Towards a nudging of stratospheric overshoots in the 3D mesoscale BRAMS model

Water repartition in the stratosphere is a key compound in the atmospheric chemical and
radiative equilibrium. Since the 80’s, an increase of the water concentration in the
stratosphere has been observed.This presence in the stratosphere can be explained by the
slow ascent of air mass above convective clouds in tropical regions. The amount of water
vapor entering in the stratosphere depends on the coldest temperature and countered
during this slow ascent because it can lead to ice cristal formation that sediment and
dehydrate the air masses. But some other processes may contribute to the stratospheric
water budget, especially to explain the increase of water vapor. Stratospheric overshoots
phenomenon can take part in the stratospheric hydratation, by injecting directly water ice in
the stratosphere. Injected ice water, by sublimation, will hydrate stratosphere locally. The
local role of overshoots is better known but their contributions at the global scale steal need
to be quantified. In order to estimate this contribution, previous studies have used the 3D
simulation mesoscale model BRAMS to show overshoot impact in the upper Tropical
Tropopause Layer (TTL). These studies are the starting point of our study.

The aim of this paper is to present the new development inside BRAMS to nudge
stratospheric ice injection by overshoots. It uses an overshoot occurrence climatology from
MHS (Microwave Humidity Sounder) satellite measurement. Ice injection in the model is
made according to ice model categories previously shown to be present in the overshoot
plumes with ratios already diagnosed in previous studies. Ice injection is made between two
layers of TTL’s stratospheric part: between 380 and 385K and between 385 et 400K. Nudging
is triggered only if, in the grid mesh (20 x 20 km) where MHS has detected an overshoot,
BRAMS computes a cumulonimbus with a top above 13.5km. For the layer above 385 K
isentrope, a subgrid box of 2 km x 2 km is considered for the computation of ice injection.
Sensibility test of this nudging scheme will be presented in this presentation.