The coastal boundary layer diurnal cycle along north-central Chile: observations and model sensitivities

The coastal strip along north-central Chile (19-35oS, 70-71oW) is located in a region of considerable climate interest. To its west the subtropical anticyclone of the SE Pacific maintains an extensive and persistent deck of low-level stratocumulus clouds that have a large impact in the planetary radiative budget. To the east, topography rises up to the peaks of the Andes Cordillera (heights >3000 m) in less than 200 km, leaving in between a region encompassing the extremely arid Atacama Desert. In this work we analyze available data and perform numerical modelling aimed at describing and understanding the dynamics of the diurnal cycle of the marine boundary layer (MBL) along this coastal region.

The observational part describes the main features of the diurnal cycles of winds and cloudiness as deduced from available satellite-derived datasets (scatterometer winds and GOES visible and infrared fields), as well as from a few coastal automatic meteorological stations. Main features of the diurnal phase of the cycles are the intensification of the southerly (along the coast) winds and the clearing of a coastal strip. In the nocturnal phase, on the other hand, winds at the coast decrease significantly, inducing a coastal rotor important for oceanic upwelling. The modelling part of the work is based on MM5 model results for a 2 month period in the austral spring of 2003, when 4-daily satellite passes with scatterometer wind data are available. Detailed analyses of the momentum budget for coastal points near the coast are performed, in order to understand the forcing of the meridional wind diurnal cycle. It is found that enhanced afternoon subsidence occurring along the coast in the lower troposphere may play a role in this diurnal cycle, as suggested also by past field measurements in the region. Sensitivity model runs are performed in order to describe the dependency of the coastal MBL structure and dynamics to the sea surface temperature.