weak southeasterlies from Bay of Bengal and western Pacific together with enhanced Walker circulation contributed to the reduction  in dry bias over the Indian landmass. In addition to the above, the midlatitude circulation contribution by enhancing the strength of Subtropical High in the North Pacific resulted enhanced precipitation over the Indian landmass. The initialization strategy used here would be highly useful for improving the seasonal monsoon forecast.

How to cite: Thottuvilambil Shahulhameed, F., Chellappan, G., Halder, S., Kakatkar, R., Sriranga Chowdary, J., Patekar, D., and Parekh, A.: A new approach for seasonal prediction using the coupled model CFSv2 with special emphasis onIndian Summer Monsoon Rainfall, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14969, https://doi.org/10.5194/egusphere-egu21-14969, 2021.