Effects of convection on the Tropical Tropopause Layer -- a model study

Water and clouds in the tropical tropopause layer (TTL) are important because: (1) the TTL's positive radiative balance allows it to control water vapor input into the lower stratosphere; and (2) TTL cloud formation has a significant impact on the global radiative balance. The bottom of the tropical tropopause layer is also above the altitude of most tropical convective detrainment. However, enough detrainment occurs within the TTL to make convective turnover times comparable to transit times via radiatively induced ascent. We thus can expect a fairly complex interaction between hydration/dehydration associated with convection, and dehydration due to horizontal flushing through cold regions. Still, comprehensive microphysical/dynamical models have been able to reproduce TTL water vapor and clouds well without any convective injection at all. The caveat to this is that the TTL cloud and water vapor measurements with the kind of vertical resolution needed to really constrain the models has been limited to long term climatologies from solar occultation methods. Also, water isotope measurements make a compelling case that convection must play a role in the water budget of the TTL. The ultimate goal of this work is to understand the relative importance of convective processes and in-situ dehydration in the TTL.

This paper presents results that include convective injection in a trajectory curtain based complete microphysical model. Microphysical calculations for 648 trajectory curtains are completed with convective injection along each trajectory. The altitude of convective injection along the trajectory is derived from a comparison of temperature profiles from meteorological analyses and brightness temperatures from 3-hourly geostationary meteorological satellite data. A 7K offset is included in the geostationary satellite imagery to reflect the underestimation of cloud top altitudes from thermal imagery. This yields convective turnover times that are at least comparable to previous studies using thermal imagery and trace constituent analyses. The major results so far are: (1) significant hydration (50 to 100%) in the lower part of the TTL, with perhaps 10% hydration at levels above 365K; (2) significant reduction (30-40%) in convective effects due to subsequent dehydration from horizontal flushing through cold regions; and (3) uniform distribution of hydration across the tropics. Further calculations will: (1) refine the altitudes of convective injection; and (2) examine the sensitivity to particle size distributions within convective outflows.