A study of the effect of overshooting deep convection on the water content of the TTL and lower stratosphere from Cloud Resolving Model simulations D. P. Grosvenor1, T. W. Choularton1, H. Coe1, and G. Held2 1The University of Manchester, Manchester, UK 2Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista, 17015-970 BAURU, S.P., Brasil
Abstract. Simulations of overshooting, tropical deep convection using a Cloud
Resolving Model with bulk microphysics are presented in order to examine
the effect on the water content of the TTL (Tropical Tropopause Layer) and lower stratosphere. This case
study is a subproject of the HIBISCUS (Impact of tropical convection on the
upper troposphere and lower stratosphere at global scale) campaign, which
took place in Bauru, Brazil (22° S, 49° W), from the end of January to early
Comparisons between 2-D and 3-D simulations suggest that the use of 3-D
dynamics is vital in order to capture the mixing between the overshoot and
the stratospheric air, which caused evaporation of ice and resulted in an
overall moistening of the lower stratosphere. In contrast, a dehydrating
effect was predicted by the 2-D simulation due to the extra time, allowed by
the lack of mixing, for the ice transported to the region to precipitate out
of the overshoot air.
Three different strengths of convection are simulated in 3-D by applying
successively lower heating rates (used to initiate the convection) in the
boundary layer. Moistening is produced in all cases, indicating that
convective vigour is not a factor in whether moistening or dehydration is
produced by clouds that penetrate the tropopause, since the weakest case only
just did so. An estimate of the moistening effect of these clouds on an air
parcel traversing a convective region is made based on the domain mean
simulated moistening and the frequency of convective events observed by the
IPMet (Instituto de Pesquisas Meteorológicas, Universidade Estadual
Paulista) radar (S-band type at 2.8 Ghz) to have the same 10 dBZ echo top
height as those simulated. These suggest a fairly significant mean moistening
of 0.26, 0.13 and 0.05 ppmv in the strongest, medium and weakest cases,
respectively, for heights between 16 and 17 km. Since the cold point and WMO
(World Meteorological Organization) tropopause in this region lies at
~15.9 km, this is likely to represent direct stratospheric moistening.
Much more moistening is predicted for the 15–16 km height range with
increases of 0.85–2.8 ppmv predicted. However, it would be required that
this air is lofted through the tropopause via the Brewer Dobson circulation
in order for it to have a stratospheric effect. Whether this is likely is
uncertain and, in addition, the dehydration of air as it passes through the
cold trap and the number of times that trajectories sample convective regions
needs to be taken into account to gauge the overall stratospheric effect.
Nevertheless, the results suggest a potentially significant role for
convection in determining the stratospheric water content.
Sensitivity tests exploring the impact of increased aerosol numbers in the
boundary layer suggest that a corresponding rise in cloud droplet numbers at
cloud base would increase the number concentrations of the ice crystals
transported to the TTL, which had the effect of reducing the fall speeds of
the ice and causing a ~13% rise in the mean vapour increase in
both the 15–16 and 16–17 km height ranges, respectively, when compared to
the control case. Increases in the total water were much larger, being 34%
and 132% higher for the same height ranges, but it is unclear whether the
extra ice will be able to evaporate before precipitating from the region.
These results suggest a possible impact of natural and anthropogenic
aerosols on how convective clouds affect stratospheric moisture levels.
Citation: Grosvenor, D. P., Choularton, T. W., Coe, H., and Held, G.: A study of the effect of overshooting deep convection on the water content of the TTL and lower stratosphere from Cloud Resolving Model simulations, Atmos. Chem. Phys., 7, 4977-5002, doi:10.5194/acp-7-4977-2007, 2007.