Journal article

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

Liu X, Riviére E, Marécal V, Durry G, Hamdouni A, Arteta J, Khaykin S ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 17 (2010) pp. 8267-8286

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The aim of this paper is to study the impacts of overshooting convection
at a local scale on the water distribution in the tropical UTLS.
Overshooting convection is assumed to be one of the processes
controlling the entry of water vapour mixing ratio in the stratosphere
by injecting ice crystals above the tropopause which later sublimate and
hydrate the lower stratosphere. For this purpose, we quantify the
individual impact of two cases of overshooting convection in Africa
observed during SCOUT-AMMA: the case of 4 August 2006 over Southern Chad
which is likely to have influenced the water vapour measurements by
micro-SDLA and FLASH-B from Niamey on 5 August, and the case of a
mesoscale convective system over A < r on 5 August 2006. We make use of
high resolution (down to 1 km horizontally) nested grid simulations with
the three-dimensional regional atmospheric model BRAMS (Brazilian
Regional Atmospheric Modelling System). In both cases, BRAMS succeeds in
simulating the main features of the convective activity, as well as
overshooting convection, though the exact position and time of the
overshoots indicated by MSG brightness temperature difference is not
fully reproduced (typically 1A degrees displacement in latitude compared
with the overshoots indicated by brightness temperature difference from
satellite observations for both cases, and several hours shift for the A
< r case on 5 August 2006). Total water budgets associated with these
two events show a significant injection of ice particles above the
tropopause with maximum values of about 3.7 ton s(-1) for the Chad case
(4 August) and 1.4 ton s(-1) for the A < r case (5 August), and a total
upward cross tropopause transport of about 3300 ton h(-1) for the Chad
case and 2400 ton h(-1) for the A < r case in the third domain of
simulation. The order of magnitude of these modelled fluxes is lower but
comparable with similar studies in other tropical areas based on models.
These two estimations exhibit significant differences and highlight
variability among the cases of the impact of overshooting convection in
hydrating the lower stratosphere. We show that the regional enhancement
of water above the tropopause is between 0.21 to 0.67 ppmv between 380
and 400 K, generally in the range of other model estimations. The amount
of water which remains in the stratosphere after the overshoot is
estimated for both cases. A range of 330 to 507 tons is found for the
Chad case and an upper limit of 200 tons is found for the A < r case.
Finally we emphasize that the hydrated area in the LS by overshooting
convection can be advected relatively far away from the overshoot
initial location, with locally mixing ratios of more than 3 ppmv higher
than the background level, which is compatible with the balloon borne
measurements performed above Niamey in the same air mass, 30 h after the

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