Topographically trapped lows along the subtropical westcoast of South America. Part II: Mesoscale simulation of a typical episode.

Coastal lows (CLs) are the most relevant topographically trapped disturbances along the subtropical west coast of South America, producing most of the day-to-day weather variability offshore, along the coast and over the western slope of the Andes. In a companion paper we present climatological aspects of CLs based on station and gridded data. Nevertheless, lack of observations off the coast and upper air data hinders a complete three-dimensional description of these mesoscale systems. To gain further understanding on the structural evolution of CLs, a typical episode (6-10 June 1994) was simulated using the PSU-NCAR mesoscale model (MM5). After validation, we have used the model output as a physically consistent, high-resolution dataset useful in diagnosing some aspects of the CL. Our non-hydrostatic simulation used two domains with horizontal resolutions of 54 and 18 km nested with a one-way grid interface and boundary conditions from NMC analyses.

The model realistically simulated many synoptic-scale features of the CL, including distinctive patterns of surface pressure and low-level wind and cloudiness. Tropospheric warming in the first 1.5 km above the surface accounts for most of the simulated surface pressure drop along the coast (about 70% of the observed pressure drop). The low-level warming was in turn produced by the enhanced subsidence over the western slope of the Andes and subsequent horizontal warm advection by offshore winds. Initially, weak offshore winds at about 30°S appears along the coast as a geostrophic response to the inland migration of a surface anticyclone farther south, and leads to downward flow over the steep western slope of the Andes. As the CL deepens, the meridional pressure gradient becomes steeper and easterly winds intensifies to the south of the pressure minimum. Thus, topographic Rossby waves superimposed on the evolving synoptic-scale pattern seem responsible for the southward propagation of the CLs. The model also simulated an ageostrophic southerly low-level jet to the south of pressure minimum and the subsequent wind relaxation (without wind reversal in this case). After the passage of the CL there is a gradual onshore return of the MBL suggesting a Kelvin wave mechanism in this later stage.