Dependence of boundary layer structure near an ice‐edge coastal front upon geostrophic wind direction

Abstract

The dependence of boundary layer (BL) features within a “coastal front” (created by temperature and roughness differences across a surface interface) upon the direction of a large‐scale geostrophic wind is examined with idealized simulations, using parameters appropriate to an ice edge. Maximum vertical velocities and strongest horizontal gradients occur when the vertically averaged BL velocity near the ice edge parallels the ice edge; this occurs at the on‐ice geostrophic angle for which frictional modification of the geostrophic wind optimally counters thermally induced cross‐interface flow at the ice edge. This optimal angle is greater for “ice‐on‐right” (when looking downwind, assuming northern hemisphere rotation) flow than for “ice‐on‐left” flow due to asymmetry introduced by surface friction. The associated vertical velocity maxima also vary asymmetrically with geostrophic wind direction. The optimal angle depends upon the average surface roughness at the interface, whereas the vertical velocity maxima depend upon the differential surface roughness across the interface. For a prototype ice edge, the optimal on‐ice angle is 20° larger and its vertical velocity maximum 3 times larger for ice‐on‐right flow, with differential surface roughness accounting for nearly half of the magnitude asymmetry. Along‐ice velocity jets develop only when the large‐scale geostrophic wind nearly parallels the ice edge. The curl of the surface stress is sensitive to changes in geostrophic wind direction, changing sign over the water when the large‐scale geostrophic wind approximately parallels the ice edge. Because of frontal collapse the width of the surface transition region has a relatively minor influence on these results.