Orographic Precipitation in the Tropics: The Dominica Experiment (DomEx)

The triggering of convection in conditionally unstable air and the physics of precipitating clouds remain two of the most challenging problems in atmospheric science. Progress will require the study of the simplest possible natural system, as a bridge to more complex systems. To pursue this reductionist strategy, the island of Dominica (15N) has recently been identified as an accessible “natural laboratory” for orographically forced moist convection in the tropics. While easterly weather disturbances are important in the summer and fall, the dominant process of local terrain-forced convection occurs throughout the year over Dominica (Smith et al, JAS, 2009). There is little diurnal modulation of this process. Observations and Large Eddy Simulation (LES) of convective initiation in Dominica (Kirshbaum and Smith, JAS, 2009) reveal that

• Upstream fluctuations of specific humidity (qv), associated with gentle oceanic trade wind convection, seed the convection over Dominica. Terrain-forced uplift generates buoyancy differences as moist and dry parcels lift along different adiabats (Woodcock, 1960). • Dominica convection is vigorous but mostly confined below the inversion. It has an unusually large cloud fraction (CF~.5). The terrain not only initiates the convection but retains control of its growth and decay. The convection dies suddenly, just downstream of the ridge crest. • With seven meters of annual precipitation, Dominica has one of the largest orographic enhancements of precipitation and the largest spatial gradient of precipitation yet discovered. In spite of the large annual rainfall and large rain rates in showers, less than 1% of the trade wind water vapor flux is rained out. The convection transports more water upwards than it precipitates to the ground.

These results provide a foundation for more detailed aircraft observations to occur in May 2011 using the Wyoming King Air. The King Air will be equipped with cloud radar and cloud lidar. The overall science objective is to understand the physics of convective initiation and precipitation in a simple steady tropical environment. The observational goals of an aircraft field project in Dominica are to determine

• The magnitude and scale of the upstream T and qv fluctuations. • The degree of conditional instability in the ambient airstream. • The amount of lateral flow divergence around the island • The intensity and statistical properties of the upslope convection, including the cloud fraction, plume buoyancy, velocity, and turbulence, and downdraft strength. • The magnitudes of the rainfall, heat and water vapor fluxes in the convection. Does the upward water vapor transport exceed the rainfall? • The pattern of ascent and descent of the trade wind inversion. Does it stay level upstream and then plunge over the lee slope? • The mechanism of cloud dissipation on the lee slope (e.g. descent, overshoot rebound or layer stabilization) • The change in the tropical airmass profile (e.g. temperature, humidity, instability) and water vapor flux caused by the island convection • Aerosol concentration and properties upstream and downstream of the island • How do the cloud and raindrop size distributions change as the flow ascends the hill? • Do the orographic clouds generate higher mean rainfall intensities than the oceanic clouds, or are they just more frequent? • Are the orographic clouds generally wider and, as a result, less susceptible to buoyancy dilution via entrainment? Are in-cloud liquid water contents generally larger over the mountain?

The King Air deployment will examine 10 to 15 cases of undisturbed orographic convection. These measurements will allow physical analysis of forced convection and the testing of new-generation numerical eddy-resolving, binned microphysics and aerosol-sensitive models.