Abstract
Abstract. The possible impact of deep convective overshooting over land has been explored by six simultaneous soundings of water vapour, particles and ozone in the lower stratosphere next to Mesoscale Convective Systems (MCSs) during the monsoon season over West Africa in Niamey, Niger in August 2006. The water vapour measurements were carried out using a fast response FLASH-B Lyman-alpha hygrometer. The high vertical resolution observations of the instrument show the presence of accumulation of enhanced water vapour layers between the tropopause at 370 K and the 420 K level. Most of these moist layers are shown connected with overshooting events occurring upwind as identified from satellite IR images over which the air mass probed by the sondes passed during the three previous days. In the case of a local overshoot identified by echo top turrets above the tropopause by the MIT C-band radar also in Niamey, tight coincidence was found between enhanced water vapour, ice crystal and ozone dip layers indicative of fast uplift of tropospheric air across the tropopause. The water vapour mixing ratio in the enriched layers exceeds frequently by 1–3 ppmv the average 6 ppmv saturation ratio at the tropopause and by up to 7 ppmv in the extreme case of local storm in coincidence with the presence of ice crystals. The presence of such layers strongly suggests hydration of the lower stratosphere by geyser-like injection of ice particles over overshooting turrets. The pile-like increase of water vapour up to 19 km seen by the high-resolution hygrometer during the season of maximum temperature of the tropopause, suggests that the above hydration mechanism may contribute to the summer maximum moisture in the lower stratosphere. If this interpretation is correct, hydration by ice geysers across the tropopause might be an important contributor to the stratospheric water vapour budget.
Highlights
Water vapour is a key player in stratospheric climate and chemistry
In the case of a local overshoot identified by echo top turrets above the tropopause by the MIT C-band radar in Niamey, tight coincidence was found between enhanced water vapour, ice crystal and ozone dip layers indicative of fast uplift of tropospheric air across the tropopause
A series of six balloon flights combining a fast response FLASH-B Lyman-alpha hygrometer and particles and ozone measurements supported by C-band ground-based radar observations has been carried out in West Africa during the monsoon season
Summary
Water vapour is a key player in stratospheric climate and chemistry. It is the most important greenhouse gas controlling partly the temperature of the stratosphere and upper troposphere (Forster and Shine, 2002), as well as a source of hydroxyl radicals and polar stratospheric clouds involved in ozone depletion (Solomon et al, 1986). The structures shown by the high resolution in-situ measurements, the variability of the concentration compared to the campaign mean, and the fast moisture changes, for example by 3 ppmv at 17 km between 3 and 5 August at the beginning of the convective season, appear of little consistency with a slow ascent process from the tropopause to 19 km by radiative heating (Rosenlof et al, 2001), which would take about 6 months during which zonal mixing could be expected These features would be better compatible with fresh injections of water across the tropopause at altitude up to 18 km (420 K). The moisture of the layers frequently higher than the average 6.5 ppmv saturation ratio at the tropopause suggest that the injected water was in the form of ice crystals as those observed by Kelly et al (1993), Chaboureau et al (2007), Nielsen et al (2007), Corti et al (2008) and De Reus et al (2008) near deep convective overshooting systems
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