Orographic initiation of moist convection in the tropics

The nature of triggered moist convection in the tropics is a multifaceted subject. When orography is present, two triggering mechanisms dominate: thermal forcing and mechanical forcing. Both produce increased convection over the orography. However, the characteristics of the convection are quite different depending on the forcing mechanism. While it appears that wind speed may be the regulating factor between the thermal and mechanical forced mechanisms, a complete understanding of how moist convection is triggered is more complex. A full description involves the inclusion of upstream air properties, both in the cloud and sub-cloud layer, along with aerosol concentration particularly over the land. In this study we seek a better understanding of the controls on shallow moist convection and the dynamics of how it is initiated.

Observational data from the field phase of the DOMEX (Dominica Experiment) project has been used to test our hypotheses of triggered moist convection. The project centered on the Commonwealth of Dominica, a small island in the trade wind belt (15°N, 61°W). Its mountainous peaks extend above the lifting condensation level but below the trade-wind inversion. Lifting of conditionally unstable air by the windward slopes initiates new convection and enhances already existing convection incoming with the trade wind flow (Kirshbaum and Smith 2009, and Smith et al. 2009). These conditions make Dominica an ideal place to study the triggering of moist convection. In-situ measurements around Dominica from 21 fixed research flight tracks by the Wyoming King Air aircraft along with fixed observational platforms and WRF modeling allow us to focus on a few key questions.

1. How do convective “seeds” and wind speed in the upstream environment control convection and precipitation over the island? Are the properties of the upstream environment combined with uplift enough to produce the observed over-island convection strength? Is the upstream control sufficient to give predictability?

2. What effect do island derived aerosols have on convection and precipitation?

3. What fraction of the over-island convection can be contributed to sub-cloud moisture anomalies versus the enhancement of already formed shallow cumulus?

4. How much entrainment occurs in the over-island convective plumes?

With a combination of observational data from aircraft, radar, lidar, and radiosondes, together with WRF modeling, we are interested in understanding the physical nature of convective initiation and its dependencies.

See the project website for background information: http://domex2011.com/.