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

Speaker: Stephanie Evan, NOAA ESRL CSD & CU CIRES

When: Wednesday, May 29, 2013, 3:30 p.m. Mountain Time
Location: Room 2A305, DSRC (NOAA Building), 325 Broadway, Boulder
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The tropical tropopause layer (TTL), a transition layer between the troposphere and stratosphere, is important for global climate and atmospheric chemistry. Previous numerical studies that have focused on TTL processes have been conducted either using cloud-resolving models or global circulations models. These studies yield contradictory results on the key physical processes controlling the TTL temperature and water vapor.

The present study attempts to bring further understanding to the TTL problem by use of a relatively high-resolution of the Weather Research and Forecasting (WRF) model.

A major purpose of this work is to gain an enhanced understanding of the sensitivity of the tropical temperature profile, and specifically the cold point tropopause temperature, to mesoscale processes that may not be well represented or parameterized in coarse grid climate models. The end goal is to identify key physical processes that are important for an accurate representation of the TTL in regional or global climate models.

A series of WRF experiments were conducted for boreal winter conditions (December 2005 to January-February 2006). The model domain is configured as a tropical channel with a horizontal grid-spacing of 36 km, a vertical grid-spacing of 500 m and a top at 0.1 hPa. Initial and boundary conditions are set using the ERA-Interim reanalysis dataset. An ozone distribution computed from satellite and ozonesonde measurements is used for radiative forcing calculations. The model’s ability to replicate observed TTL temperatures is evaluated via comparisons with radiosonde data and reanalyses (MERRA and ERA-Interim). The Microwave Limb Sounder (MLS) water vapor measurements are used to evaluate WRF simulated water vapor in the TTL.

Results of the simulations show that the model reproduces the mean temperature and its variability above 50 hPa as well as the tropical tropopause height. However, the model cold point tropopause temperature is colder than the reanalyses by ~1.2 K. The model captures the location of TTL water vapor minimum in the Western Pacific, but is drier than the MLS observations in the TTL. To assess possible reasons for the tropopause temperature discrepancy, an additional WRF experiment was conducted using analysis nudging for water vapor. This experiment produces more tropical cirrus clouds in the upper troposphere and a warming of ~1.5 K of the cold point tropopause. This suggests that the radiative effects of cirrus clouds and water vapor must be considered for accurate temperature simulations in the TTL.

Furthermore we discuss the role of microphysics in WRF simulation of TTL temperatures and water vapor. Several experiments are performed to assess the model sensitivity to a number of specified microphysical parameters used in the microphysics scheme.

Finally the impact of a Sudden Stratospheric Warming (SSW) event in January 2013 on the tropics is investigated based on satellite data (COSMIC, MLS and MODIS).

The tropical cold point tropopause temperature and water vapor at 82 hPa decreased by about 2K and 1.5 ppmv respectively within the first 15 days of January. Changes in tropical clouds are also observed together with the occurrence of the SSW.