In the tropics, atmospheric composition in general and the temperature profile in particular are rather different above and below the tropopause e.g.,. Below the tropical tropopause the atmosphere is of tropospheric nature and is vertically strongly mixed, above the tropopause the air is more of stratospheric nature; in particular the air shows lower mixing ratios of water vapour and higher mixing ratios of ozone e.g.,.

The tropopause is classically defined by the atmospheric lapse rate (i.e., the location of a significant change in the atmospheric temperature lapse rate); the lapse rate tropopause is also referred to as the thermal tropopause. Overshoots of convection across the lapse rate tropopause occur e.g.,, in particular in the monsoon regions. This leads to transport of water vapour across the tropopause into the lowermost stratosphere. The transport of water vapour into the lowermost stratosphere and the abundance of water vapour in the lower stratosphere are important for tropospheric climate and dynamics; processes in this regard are often not well simulated in models . Lower stratospheric water vapour is a key radiative agent in the Earth's climate system; in particular, the regional Pacific moist bias in stratospheric models can be reduced through a Lagrangian transport scheme, which could be important for improving the simulation of regional circulation systems in the Asian monsoon region .

The tropopause as the separation between the troposphere and the stratosphere is a notion that has long been established e.g.,. However, the tropopause is still a topic of current research e.g.,. There are different ways to determine the location of the tropopause e.g.,. There are also variations over long time scales (decades) of tropopause height and temperature e.g.,.

The role of the ASMA in transport across the tropopause has been discussed since many years and relevant things have been discovered. Already early model studies predicted the role of the ASMA in the entry of water vapour into the stratosphere . and showed early evidence for the large-scale transport of moist air across the subtropical jet stream in association with upper level monsoon anticyclones. And exchange between the upper tropical troposphere and the lower stratosphere (and vice versa) was investigated in aircraft observations of water, ozone, wind, and temperature in the potential temperature range of ∼360 to 420 K potential temperature .

The tropical tropopause is observed see also Table  at potential temperature values greater than the maximum moist static surface energies about 355 K, indicating that adiabatic transport of potentially warm air from the midlatitude stratosphere to the tropics occurs . Here potential temperature θ is defined as 1θ=T⋅p0pκ with κ=R/cp, R is the gas constant for dry air, cp is the specific heat capacity at constant pressure, and p0 is a reference pressure (1000 hPa for the calculations reported here).

Often, the location of the tropopause is considered as a cold point (the point in the profile showing the lowest temperature) or as the lapse rate tropopause . These two different ways of locating the tropopause lead to different conclusions on the properties of air masses in the vicinity of the tropopause . Observations in the vicinity of the tropopause were also made earlier in flights between the surface and 18 km in late January 2004 from Costa Rica 10° N,. The cold point and the lapse rate tropopause are sometimes collocated, but often the tropical cold point is located substantially above the lapse rate tropopause . Both the cold point and the lapse rate tropopause were analysed based on aircraft measurements in January–March 2014 in the western Pacific ; finding that the air is drier on average at the cold point than at the lapse rate tropopause. Further, found a significant air mass discontinuity at the lapse rate tropopause. They conclude that there is no local barrier for transport across the lapse rate tropopause, but rather that this boundary reflects the with altitude diminishing role of convectively driven transport and mixing. This is important as the tropical west Pacific in winter is a key entry region for tropospheric air into the stratosphere. e.g.,.

Likewise, the ASMA is an important entry region to the stratosphere, which will be the focus here. We use measurements on board of a high flying research aircraft (Geophysica) in July and August 2017 (that is during the Asian summer monsoon peak season), during the StratoClim campaign based in Kathmandu/Nepal e.g.,. There were eight science flights out of Kathmandu during this period and the flights covered the altitude range from the ground to about 475 K. Trace gas, water vapour, and particle measurements during this campaign have been analysed before e.g.,. Also, the moistening of the lower stratosphere in the ASMA observed in a specific flight (8 August 2017) during the StratoClim campaign was investigated . Here we focus on tropopause characteristics in the ASMA region during the StratoClim campaign in Kathmandu in summer 2017.

The ASMA exists in Northern hemisphere summer in the upper troposphere and lower stratosphere e.g.,. The ASMA stretches over a large area, between the Eastern Mediterranean and Northern India and the Tibetan plateau e.g.,. In the Asian monsoon region and season, the lapse rate tropopause is typically located at greater altitudes (i.e. at lower pressures) than the tropopause outside the monsoon region by about 0.5 to 1 km or 20 K in potential temperature;. Further, there is a strong year-to-year variability in the altitude and the temperature of the lapse rate tropopause in the Asian monsoon region .

Upward transport in the Asian summer monsoon is dominated by convective activity. The fact that thunderstorms may inject substantial amounts of water vapour into the lowermost stratosphere was already found in early airborne measurements aircraft flights around a thunderstorm top near Amarillo, Texas, in 1972,. There are sources of tropospheric perturbations at lower altitudes (e.g., ozone, ozone depleting substances, water vapour, CO, and CO2) which are detectable in the ASMA at greater altitudes, above about 370 K potential temperature . When investigating the impact of convection on the stratospheric composition in the ASMA it is important to consider the influence of transport across the lapse rate tropopause .

The impact of convective activity in the North American and Asian Monsoon regions on stratospheric humidity has been investigated based on satellite observations ; it is concluded that stronger convection leads to a drier stratosphere, whereas weaker convection leads to a more humid stratosphere . Dehydration in the upper troposphere and lower stratosphere over the Asian summer monsoon caused by tropical cyclones has also been observed . Further, moistening of the global lower stratosphere through sublimating ice particles (a process showing a substantial regional variability) can be detected as an isotopic enhancement of the HDO/H2O ratio .

Moreover, the altitude of the tropopause and its characteristics are very different in the extra-tropics and in the tropics; in the tropics the transition from the troposphere to the stratosphere occurs in a layer, rather than at a sharply defined point referred to as “tropopause”. This fact has led to the concept of a tropical tropopause layer TTL,; with a bottom at ∼150 hPa, 355 K, 14 km (pressure, potential temperature, and altitude) and a top at ∼70 hPa, 425 K, 18.5 km . The existence of a TTL (but not using this term) was discussed earlier based on observations . There is further a longitudinal variation of the tropopause altitude, in particular associated with the ASMA and with El Niño-southern oscillation events .

In the upper troposphere and lower stratosphere, temperature (T) altitude (z), pressure (p) and potential temperature (θ) are related e.g.,. Pressure decreases with altitude and potential temperature increases with altitude see also Appendix . The relation in detail can be different above and below the tropical tropopause, where the tropical tropopause temperature is close to the minimum in temperature e.g.,. In any case, in discussions of Asian monsoon issues, altitude, pressure and potential temperature are all used frequently; therefore we provide here relations between these quantities based on in-situ measurements in the ASMA in 2017 (see Appendix ).

We focus here on airborne observations of water vapour, ozone, temperature, other meteorological parameters, and cloud occurrence (total water); all these quantities are accessible also through balloon-borne observations. Obviously, the measurements during StratoClim in 2017 provide a wealth of further information, which will be valuable for an extension of the present work.

Here we find that the lapse rate tropopause in the Asian summer monsoon anticyclone constitutes a good estimate for the upper boundary of the well mixed tropospheric air mass and that the lapse rate tropopause and the cold point tropopause are often not co-located; these two “tropopauses” are different. The cold point tropopause is in particular important for water vapour; in the ASMA, above the cold point tropopause, mixing ratios of water vapour are ≲10 ppm. For strong convection there is substantial dehydration at the cold point tropopause and no indication of particle occurrence or enhanced mixing ratios of water vapour (≳6 ppm) above the altitude of the cold point tropopause.

Here we focus on three specific flights during the StratoClim campaign in 2017 based in Kathmandu, Nepal e.g.,. We focus on measurements of temperature, ozone mixing ratios, gas-phase water vapour mixing ratios and water vapour in the particle phase. Flight F2 on 29 July 2017 was designed as an ATAL flight, no particles were observed. Flight F7 on 8 August, designed as an ASMA survey, showed few particles and some enhancement of gas-phase water vapour at greater altitudes. Flight F8, on 10 August 2017, was probing fresh convection.

Here, with a focus on the Asian Monsoon; we confirm earlier reports for the tropical west Pacific in winter that the lapse rate tropopause identifies the transition from tropospheric air masses to stratospheric air masses. We also find that in the Asian monsoon the air is generally drier at and above the cold point tropopause than below; again consistent with earlier reports for winter in the west Pacific and near tropical central America at 10° N . It should be noted that an early stratosphere-troposphere model already predicted the role of monsoonal circulation in air entry to the stratosphere . However, the air above the cold point tropopause in the Asian summer monsoon is more humid (mixing ratios of ∼3–10 ppm) than in the west Pacific in winter ∼3 ppm,.

Ozone mixing ratios in the ASMA, increase moderately with altitude in the upper troposphere (below the lapse rate tropopause) and increase substantially above the lapse rate tropopause e.g.,. Ozone mixing ratios between the cold point tropopause and the lapse rate tropopause range between 50 and 200 ppb, consistent with measurements in the west Pacific in winter .

In the Asian monsoon region, penetration of deep convective systems to above the lapse rate tropopause and also (but to a lesser extent) above the cold point tropopause occurs. If ice particles reach the lowermost stratosphere and sublimate there (before the particles sediment) this process may cause local hydration. Indeed, layers of enhanced gas-phase water vapour (below ≈10 ppm) above the lapse rate and the cold point tropopause were observed on 29 July and 8 August 2017 see also Figs. , , and . There are also observations of enhanced water vapour mixing ratios of ≈10 ppm in the lowermost stratosphere over North America during and before the American monsoon . However, such conditions of enhanced water vapour are not observed frequently.

On the flight on 8 August 2017 (Fig. ), the occurrence of ice particles was observed at ∼390 K (17 km) and an enhancement in gas-phase water vapour at ∼400–410 K (18–19 km) above the cold point tropopause in agreement with earlier reports . In moist, supersaturated air parcels, prevailing above the cold point tropopause, formation of ice particles is possible. However, it is not obvious that these ice clouds cause substantial dehydration because of a limited spatial and temporal extent of such air parcels and because of a limited size (causing a low sedimentation velocity) of the ice particles .

Overall, we observe a certain variability of gas-phase water vapour in the Asian summer monsoon region as both dehydration of air caused by freeze drying (water vapour mixing ratios below about 3 ppm) and hydration caused by convective overshoots (water vapour mixing ratios ≲10 ppm) of the cold point tropopause occurs consistent with the findings of.

The scientific flight on 10 August 2017 allowed the tropopause conditions of strong convection to be investigated. report that the cold point tropopause under such conditions is strongly tied to the convective cooling maximum. This is consistent with substantial ice particle occurrence around the cold point tropopause as observed during the flight on 10 August 2017 (Figs.  and ). The ice particles at the cold point exist in strongly supersaturated air (Fig. ) and will thus likely sediment. This would lead to dehydration at the cold point tropopause until a saturation of one is reached, again consistent with the conclusions of . The findings by that the cold point tropopause is strongly tied to convection suggests that dehydration processes at the cold point can be assumed as abrupt.

Our analysis for the flight on 10 August indicates that stratospheric water vapour mixing ratios in the Asian summer monsoon region are regulated by cold point temperatures, and that deep convection overshooting the lapse rate tropopause plays a relatively minor role in moistening the stratosphere supporting conclusions in earlier work based on remote sensing observations,. Further, the conclusion of very rapid transport from the ground up to the cold point on 10 August 2017 is also supported by the CO2 measurements during this flight: CO2 mixing ratios in the vicinity of the cold point (about 375 K, Fig. ) are of lower tropospheric character mixing ratios below about 400 ppm,their Fig. A3.

The lapse rate tropopause, in the ASMA, marks the top boundary of tropospheric air. Tropospheric air is vertically mixed, humid and relatively low in ozone; insofar, the lapse rate tropopause in the Asian summer monsoon constitutes an air mass boundary. Further, the lapse rate tropopause (in contrast to the cold point tropopause) constitutes a clear discontinuity in the profile of temperature against potential temperature (Fig. , bottom, and electronic supplement). Nonetheless, the demarcation between the troposphere and the stratosphere is different for different trace species, for example there is freeze out of water vapour at the lowest temperatures, chemical production of ozone in the stratosphere and limited inmixing of aged lower stratospheric air into tropospheric air ascending into the lowermost stratosphere. Vertical ascent (θ˙) in the ASMA through layers of constant potential temperature θ occurs according to the atmospheric heating rate (Eq. ) with no local (long-term) obstacle for vertical transport. The location of the lapse rate tropopause in the Asian summer monsoon is thus not determined by local processes.

Thus, the air masses below and above the lapse rate tropopause are very different. In other words, the lapse rate tropopause and the cold point tropopause are two different things (in spite of both being referred to as “tropopause”); they constitute boundaries of different air masses. Commonly, the cold point is located substantially (about 1 km) above the lapse rate tropopause. Above the cold point tropopause in the Asian Monsoon, the air is largely stratospheric; i.e., the air is dry and ozone mixing ratios increase with altitude.

Therefore, it is necessary to specify, whether convective systems are overshooting the lapse rate or the cold point tropopause. On some occasions, deep convective systems may penetrate the cold point tropopause. Under such conditions, sublimating cloud particles can humidify the air above the cold point tropopause. Such conditions were observed during the flight on 8 August 2017 (Fig. ) with total water mixing ratios of ≲7 ppm ∼1 km (∼25 K) above the cold point tropopause.

Moist plumes caused by convection above the cold point tropopause may reach a layer characterised by radiatively driven upward motion of ≈1 Kd-1 . Thus, such moist air parcels – as long as they remain unmixed – will rise to greater potential temperatures (altitudes). This type of air parcels was also detected in balloon measurements in the ASMA above the cold point tropopause in 2016 and 2017 . However, a hydration of the lowermost stratosphere is not regularly observed. Further, in such moist air parcels, new ice cloud formation is possible .

When convection is strong (flight on 10 August 2017), particle occurrence and oversaturation is observed in close proximity of the cold point tropopause (Figs. , top right, , and ). A very strong enhancement of total water (i.e., ice particle occurrence) is noticeable close to the cold point tropopause at about 17.1 km (about ±100 m around the cold point, Fig. ). The coincidence of the cold point tropopause and the ice particle occurrence at an altitude of ∼17.1 km leads to the hypothesis that the same processes are responsible for the formation of the cold point and for the formation of cloud particles. Thus strong convection should lead to dehydration at the cold point tropopause and not to a moistening of the stratosphere.

Further, a clear enhancement of ozone mixing ratios (up to about 250 ppb) is visible at the location of the cold point tropopause (Fig. , top left). There is no indication in the Geophysica measurements in the Asian summer monsoon in 2017 of humidification of stratospheric air exceeding mixing ratios of ∼10 ppm of water vapour above the cold point tropopause. Overall, the air in the lower stratosphere above the Asian monsoon anticyclone is moister (water vapour mixing ratios of ∼3–10 ppm) than in winter over the west Pacific (water vapour mixing ratios of ∼3 ppm).

Here we investigate solely measurements in the Asian monsoon anticyclone in 2017. However there is a substantial interannual variability of the monsoon (e.g., ). To address this variability, the analysis of measurements in other years is required. Moreover, it will be possible to further exploit the aircraft measurements in 2017 by extending the analysis to other measurements on the plane.

Measurements of the species and quantities considered here (namely ozone, water vapour, temperature and cloud occurrence) are in principle also accessible through balloon-borne measurements e.g.,. Measurements of cloud occurrence are not straightforward as it is difficult to cover the entire size range of cloud particles occurring in the atmosphere in measurements. In the future, when fewer satellite observations will be available networks of balloon-borne observations might become increasingly important . Therefore, in future work, an extension of the present analysis for the Asian monsoon will be possible (based on balloon-borne measurements), which will feature more profiles (and thus a better statistics) and will allow inter-annual as well as intra-seasonal variability to be addressed.