Surface Dynamics of Observed Maritime Fronts

Abstract

The problem of establishing the important mechanisms and processes in surface fronts is examined. It is shown that boundary layer processes may modify the details of frontal dynamics near the surface and that at the same time fronts may alter boundary layer solutions. A similarity model that assumes a horizontally homogeneous steady-state planetary boundary layer is used to test the effects of baroclinity and stratification on the ageostrophic vorticity. Model response to baroclinity and stratification is tested. A boundary layer positive feedback mechanism that stems from the variation in stratification and thermal advection across a front and favors the creation of stronger temperature gradients in cold fronts is proposed. A compatible model that includes the effects of horizontal inhomogeneity is derived and used in conjunction with scatterometer data to observe the behavior of five surface fronts. Observations are compared to results of numerical models by several authors and it is concluded that a few scenarios for the evolution of surface fronts exist. Surface friction is important in frontal processes through its role in determining the dissipation and the ageostrophic terms in the vorticity and divergence equations. The former normally acts to weaken a front while the latter normally acts to strengthen it. The magnitude and importance of these terms vary considerably at different stages and with stratification, thermal advection, and the deformation and shear forcing. Except for one case of an explosive storm where wave activity is suspected, the convergence coincides with the cyclonic vorticity in frontal zones. The inclusion of the inertial terms representing the deformation in the Ekman solution tends to concentrate gradients by increasing frontogenesis in vorticity at or slightly ahead of the front while diminishing it elsewhere. The nonlinear terms in the divergence equation are frontogenetic for all fronts. Their relative importance varies considerably for different fronts, however.