The Tropopause Inversion Layer in the GEOS-5 Data Assimilation: Sensitivity to the Observing System

The Tropopause Inversion Layer (TIL) is a feature of the extratropical lower stratosphere characterized by a positive temperature lapse rate and a local maximum of static stability within a shallow layer directly above the tropopause during all seasons. The existence of the TIL has been reported in radiosonde data, Global Positioning System Radio Occultation measurements and in general circulation models. It has been shown that both radiative (heating and cooling by ozone and water vapor, respectively) and dynamical (eddy potential temperature transport) processes are likely to contribute to the formation of the TIL but the exact mechanism responsible for the feature remains unknown.

Older global atmospheric data assimilation systems produce only a weak TIL in disagreement with observations. This has been attributed to excessive smoothing inherent in the data assimilation methodology. In this presentation we will demonstrate that, by contrast, the TIL is well represented in the Goddard Earth Observing System, version 5 (GEOS-5) analyses in terms of the season-dependent magnitude, vertical structure and extent, as permitted by the model resolution but there is a strong dependence of the analysis TIL on the observing system used. In particular, the insertion of conventional and hyperspectral radiance such as from the Atmospheric Infrared Sounder (AIRS) instrument data leads to a more realistic TIL. We will also briefly discuss the statistical distribution of Brunt–Väisälä buoyancy frequency above the tropopause as a measure of static stability and the sharpness of the TIL. Next, we will use the temperature tendencies computed by the GEOS-5 general circulation model in an attempt to separate dynamical and radiative contributions to the formation of the temperature inversion above the extratropical tropopause.