Trade-wind convection over tropical islands

Dr Dan Kirshbaum

Shallow trade-wind cumuli lying beneath strong inversions are widespread over the tropical oceans. Emerging observations from the small mountainous Caribbean island of Dominica (15N) suggest that these clouds can rapidly build into rain showers as they make landfall even when little to no precipitation falls over the surrounding sea. The persistence of this regime leads to strong spatial variability in the island climate with a precipitation maximum on the high terrain and a pronounced lee-side rain shadow. These data also suggest that diurnal heating/cooling is unimportant for triggering the showers as there is little diurnal variation in precipitation. To describe the physical mechanisms underlying the island precipitation enhancement, we have conducted large-eddy simulations of trade-wind flow over an idealized ridge that capture the formation of turbulent clouds over the ocean and their subsequent evolution over land. These simulations reveal that three mechanisms are responsible for the large island precipitation enhancement: (1) orographic lifting initiates more cumuli, (2) differential adiabatic cooling within the saturated and unsaturated regions enhances the buoyancy within preexisting clouds and (3) abrupt lifting of boundary-layer moisture structures results in an enlargement of the cumuli that weakens the suppressive impacts of entrainment. It is worth noting that the trade-wind inversion, which caps cumulus cloud tops over the open ocean, does not rise over the high terrain, but sinks in a hydraulic-like response. Thus the forced lifting does not directly enable clouds to rise any higher over the mountains than over the ocean. But the clouds become more vigorous due as a consequence of the mechanisms listed above.

To evaluate the above hypotheses derived from numerical simulation, we will be conducting an observational field campaign over Dominica in spring 2011. The NSF-funded DOMinica EXperiment (DOMEX) will use the Wyoming King Air research aircraft to observe cloud dynamics, thermodynamics, and microphysics within cumuli over the open ocean and the complex island terrain. From this experiment we hope to derive general theories for the evolution of cloud fields subject to bulk lifting.