Structure of the marine boundary layer during episodes of strong southerly winds along the coast of central Chile

Sea-surface winds derived from Version-3 Quikscat data are used to characterize the marine boundary layer (MBL) off the Chilean coast between 20-40 S. The mean wind field shows a region of maximum southerly winds extending between 27-37 S and 72-75 W. This southerly jet has a strong annual cycle peaking during the warm season. The jet has also large synoptic variability, but a remarkably small diurnal cycle. We have used the MM5 mesoscale model to simulate a period of 20 days in October 2000, during which several days presented a strong southerly jet located at 30 S. The model is run with a TKE boundary layer scheme and with high vertical resolution (30 m) in the lowest 1500 m above the surface. The surface wind field and the height of the MBL compare well with Quikscat data and other special observations available for the period. Model results are used to diagnose the momentum balance supporting the coastal jet and to characterize the structure of the MBL. The modeled turbulence balance is presented for the MBL and for the inversion layer capping it. Shear production, buoyancy production-consumption and dissipation are all important terms in the balance, but they have different relative strengths in the MBL compared to the layer above it. Although the MBL height at the coast is approximately the same to the north and to the south of the jet, we find in the model results as well as in the climatology that there is a consistent increase (decrease) of cloud cover to the north (south) of the jet. Boundary layer dynamics and horizontal zonal advection appear to explain this behaviour.