Rectification of the Diurnal Cycle Over Small Islands in Radiative-Convective Equilibrium

Convective heating over the Maritime Continent plays a major role in the atmospheric general circulation, but is poorly represented in general by global models. Observations that precipitation is much greater over islands as compared to the surrounding ocean regions, together with the strong observed diurnal cycle in precipitation, have led several authors to hypothesize that the diurnal cycle may rectify into an enhancement of the time-mean precipitation and ascent over land through complex land-sea and mountain-valley breezes. However, the complexity of such circulations in real terrain, especially in concert with other differences between the land and ocean lower boundaries, makes precise identification of the mechanism for such rectification quite difficult. Here, we attempt to strip down the problem to its most basic form: Can the lower heat capacity of an island, by itself, induce rectification of the diurnal cycle and precipitation enhancement over an island compared to the surrounding ocean?

We present results from idealized simulations of radiative-convective equilibrium in the System for Atmospheric Modeling cloud-resolving model, in which a “swamp island” of shallow slab-ocean is embedded in a larger region of deeper slab-ocean. The domain is doubly-periodic and non-rotating, with interactive surface temperatures everywhere and weak background flow. For a low-heat capacity island 96 km by 96 km in size, precipitation is roughly doubled over the island relative to the domain-average, with strong time-mean ascent in the mid- and upper- troposphere over the island compensated by a dry ring of subsidence over the nearby ocean. Surface skin temperature differences between island and ocean, and cloud radiative forcing, both contribute significantly to the atmospheric energy surplus over the island that drives the mean ascent and precipitation enhancement. The impacts of island size and geometry on island precipitation enhancement are also discussed.