Simulations of the Amazon basin circulation with a regional model

Numerical simulations of the atmospheric circulation over tropical South America are performed with a regional model developed at The Pennsylvania State University and the National Center for Atmospheric Research and commonly referred to as the MM4. The authors focus on a 5-day period beginning at 1200 UTC 27 February 1990. The observed circulation is evaluated in terms of initialized analyses of standard meteorological variables from the National Meteorological Center, outgoing longwave radiation from polar orbiting satellites, and surface observations. The NMC analyses are also used to specify the initial conditions, as well as provide the lateral boundary conditions, for the 5-day simulations. The impacts on the simulated circulation of major changes to the standard MM4 are assessed. When an improved treatment of radiative processes is included, excessive rainfall develops over the Andes Mountains and over the Amazon Basin. The excessive rainfall is concentrated in ''gridpoint'' storms that are not eliminated when the surface physical parameterizations are improved. Modifications to the treatment of the vertical transport of moisture are required to diminish the excessive rainfall. Even with these and other changes included in the model, the simulated basin-averaged rainfall continues to exhibit unrealistic features. The improved, though still imperfect, model simulations are used to diagnose the temporal and spatial evolution of the circulation with an emphasis on equatorial-subtropical interactions.