The diurnal cycle of rainfall in northwestern South America has been investigated with a nested-grid model (MM5), using grids as fine as 2km in the innermost domain. The model reproduces the diurnal cyclewell enough for meaningful physical process diagnosis and experiments.

Daytime heating over land produces a sea breeze, with convergence at its inland edge (the "sea-breeze front"), yielding a familiar convective rainfall maximum ~50-100 km inland in the late afternoon. After midnight, convection erupts offshore, not along an analogous "land-breeze front", but almost simultaneously over a mesoscale region. This envelope of rainfall propagates offshore with a speed of ~20 m/s.

The model reveals that this rapidly propagating convection follows the cool phase of a gravity-wave temperature anomaly centered at about 2-3 km altitude. This temperature anomaly is at the right altitude to uncap the boundary layer, unleashing deep convection. The source of the wave is the deep daytime mixed layer over high mountains, which acts as an oscillating heat source which protrudes into layers of the atmosphere that are stratified in adjacent lowland and oceanic regions.