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

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 night. 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.