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Stratospheric water vapour budget and convection overshooting the tropopause: Modelling study from SCOUT-AMMA

by X. M. Liu, E. D. Riviére, V. Marécal, G. Durry, A. Hamdouni, J. Arteta, S. Khaykin
Atmospheric Chemistry and Physics ()
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The aim of this paper is to study the impacts of overshooting convection\nat a local scale on the water distribution in the tropical UTLS.\nOvershooting convection is assumed to be one of the processes\ncontrolling the entry of water vapour mixing ratio in the stratosphere\nby injecting ice crystals above the tropopause which later sublimate and\nhydrate the lower stratosphere. For this purpose, we quantify the\nindividual impact of two cases of overshooting convection in Africa\nobserved during SCOUT-AMMA: the case of 4 August 2006 over Southern Chad\nwhich is likely to have influenced the water vapour measurements by\nmicro-SDLA and FLASH-B from Niamey on 5 August, and the case of a\nmesoscale convective system over A < r on 5 August 2006. We make use of\nhigh resolution (down to 1 km horizontally) nested grid simulations with\nthe three-dimensional regional atmospheric model BRAMS (Brazilian\nRegional Atmospheric Modelling System). In both cases, BRAMS succeeds in\nsimulating the main features of the convective activity, as well as\novershooting convection, though the exact position and time of the\novershoots indicated by MSG brightness temperature difference is not\nfully reproduced (typically 1A degrees displacement in latitude compared\nwith the overshoots indicated by brightness temperature difference from\nsatellite observations for both cases, and several hours shift for the A\n< r case on 5 August 2006). Total water budgets associated with these\ntwo events show a significant injection of ice particles above the\ntropopause with maximum values of about 3.7 ton s(-1) for the Chad case\n(4 August) and 1.4 ton s(-1) for the A < r case (5 August), and a total\nupward cross tropopause transport of about 3300 ton h(-1) for the Chad\ncase and 2400 ton h(-1) for the A < r case in the third domain of\nsimulation. The order of magnitude of these modelled fluxes is lower but\ncomparable with similar studies in other tropical areas based on models.\nThese two estimations exhibit significant differences and highlight\nvariability among the cases of the impact of overshooting convection in\nhydrating the lower stratosphere. We show that the regional enhancement\nof water above the tropopause is between 0.21 to 0.67 ppmv between 380\nand 400 K, generally in the range of other model estimations. The amount\nof water which remains in the stratosphere after the overshoot is\nestimated for both cases. A range of 330 to 507 tons is found for the\nChad case and an upper limit of 200 tons is found for the A < r case.\nFinally we emphasize that the hydrated area in the LS by overshooting\nconvection can be advected relatively far away from the overshoot\ninitial location, with locally mixing ratios of more than 3 ppmv higher\nthan the background level, which is compatible with the balloon borne\nmeasurements performed above Niamey in the same air mass, 30 h after the\novershoot.

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