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Abstract
Frontogenesis and the propagation of fronts, a location with a sharp horizontal density gradient, impact water quality as well as many biological processes. Researchers have studied fronts for over three decades; however, other than local maximums in the density gradient, few studies have looked at the mechanisms leading to frontogenesis.
The present study investigates the dependence of front dynamics on the momentum flux at the inflow boundary of a narrow estuary. The author identifies three mechanisms leading to frontogenesis: local maximum in the density gradient, convergent zones in the lateral velocity component, and horizontal fluctuations in the vertical velocity component. In this work, the latter two are shown to develop in shear flows due to Kelvin-Helmholtz instabilities. When there is a horizontal density gradient in the surface plume, Kelvin-Helmholtz billows (KHB) lead to secondary frontogenesis if the plume layer is thin relative to the height of the KHB. Flows with multiple fronts are shown to have different flow dynamics than plumes with only a single front. In single front flows the surface layer spreads with an internal Froude number (Fr) around 1.3, similar to previous observations; while flows with multiple fronts travel with a larger Froude number and show an increase in turbulent mixing.
Furthermore, we outline a numerical method that utilizes embedded boundaries (EB) to deal with the complex geometry of the domain and adaptive mesh refinement (AMR) to correctly resolve shear instabilities. We give particular attention to the boundary conditions of the projection operator. The method is second-order accurate in both time and space.
