Marine boundary layers above heterogeneous SST: Across-front winds

Abstract

Turbulent flow in a weakly convective marine atmospheric boundary layer (MABL) driven by geostrophic winds Ug = 10ms−1 and heterogeneous sea surface temperature (SST) is examined using fine mesh large eddy simulation (LES). The imposed SST heterogeneity is a single-sided warm or cold front with temperature jumps Δθ= (2,−1.5)K varying over a horizontal distance between [0.1 − 6] km characteristic of an upper ocean mesoscale or sub-mesoscale regime. A Fourier-fringe technique is implemented in the LES to overcome the assumptions of horizontally homogeneous periodic flow. Grid meshes of 2.2×109 points with fine resolution (horizontal, vertical) spacing (δx = δy, δz) = (4.4, 2)m are used. Geostrophic winds blowing across SST isotherms generate secondary circulations which vary with the sign of the front. Warm fronts feature overshoots in the temperature field, non-linear temperature and momentum fluxes, a local maximum in the vertical velocity variance and an extended spatial evolution of the boundary layer with increasing distance from the SST front. Cold fronts collapse the incoming turbulence but leave behind residual motions above the boundary layer. In the case of a warm front, the internal boundary layer grows with downstream distance conveying the surface changes aloft and downwind. SST fronts modify entrainment fluxes and generate persistent horizontal advection at large distances from the front.