Interaction of cold-air blocking and upper-level potential vorticity anomalies during lee cyclogenesis

Abstract

A two-dimensional model of the interaction of cold-air blocking by a mountain and of an anomaly of potential vorticity (PV) moving over the mountain during lee cyclogenesis, is presented. The model is based on the semigeostrophic equations for a three-layer atmosphere with piecewise constant distributions of potential vorticity. A geostrophic mean flow U normal to the mountain is imposed. Initially, an upper-level PV-anomaly is assumed above a cold layer confined by a surface front. The near-surface circulation is anticyclonic underneath the anomaly. Both the front and the anomaly move with speed ~ U towards the mountain, where the cold air is blocked while the anomaly crosses the mountain and induces a cyclone in the lee.