Modeling Clouds and Water Vapor in the Upper Tropical Troposphere

The tropical tropopause layer (TTL, ~15-17km) is a unique atmospheric region with persistent high relative humidity, regions of super saturation and extensive cirrus. These properties are not surprising since this is the region where air is dehydrated before entering the stratosphere. We focus on model simulations of this regions using the GEOS-5 model with two-moment (2-M) cloud physics and the forward domain filling model Lagrangian model (FDF) with a simplified cloud model. FDF does a credible job simulating this region when compared to aircraft and satellite observations. The model cloud fraction agrees with CALIPSO observations and the TTL relative humidity is similar to that observed by both aircraft and satellite. The GEOS-5 model with 2-M shows distinct improvements over single moment model simulations of the same region, but 2-M does not reproduce the level of super-saturation observed by aircraft. FDF runs with and without convection show that convection plays little role in determining the water vapor concentration in the upper TTL.