The representation of the TTL in a tropical channel version of the WRF model

The NCAR Weather Research Forecast (WRF) model was initially developed and tested for regional simulations and weather forecasting in the troposphere. Little has been reported on WRF performance in simulations of the Tropical Tropopause Layer (TTL) and the lower stratosphere (LS). To address WRF ability to resolve the temperature and water vapor distribution in the TTL, we conducted a series of numerical experiments in the tropics for the boreal winter 2006 (December-January-February). The model domain is configured as a tropical channel with a horizontal grid-spacing of 37km, a vertical grid-spacing of 500m and a top at 0.1hPa. The European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis Interim (ERA-I) data provide initial and boundary conditions. The model is forced with an ozone distribution that corresponds to the climatology of observed ozone developed by Hassler et al. (2008). The model performance for TTL temperature variability is evaluated via comparisons with radiosonde data in the Western Pacific/South America and the MERRA and ERA-I reanalyses. The MLS water vapor is also used to evaluate WRF simulated water vapor in the TTL. The model is shown to have a realistic representation of tropical precipitation variability, mean tropical ascent, and evolution of the TTL zonal mean wind and temperature. The WRF simulations of the cold point tropopause show reasonable agreement with the reanalyses. The model captures the location of TTL water vapor minimum in the Western Pacific. However, the model simulation is drier than the MLS observations. Therefore we discuss the sensitivity of the WRF model in simulating TTL water vapor and temperature to the choice of the cumulus convection parameterization schemes.