The vertical cross sections of water vapour and ice content prove that the large amounts of water vapour and ice are injected above the 380 K level by the convective overshoots that occurred during 15:00–21:00 UTC on 6 August. At 13:00 UTC (Fig. 6a), just before the overshoot development, a strong upward motion is seen at 16–18 km altitudes, while the cloud top (black solid line) is located in the IL (about 17.5 km), just below the CPT. At 15:00 UTC (Fig. 6b), a large amount of water vapour (≥15 ppmv) is seen in the ML above 410 K level, while a large ice content in excess of 120 eq. ppmv is found in the IL, between the 380 and 410 K levels (Fig. 7b). Figure 8a and b show that during 15:00–17:00 UTC the concentration of the tropospheric tracer increases in both the ML and IL, with values of 4 % and 30 %, respectively.

At 17:00 UTC an even higher cloud top is apparent at the ∼19.5 km altitude (Fig. 6c), a large amount of water vapour (≥18 ppmv) rises to ∼20 km, at around 103∘ E, and a large ice content (≥120 eq. ppmv) stays below 18 km altitude (Fig. 7c). The large amount of water is directly injected by convective overshoots mainly in the form of ice, as the ice-laden air within the convective overshoots mixes with the entrained stratospheric air during the collapse of the overshooting top. The warm, subsaturated stratospheric air causes the ice to rapidly sublimate into water vapour at the top of the overshoot, moistening the layer. It is worth noting that the water injected by the convective overshoots at 15:00 UTC is still apparent in the ML at 17:00 UTC at around 102∘ E, with a water vapour mixing ratio above 9 ppmv (Fig. 6c). In a similar way, the convectively injected large moisture at 17:00 UTC at around 103∘ E (Fig. 6c) is found in the ML at 19:00 UTC at around 102.5∘ E, with a water vapour mixing ratio larger than 15 ppmv (Fig. 6d). At 19:00 UTC (Fig. 6d), the strong convective updraughts perturb the isentropic surfaces (red solid lines), descending at the 410 K level largely from about 18.5 to 17.5 km. At 21:00 UTC a higher cloud top is found above ML in a wide area (102.3–103.3∘ E). The injected water vapour (≥18 ppmv) is transported above 20.5 km (Fig. 6e), while the concentration of 0.1 %–0.5 % of the tropospheric tracer is seen in the water vapour pocket. The large ice content exceeding 120 eq. ppmv is distributed mostly in the IL (Fig. 7e). During 15:00–21:00 UTC (Fig. 8b–e), a concentration of 2 %–20 % of the tropospheric tracer is consistently seen in the ML, while a higher concentration of 40 % is found in the IL. During 17:00–21:00 UTC (Fig. 9c–e), the large turbulent kinetic energy (TKE) of 0.2–0.9 m2 s-2 is apparent in a limited area of cloud top (∼103 ∘ E).

From 23:00 UTC on 6 August to 06:00 UTC on 8 August 2017, the convective overshoots gradually diminish in the region of ∼28–25∘ N, ∼98–85∘ E (see Fig. 4). At 23:00 and 00:00 UTC, the anvil-shaped cloud above the 16 km altitude presents a rather flat cloud top around 19 km (Fig. 7f and g). The injected large amount of water vapour ≥18 ppmv is evident in the ML, even at higher altitudes up to 20.5 km (Fig. 6f and g), whereas the large ice content ≥120 eq. ppmv is no longer apparent in the IL (Fig. 7f and g). Within the anvil cloud, a still-large TKE value of 0.2–0.9 m2 s-2 is seen (Fig. 9f and g). During 06:00–18:00 UTC on 7 August, the water vapour mixing ratio in the ML gradually decreases from 15 to ∼9 ppmv; meanwhile the air mass in the IL becomes dry, with a water vapour mixing ratio below 4 ppmv (Fig. 6h–j). During the same period, the increase in tropospheric tracer concentration and TKE are evident in both the ML and IL. The air mass with a concentration higher than 40 % is apparent in the IL, while the air mass with a lower tropospheric concentration of around 2 %–30 % is seen in the IL (Fig. 8h–j).

The air mass with a high tropospheric tracer concentration of 2 %–40 % consistently exists in the ML and IL from 00:00 to 06:00 UTC on 8 August 2017 (Fig. 8k and l), while the TKE of 0.2–0.9 m2 s-2 exists in a wide area between the altitudes of 16 and 18 km (Fig. 9k and l). During this period, the hydration patch is further narrowed and widened in the ML (Fig. 6k and l), and the air mass becomes drier in the IL (≤3 ppmv). Note that even the low numerical diffusion of the PPM scheme also contributes to the dispersion of the hydration patch. At 00:00 UTC (Fig. 7k), new convection tops are apparent in altitudes of 16–17 km, and an increase in ice content above 3 eq. ppmv is seen in the IL. Then a decreased ice content of 0.1–1 eq. ppmv distributes in the IL at 06:00 UTC where a large ice content of around 1–1.9 eq. ppmv was measured (see Fig. 2b).

The hourly evolution of the average water vapour and the ice content along the pathway of the hydrated layer demonstrates the hydration in the TTL by the convective overshoots (Fig. 10). During the development of the convective overshoots between 14:00 and 21:00 UTC on 6 August 2017, the average water vapour mixing ratio increases to 5.7 ppmv in the IL (yellow solid line; Fig. 10a), while a large mixing ratio of 6.5 ppmv is seen in the ML (blue solid line in Fig. 10a). The ice content reaches more than 200 eq. ppmv in both layers and more than 300 eq. ppmv in the IL (Fig. 10b). Until 17:00 UTC, the temperature increases in both layers (solid lines in Fig. 10c), indicating the mixing with the warmer stratospheric air. Because of this entrained stratospheric air, RHice decreases largely below 60 % in the ML (blue line with symbols in Fig. 10c) and down to 90 % in the IL (yellow line with symbols). Due to the mixing with entrained warmer stratospheric air, the enriched water vapour layer then remains at this higher isentropic level after the overshoot collapses. The conditions and timescale of the detailed process trapping the enriched water vapour in the TTL were demonstrated by Dauhut et al. (2018). Thanks to a fine temporal resolution of 1 min, they revealed that this process occurs on short timescales within 20 min. The active mixing of the convective overshoots with the stratospheric air between 14:00 and 21:00 UTC is also evidenced by the evolution of vertical profiles of tropospheric tracer (Fig. 11a). The tropospheric tracer concentration increases from 0 % to 5 % in the ML (yellow and green lines in Fig. 11a), while the stratospheric air concentration (1 minus tracer) increases by 5 % in the IL. The temperature increases in both the ML and IL (yellow and green lines in Fig. 11b) where the relative humidity decreases (yellow and green lines in Fig. 11c).

The scatter plot of the tropospheric tracer and water vapour mixing ratio (Fig. 12) evidences the large mixing of tropospheric and stratospheric air masses in the TTL (14–22 km altitudes). A large evolution of the tropospheric tracer–water vapour diagram is found from 13:00 to 21:00 UTC on 6 August (Fig. 12a and b). At 13:00 UTC, before the development of the convective overshoots, the air mass with a potential temperature (θ) of 410–420 K (yellow circles), corresponding to ML, is relatively dry with a water vapour mixing ratio of 5–7.2 ppmv (Fig. 12a) and very small compounds of tropospheric air (tracer ≤ 0.1 %). The air mass with a θ of 380–390 K (black circles), corresponding to the IL, has a low water vapour mixing ratio of 3–7.5 ppmv. At 21:00 UTC (Fig. 12b), the air mass with a θ between 410 and 420 K becomes very humid (5.5–13.6 ppmv of water vapour), and the concentration of tropospheric tracer increases to 0.2 %–8 %. Moreover the air mass with a very high θ of 450–460 K (purple circles) is moistened largely, as shown by a water vapour mixing ratio above 15 ppmv. So does the air mass with a θ between 390 and 410 K, which is both moistened and enriched by the tropospheric tracer with a concentration of 5 %–60 % (red and orange circles; Fig. 12b). The convective overshoots also impact the air mass below the CPT by widening the range of the water vapour mixing ratio with a θ between 370 and 380 K (grey circles in Fig. 12a and b) from 3.2–13.9 ppmv at 13:00 UTC (Fig. 12a) to 0–18.8 ppmv at 21:00 UTC (Fig. 12b).

From 17:00 UTC on 6 August to 02:00 UTC on 7 August, Fig. 9c shows that the temperature decreases gradually (solid lines), while the relative humidity increases (lines with symbols). In the ML and IL, a great part of ice contents, especially snow and graupel, falls quickly (dashed line in Fig. 10b), and the rest sublimates. Meanwhile, with the ice sediment out, still there is a low concentration of cloud ice in both the ML and IL, and the water vapour concentration slightly decreases (blue solid line in Fig. 10a). The continued presence of cloud ice in the ML suggests that the ice may have formed in situ in response to wave-driven temperature oscillations that locally drive the RH to ice saturation. The ice microphysics might play a pivotal role in controlling the eventual moisture content, since ice nucleation and the subsequent ice-growth process deplete the ML slowly. In the ML, the relative humidity increases (in range of 65 %–80 %) mainly due to the temperature decrease (in range of -78 and -82 ∘C; solid lines; Fig. 10c). During this period, the easterly wind is nearly constant, with the relatively weak speed of ∼15 m s-1 in the ML and ∼16.5 m s-1 in the IL (yellow and green lines; Fig. 11f). After 7 August, in the ML, the relative humidity of less than 80 % indicates strong subsaturation where a very small amount (0.1–0.3 eq. ppmv) of cloud ice still resides. This is probably induced by the domain-averaged analysis.

After the development of the convective overshoots, the hydration patch travels westward across India and northern Bangladesh from 21:00 UTC on 6 August to 06:00 UTC on 8 August (Fig. 4). During its travel, the air mass in the ML and IL has a smaller and smaller amount of water vapour and ice content (Fig. 10a and b).

Between 21:00 UTC on 6 August and 12:00 UTC on 7 August, the concentration of tropospheric tracer increases at high altitudes, with a θ between 410 and 420 K up to 18 % (yellow circles; Fig. 12b and c). At the same time, the water vapour decreases by a factor of 2, in the range of 5–9.6 ppmv. This can be also seen in the vertical profiles for which the concentration of tropospheric tracer increases at 12:00 UTC on 7 August in both the ML and IL (green and blue lines; Fig. 11a) and the water vapour decreases (green and blue lines; Fig. 11d). The two layers become colder by ∼3 ∘C (green and blue lines in Fig. 11b) and dehydrated compared to the initial state of 13:00 UTC on 6 August (yellow line in Fig. 11d and e). This includes a much-reduced amount of ice content (green and blue solid lines in Fig. 11e), but still ice content ≥1 eq. ppmv and cloud ice ≥0.5 eq. ppmv exist at 12:00 UTC on 7 August (blue dashed line; Fig. 11e). In the IL (Fig. 10c), RHice oscillates, mostly driven by temperature variation. Over time, the air mass in the ML and IL gets colder and less humid by the lowered cloud top below the 17 km altitude. In both the ML and IL, the easterly winds weaken below 15 m s-1. Moreover, the TKE increases from 0.1–0.3 m2 s-2 at 21:00 UTC to 0.2–0.9 m2 s-2 at 12:00 UTC in the ML and IL (Fig. 9e and i). These results suggest that the water vapour concentration in the ML and IL decreases due to the turbulent diffusion in both the vertical and the horizontal direction, consistent with the increase in tropospheric tracer. Also, the vapour-scavenging effect by ice nucleation and particle growth within the IL contributes to reducing the water vapour deriving the dehydration. The rapid decrease in ice content in the IL due to both sublimation and sedimentation (Fig. 7f–i) results in the lowering of the cloud top from 17 to below 16 km at 97–101∘ E at 06:00 UTC (Fig. 7h) and finally to 15 km at around 95.5∘ E at 12:00 UTC (Fig. 7i).

Further increased tropospheric tracer concentration can be seen from 12:00 UTC on 7 August to 06:00 UTC on 8 August 2017 in the ML and IL (blue and red lines; Fig. 11a). Moreover the tropospheric tracer concentration reaches about 30 % and 70 % in the ML and IL, respectively, while the water vapour decreases (Fig. 11a and d). During this time, the cloud top height of convective cloud descends below 14 km (Fig. 6i–l), where RHice dramatically decreases (Figs. 2d and 11c). The entrained cold tropospheric air (and/or colder background air) and the hydrostatic adjustment decrease the temperature in the ML and IL (Fig. 11b). It is worth noting the shape of the temperature profile that becomes straight upward in the altitudes of 17–18.5 km during the overshoot activity (green line; Fig. 11b). Also it is worth noting the decrease in ice content of less than 0.3 eq. ppmv (blue and red line; Fig. 11e) and the large decrease in relative humidity in altitudes below 17.5 km.

The increased tropospheric tracer concentration in the ML and IL is seen as well by the tracer–vapour diagram in Fig. 12c and d. The concentration of tropospheric tracer increases at high altitudes with θ between 410 and 420 K (yellow circles) up to 20 % at 06:00 UTC on 8 August 2017; meanwhile the tropospheric air concentration increases up to 50 % at the altitudes with θ between 390 and 400 K (red circles in Fig. 12d). During the period (12:00 UTC on 7 August to 06:00 UTC on 8 August), the water vapour decreases in all altitudes with a θ value above 380 K (Fig. 12c and d). It decreases from 9.6 to below 6.2 ppmv in the ML (θ between 410 and 430 K; yellow and green circles), while dropping below 5 ppmv in the IL (θ between 380 and 400 K; red and black circles). The reduced ice content in the ML and IL might be induced by sublimation due to the mixing with the dry tropospheric air (RHice ∼ 50 %–70 %) of below the 16 km level (red line in Fig. 11c and crosses in Fig. 2d). The air mixing of tropospheric and stratospheric air masses might be induced by the vertical wind shear, with the maxima wind speeds in excess of 30 m s-1 at ∼17 and 18.5 km altitudes (see Fig. 2e; average value in range of 18–25 m s-1 of red line in Fig. 11f). With the strengthened easterlies, the air mass in the IL is well-mixed rather than conserved in this layer. Also, this wind shear layer with a large gradient of wind speed (25–35 m s-1) is located below and above the CPT (Fig. 2c and e); thus it results in the straight upward temperature profile with the constant value of about -80 ∘C at 06:00 UTC on 8 August, as seen in Fig. 11b (red line). The air mass in the ML and IL has large TKE values of 0.5 m2 s-2 (Fig. 9l).