Composite analysis of the tropopause inversion layer in extratropical baroclinic waves

Abstract. The variability and similarities in the evolution of the tropopause inversion (TIL) layer during cyclongenesis in the North Atlantic storm track are investigated using operational meteorological analysis data (Integrated Forecast System from the European Centre for Medium-Range Weather Forecasts). For this a total amount of 130 cyclones have been analysed which evolved during the months August through October between 2010–2014 over the North Atlantic. Their paths of migration along with associated flow features in the upper troposphere/lower stratosphere (UTLS) have been tracked using the mean sea level pressure. Subsets of the 130 cyclones have been used for composite analysis using minimum sea level pressure to filter the cyclones based on their strength. The composite structure of the TIL strength distribution in connection with the overall UTLS flow strongly resembles the structure of the individual cyclones. Key results are that a strong dipole in tropopause inversion layer strength forms in regions of cyclonic wrap-up of UTLS air masses of different origin and isentropic potential vorticity. These air masses are associated with the cyclonic rotation of the underlaying cyclones. The maximum values of enhanced static stability above the tropopause occur north and northeast of the cyclone centre, vertically aligned with outflow regions of strong updraft and cloud formation up to the tropopause, which are situated in anticyclonic flow patterns in the upper troposphere. These regions are colocated with a maximum of vertical shear of the horizontal wind. The strong wind shear within the TIL results in a local minimum of Richardson numbers, representing the possibility for turbulent instability and potential mixing (or air mass exchange) within regions of enhanced static stability in the lowermost stratosphere.