A Dimensionless Analysis of Coastal Circulation Dynamics

Abstract:

Earth surface heterogeneity is ubiquitous, and covers a wide range of scales. Particularly challenging classes of heterogeneous surfaces are the ones that generated strong secondary circulations that can dominate flow dynamics. In this study, we focus on land-sea breeze circulations near land-water interfaces. These circulations are of particular interest since they have a direct impact on coastal zones that house over 50% of the world’s population and 70% of its cities. The increased interest in offshore wind farms is another motivation to better understand the dynamics and scales of these unsteady flows. A specific challenge is to understand the relative importance of the physical mechanisms that drive land-sea breeze dynamics, as well as the fine details of the flow and transport patterns. Previous research has focused on the temperature contrast and how its magnitude modulates the flow, but other physical drivers can strongly influence the resulting circulations. We hypothesize that the dynamics can be captured by a reduced set of dimensionless parameters, namely: land-sea contrasts in surface roughness [Π₁=roughness length of land/roughness length of sea], temperature [Π₂=(T_land−T_sea)/T_sea], Π₃=angle between shore and geostrophic wind, and Π₄= geostrophic wind magnitude/max(convective velocity over land, convective velocity over sea). The hypothesis is then tested with the aid of the turbulence-resolving large eddy simulation (LES) technique, with cases designed to vary the 4 non-dimensional groups identified independently. The results elucidate the complex interaction between mean flow, secondary circulations, and turbulence, and their influence on flow and transport in coastal zones, with implications for offshore wind energy and coastal urban environments, among others. This hierarchical analyses framework paves the way to the development of succinct parametrizations of coastal heterogeneity effects in coarser geophysical models.