Diurnal patterns of rainfall in northwestern South America. Part III: Diurnal gravity waves and nocturnal convection offshore

Afternoon/evening near-coastal convection over land is easily understood as a response to solar heating of the land, turbulent transfer of heat and moisture to the boundary layer, and lifting of air by vigorous sea-breeze fronts. Subtler processes apparently underlie the late night and morning convection that is prevalent over coastal waters throughout the Tropics. Sensitivity tests using the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5), and further diagnoses of the control run described in Part II, are used to explore these processes. Prior studies have speculated that "land-breeze" circulations, analogous but opposite to the sea breeze, drive offshore convection at high. However, nighttime radiative cooling of land and the associated thermal breezes are much weaker than the corresponding daytime processes, especially under humid tropical skies. Analysis of model mean soundings reveals that modest (fractions of a degree Celsius) temperature changes near the 800-hPa level change the sign of the buoyancy of low-level air, from negative (inhibited) to positive (convecting) after about midnight in the coastal zone. These diurnal temperature changes are seen to be signatures of a diurnal gravity wave with a propagation speed of ∼ 15 m s^-1, similar to the speed of motion of the diurnal seaward sweep of convection seen in satellite data as well as in the model. This wave radiates from the diurnally oscillating heat source of the daytime mixed layer, raised up into the stratified layers of the atmosphere by elevated terrain. A surprising finding is that the model mean rainfall field is almost the same in a simulation without diurnally varying solar radiation.