Unsteady land-sea breeze circulations in the presence of a synoptic pressure forcing*

Land-Sea breezes are some of more complex geophysical turbulent flows: they are unsteady, heterogeneous, and depend on the balance of thermal to mechanical pressure forcing. Studies of these flows have typically focused on time snap-shots and have often ignored the important role of synoptic scale mechanical pressure forcing. In this study, we investigate the diurnal cycles of land-sea breeze (LSB) circulations in the presence of increasingly strong synoptic pressure forcing (expressed as an equivalent geostrophic wind). The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a bulk inverse Richardson number Ri = W_*²/M_g², where M_g is the geostrophic wind magnitude and W_* is a convective buoyant velocity scale, and the angle α between the shore and geostrophic wind.

Large eddy simulations varying these two non-dimensional parameters show asymmetry in the dynamics when the geostrophic wind is oriented from sea to land, versus land to sea. The steady state circulations indicate four regimes depending on Ri and α, varying from the canonical LSB circulation at high  Ri, to a complete absence of LSB at low Ri, but only when the synoptic flows are blowing from sea to land. The unsteady simulations depict added complexities as the land-sea surface temperature difference, the mean flow, and the turbulence are found to be consistently out of equilibrium. We analyze the flow and turbulence hysteresis for these non-equilibrium conditions.

The findings are particularly relevant for the accelerating deployment of offshore wind energy, as well as for predicting pollution ventilation and urban heat island dynamics of coastal cities.