Diurnal Patterns of Rainfall in Northwestern South America. Part II: Model Simulations

Abstract

This second paper of a three-part series documents the ability of the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) to successfully replicate some known aspects of the rainfall of northwestern South America. The success of the control simulation justifies the use of the model in Part III for sensitivity experiments and more detailed diagnosis. The simulation is produced using multiply nested grids, with grid increments from 72 km down to 2 km. Three coarser-resolution grids are integrated for a period of 10 days, with the finest-resolution grid run for a 2-day subset of that period. Several features of the rainfall field are examined as metrics of the model's performance. On the largest scales, the model reasonably simulates easterly waves in the western Atlantic and Caribbean, although it tends to develop them into tropical cyclones too readily. Over the Amazon River basin, diurnally initiated rainbands are observed to propagate westward, crossing the basin in 2?3 days. The simulation contains a similar phenomenon, whose propagation appears to involve both moisture advection and ducted gravity wave dynamics in the presence of an easterly jet near the 700-hPa level. On a finer scale, the model replicates the diurnal shift in precipitation across the west coast of Colombia, with afternoon and evening convection over land, and oceanic convection that begins immediately offshore after midnight and moves out to sea through the morning. On the finest scale, the simulation contains what appears to be a hydraulic jump, as the leading edge of the cool maritime air of the sea breeze surmounts the low coastal mountain range and flows eastward into the Atrato valley, to the foot of the Western Cordillera of the Andes. A similar feature has been previously documented to occur in a narrow valley in western Colombia, between north?south-oriented ridges of similar dimension and distance inland.