Large-eddy Simulations of Sea Breezes over Mountainous Islands

The mountain-plain solenoid and the sea breeze are both thermally-driven circulations forced by differential surface heating.  Although each circulation has been studied intensively on its own, the interactions between the two remain poorly understood.  While some studies suggest an enhancement of the sea-breeze circulation and/or propagation over mountains, others suggest the opposite.  To gain insight into these interactions, a series of large-eddy simulations of diurnally heated airflow over idealized island terrain of different heights is conducted.  As the simulated island terrain height increases, the sea-breeze front accelerates faster inland but its cross-frontal circulation weakens dramatically.  Over sufficiently tall islands, the sea-breeze front vanishes entirely as it ascends the slope.  The orographic effects on the sea breeze are quantified by tracking the frontogenesis in a sea-breeze- and terrain-following reference frame. This analysis shows that the slope-parallel convergence, horizontal advection, and frontal baroclinicity all decrease sharply as the island terrain height increases. Further examination of the frontal convergence budget reveals that the convergence-generating terms (along-slope convergence and vertical advection) decrease sharply as the island terrain height increases, but the decrease is largely balanced by the reduction in the divergence-generating terms (along-slope tilting and horizontal advection). This leaves the two remaining terms in the budget, the along-slope gradients of pressure-gradient and buoyancy forces, the former (latter) of which strengthens (weakens) the convergence, as the lone factors differentiating the frontal convergence between the cases.  While the pressure-gradient term dominates over flat terrain, the magnitude of the buoyancy term gradually increases with island terrain height, to the point where it balances or even exceeds the pressure-gradient term.  This causes the frontal convergence and, in turn, the frontal circulation, to weaken.  These results can be summarized more intuitively as follows: as the terrain height increases, the increasing along-slope buoyancy draws pre-frontal air more rapidly up the slope, which weakens the cross-frontal convergence that drives sea-breeze frontogenesis.