DIABATIC HEATING IN AN AXISYMMETRICAL CUTOFF CYCLONE AND RELATED STRATOSPHERE TROPOSPHERE EXCHANGE

A mechanistic axisymmetric Eliassen balanced vortex model is presented for the investigation of the role of diabatic heating in the dynamic evolution of a cut-off cyclone and the related stratosphere-tropospher e exchange. As an initial state, a balanced upper-level vortex is chos en representing an idealized cut-off cyclone. After specifying a diaba tic heating field, which is supposed to simulate latent heat release i n deep convection, one can diagnose the induced cross vortex circulati on and calculate the ensuing vortex evolution. Tropospheric heating wi th a maximum of some 10 K d(-1) underneath the lowered tropopause lead s to the decay of the upper-level vortex within a few days. During the decay there is a sizeable, diabatically induced mass flux across the PV-defined tropopause: air in the vortex centre which initially was pa rt of the lower stratosphere gradually turns into tropospheric air. Th is stratosphere-troposphere exchange depends sensitively on the heatin g profile around the tropopause level and, hence, on the maximum heigh t reached by the convection. The inclusion of diabatic cooling to acco unt for the effect of radiation at cloud tops leaves the vortex evolut ion almost unchanged but increases the rate of stratosphere-tropospher e exchange dramatically. Cross-isentropic mixing alone may result in s trengthening rather than in weakening the vortex thus re-emphasizing t he role of diabatic heating during decay.