Orographic Precipitation in the tropics: linear theory of triggered convection

Abstract: The “natural laboratory” of mountainous Dominica in the trade-wind belt is used to study the physics of tropical orographic precipitation in its purest form; unforced by weather disturbances or by the diurnal cycle of solar heating. A cross-island line of rain gauges and 5-minute radar scans from Guadeloupe are used to describe the precipitation. The large annual precipitation at high elevation (i.e. 7 meters per year) and the large orographic enhancement factor (i.e. 2 to 8) are caused primarily by repetitive convective triggering over the windward slope. The triggering is caused by terrain-forced lifting of a mixture of wetter and dryer air parcels, quickly generating relative buoyancy for the wetter parcels (Woodcock, 1960).

The proposed mechanism of convective triggering is clarified by constructing a linear theory of orographic convection in a conditionally unstable atmosphere. The forcing term for convection is proportional to the difference between the dry and moist stability, and the mean ascent rate. The theory includes the stabilizing effect of dry downdrafts, the entrainment of momentum and buoyancy and the mean lifting of the cloud layer by three-dimensional airflow around the island. The theoretical pattern and amplitude of convection qualitatively agree with observations and LES simulations.