Surface Convergence Induced by Cold Fronts Aloft and Prefrontal Surges

Abstract

The magnitude of surface convergence, produced by the movement of cold fronts aloft and prefrontal surges, is derived by applying the linear divergence equation to observed surface pressure traces for Pacific Northwest warm occlusions and from a mesoscale model simulation of a warm occlusion?like structure in the central United States. Convergence values of approximately 10?4 s?1 are found to be generated locally for periods of about 1 h, yielding vertical displacements of 10?50 hPa. It is hypothesized that such convergences should noticeably enhance condensation rates in the widespread lower stratiform clouds associated with warm occlusions and could be a key mechanism for the triggering of squall lines by cold fronts aloft in cyclones in the central United States.