Abstract
Abstract. This paper presents an evaluation and validation of the Naval Research Laboratory's COAMPS® real-time forecasts during the VOCALS-REx over the area off the west coast of Chile/Peru 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° 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° S was correctly predicted by the model. The model's 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 grid-scale 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° 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.
Highlights
The marine boundary layer is known to play key roles in regulating large- and meso-scale atmospheric circulations as it serves as the major source of moist available energy for atmospheric circulation and the major sink of atmospheric kinetic energy (e.g., Lorenz, 1978; Palmen and Newton, 1969)
This regional comparison demonstrates that the predicted mean wind and cloud fields in general agree with the satellite observations, including the low-level jet along the west coast of Chile, the weak wind speed area present near the subtropical high center and in the coastal area near 20◦ S, the overall liquid water path (LWP) distribution pattern, and diurnal phase pattern for both clouds and wind speeds
The model was configured with three nested grid meshes (45 km, 15 km and 5 km) and 45 vertical levels to cover the large experiment area with adequate resolution
Summary
The marine boundary layer is known to play key roles in regulating large- and meso-scale atmospheric circulations as it serves as the major source of moist available energy for atmospheric circulation and the major sink of atmospheric kinetic energy (e.g., Lorenz, 1978; Palmen and Newton, 1969). In the last few decades, an increasing number of studies have focused on the regional modeling investigations of MBL clouds, including regional simulations using a simple two-layer model (Wang et al, 1993), cloud evaluation and process studies using mesoscale models in a regional climate mode (e.g., Wang et al, 2004a, b; McCaa and Bretherton, 2004; Garreaud and Munoz, 2004), investigations of cloud and dynamic processes in case studies (Mocko and Cotton, 1995; Mechem and Kogan, 2003; Thompson et al, 2005), and study of shallow cumulus effects in regional ocean-atmosphere coupled climate model (de Szoeke et al, 2006) These regional model studies demonstrate that the fidelity of the MBL cloud simulations is highly dependent on the quality of the cloud and turbulence parameterizations as well as the accuracy of the large- and mesoscale flows.
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