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A Regional Real-time Forecast of Marine Boundary Layers During VOCALS-REx

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This paper presents an evaluation and validation of the Naval Research Laboratorys COAMPS real-time forecasts during the VOCALS-REx over the area off the west coast of ChilePeru in the Southeast Pacific during October and November 2008. The analyses focus on the marine boundary layer MBL structure. These forecasts are compared with lower troposphere soundings, in situ surface measurements,and satellite observations. The predicted mean MBL cloud and surface wind spatial distributions are in good agreement with the satellite observations. The large-scale longitudinal variation of the MBL structure along 20 degrees S is captured by the forecasts. That is, the MBL height increases westward toward the open ocean, the moisture just above the inversion decreases, and the MBL structure becomes more decoupled offshore. The observed strong wind shear across the cloud-top inversion near 20 degrees S was correctly predicted by the model. The models cloud spatial and temporal distribution in the 15 km grid mesh is sporadic compared to satellite observations. Our results suggest that this is caused by gridscale convection likely due to a lack of a shallow cumulus convection parameterization in the model. Both observations and model forecasts show wind speed maxima near the top of MBL along 20 degrees S, which is consistent with the westward upslope of the MBL heights based on the thermal wind relationship. The forecasts produced well-defined diurnal variations in the spatially-averaged MBL structure, although the overall signal is weaker than those derived from the in situ measurements and satellite data. The MBL heights are generally underpredicted in the nearshore area. An analysis of the sensitivity of the MBL height to horizontal and vertical grid resolution suggests that the underprediction is likely associated with overprediction of the mesoscale downward motion and cold advection near the coast.

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  • Atmospheric Physics
  • Meteorology
  • Physical and Dynamic Oceanography
  • Fluid Mechanics

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