The influence of synoptic wind on land–sea breezes

Particularly challenging classes of heterogeneous surfaces are ones where strong secondary circulations are generated, potentially dominating the flow dynamics. In this study, we focus on land–sea breeze (LSB) circulations resulting from surface thermal contrasts, in the presence of increasing synoptic pressure forcing. The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a heterogeneity Richardson number (measuring the relative strength of geostrophic wind and convection induced by buoyancy), and the angle α between the shore and geostrophic wind. Large-eddy simulations reveal the emergence of various regimes where the dynamics are asymmetric with respect to α. Along-shore cases result in deep LSBs similar to the scenario with no synoptic background, irrespective of the geostrophic wind strength. Across-shore simulations exhibit a circulation cell that decreases in height with increasing synoptic forcing. However, at the highest synoptic winds simulated, the circulation cell is advected away with sea-to-land winds, while a shallow circulation persists for land-to-sea cases. Scaling analysis that relates the internal parameters Q_shore (net shore volumetric flux) and q_shore (net shore advected kinematic heat flux) to the external input parameters results in a succinct model of the shore fluxes that also helps explain the physical implications of the identified LSBs. Finally, the vertical profiles of the shore-normal velocity and shore-advected heat flux are used, with the aid of k-means clustering, to independently classify the LSBs into four regimes (canonical, sea-driven, land-driven, and advected), corroborating our visual categorization.