A regional real-time forecast of marine boundary layers during VOCALS-REx

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 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 • 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 genCorrespondence to: S. Wang (shouping.wang@nrlmry.navy.mil) erally 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.

Limited-are a modelling of stratocumulus over South-Eastern Pacific

“…Analysis of the WRF output suggested the boundary layer schemes diagnosed boundary layer height near the cloud base rather than near the cloud top, as one might expect for approximately well-mixed (in the sense of conserved moist variables) marine boundary layer. increased from 36 to 81, consistent with results discussed by Wang et al (2011). When the horizontal gridlength was reduced from 9 to 3 km, there was almost no change in boundary layer height.…”

“…Although there was a significant variability of the observed profiles (the largest for the horizontal velocity components within the boundary layer), the variability does not explain the systematic differences in the boundary layer height between the observations and simulations. Overall, significant underprediction of the height is consistent with previous limited-area simulations of cloud-topped marine boundary layer (e.g., Wyant et al, 2010;Abel et al, 2010;Yang et al, 2011;Wang et al, 2011). In our case, the underprediction results from a combination of two factors.…”

“…When the horizontal gridlength was reduced from 9 to 3 km, there was almost no change in boundary layer height. This is in contrast to the results presented by by Wang et al (2011), where improvement with the increasing horizontal resolution (45 km, 15 km, 5 km) was reported. Moreover, the results changed little when different boundarylayer and cloud microphysics parametrizations available in the particular version of the WRF model were used.…”

Numerical Study on the Effect of Air–Sea–Land Interaction on the Atmospheric Boundary Layer in Coastal Area

“…Within the scheme of Mellor and Yamada [24], the eddy viscosity K M and eddy diffusivity K H coefficients are computed at every model grid point as functions of the flux Richardson number, the turbulence kinetic energy and the turbulent mixing length scale following [27]. This approach to modeling the atmospheric boundary layer in COAMPS performs well on the open ocean when verified against observations [28][29][30]. This approach to modeling the atmospheric boundary layer in COAMPS performs well when verified against observations [28][29][30][31].…”

“…This approach to modeling the atmospheric boundary layer in COAMPS performs well on the open ocean when verified against observations [28][29][30]. This approach to modeling the atmospheric boundary layer in COAMPS performs well when verified against observations [28][29][30][31]. Forecasts of between one and 12 h lead time from the grid points nearest to selected latitude/longitude points were used to extract profiles to force the LES domains.…”