Altai-Sayan lee cyclogenesis: Numerical simulations

Summary In this paper we compare and analyse two cases of lee cyclogenesis observed near the Altai-Sayan massif (14–16 April 1988; 3–5 March 1986). The interpretation of the events is based on numerical simulations with an isentropic primitive equation model, where runs without orography are compared to those with realistic orography. The runs with mountains are successful in depicting the observed developments, so that the no-mountain experiments can be assumed to provide further information on the dynamics of both cases.The case of 14–16 April 1988 is dominated by rapid cyclogenesis and cut-off low formation aloft linked to the southward motion of a potential vorticity maximum towards the Altai-Sayan massif. Cyclone formation would have occurred without mountains as well. However, cyclogenesis at low levels is enhanced by the presence of the mountains through blocking of the cold air and corresponding deformation of the surface cold front. So this is a case of orographically modified cyclogenesis whereas the cyclogenesis of 3–5 March 1986 is induced by the mountain. In that case a broad and almost steady trough moves eastward over the Altai-Sayan region. Cyclone formation is absent in the no-mountain run. However, both cases exhibit some similarities with respect to the low-level developments induced by the mountains. A conceptual model is presented in order to further elucidate the key features of both cases.With 11 Figures