Formation of maritime convergence zones within cold air outbreaks due to the shape of the coastline or sea ice edge

Abstract

Maritime cold air outbreaks often feature convergence zones that provide a conducive environment for the development of polar mesoscale cyclones and polar lows. This study examines the formation mechanisms of these convergence zones in cold air outbreaks downstream of a coastline or sea-ice edge. A simplified configuration in which the coastline or sea-ice edge is approximated by a line featuring a bend with an angle is examined using analytic solutions and idealised numerical simulations. The bend of the coastline causes differences in the fetch over which air parcels travel, causing a warm wedge of air downstream of the bend due to differential airmass transformations. The warm wedge is associated with a pressure trough that leads to convergence in the presence of surface friction. The analytic model captures this mechanism and compares well with the idealised numerical simulations. Condensational heating associated with moist convection enhances vertical motions and thus intensifies the horizontal convergence. The idealised numerical simulations also reproduce an asymmetry in the vertical shear of the horizontal wind across the convergence zone, which explains the transverse cloud streets downstream to the left of the convergence zone.