A novel approach of identifying tropopause folds

Eine neue Methode zur Erkennung von Tropopausenfalten

Diploma thesis 2002
Intitute for Atmospheric and Climate Science, ETH Zürich

Vertical intrusions of the dynamical tropopause (2pvu-isosurface) into the troposphere, which are folded due to differential isentropic advection, are called tropopause folds. In this diploma thesis, a new geometric algorithm is presented, which identifies tropopause folds and divides them in three different classes, depending on their vertical extent. The data source are high-resolution ECMWF-analyses, that are available every 6 hours with a horizontal resolution of 1*1 degree on 60 vertical levels. The results from this novel methodology were succesfully verified in a detailed case-study from the 22 October 2000. With the aid of the new method, a one-year climatology (March 2000 - February 2001) has been calculated that gives a good impression about the temporal and spatial distribution of tropopause folds. During winter, folds of all categories are more frequent than during summer. The season with the weakest global fold activity is the northern hemispheric autumn. The strongest activity is during the months December until February. Shallow folds are concentrated along elongated bands in the subtropics. The probability of fold events in this region goes up to 40% over the Asian continent. They are directly associated with the strength of the horizontal temperature gradient on tropopause level. Deep folds are mainly located in the mid-latitudes and accumulate in regional clusters. The probability of deep fold events is particularly high above the North Atlantic storm track during the northern hemispheric winter months. Another issue discussed here is the connection of tropopause folds with mass exchange events across the tropopause. In the subtropics, up to 60% of the mass exchange events are coupled with tropopause folds. This amount decreases to 20% in the mid-latitudes. However, the exchange activity of tropopause folds, defined as mass exchange per fold area, is higher in the mid-latitudes than in the subtropics or tropics.