Idealized simulations of sea breezes over mountainous islands

Abstract

High‐resolution simulations of diurnally heated airflows over mountainous islands are conducted to study the impacts of island terrain on sea‐breeze circulations. As the simulated island terrain height increases, the sea‐breeze front (SBF) propagates inland faster but its frontal circulation weakens dramatically, as does the sea‐breeze flow behind it. This sensitivity is interpreted through the frontogenesis budget in a SBF‐relative reference frame. The dominant frontogenetic term (the cross‐frontal convergence) weakens over taller islands, but this trend is offset by a similar weakening of the dominant frontolytic term (the front‐relative advection) to yield minimal sensitivity of the SBF baroclinicity to island terrain height. At first glimpse, this finding appears inconsistent with the systematic reduction in SBF circulation strength over taller islands. However, as the island height is increased, baroclinicity at the SBF becomes increasingly associated not with the front itself but with the ‘background’ baroclinicity associated with elevated heating over steeper island slopes. When this latter contribution is removed, a clear weakening of the SBF baroclinicity is recovered over the taller islands. Analysis of the cross‐frontal convergence budget indicates that this reduction in cross‐frontal convergence, and hence the SBF baroclinicity, is caused by an increased slope‐parallel buoyancy gradient. As the SBF migrates inland, this gradient preferentially accelerates the air ahead of the SBF up the slope, which weakens the convergence along the front. Finally, as shown by a simple scaling of the slope‐parallel momentum equation, the weakened sea‐breeze flow over taller islands is associated with a weaker onshore perturbation pressure gradient force, owing to the protrusion of the convective boundary layer into the stable free troposphere.