6.2 Stable boundary layer modeling for local and regional-scale meteorological models

Thursday, 5 August 2010: 9:15 AM
Torrey's Peak I&II (Keystone Resort)
Jeffrey C. Weil, CIRES/Univ. of Colorado, Boulder, CO

Modeling of the stable boundary layer (SBL) is one of the more outstanding challenges of planetary boundary layer research. A recent study by Cuxart et al. (2006), showed that SBL schemes used in operational, regional-scale models overestimated the SBL depth and surface friction velocity, leading to predictions of more weakly-stable SBLs than in reality. This has important consequences for climate, weather, and air quality, e.g., underestimates of concentrations from surface releases in the SBL.

This paper presents a time-dependent SBL model for vertical profiles of mean wind, potential temperature, and turbulence over a wide range of stability. The equations for the mean horizontal winds and potential temperature are closed using an eddy-diffusivity (K) approach, which is justified because of the small turbulence length scales in the SBL as found from field data and large-eddy simulations (LES) (Weil et al., 2006). Important aspects of the model are: 1) use of the Brost and Wyngaard (1978) K profiles, 2) reduction of the model equations to a simple (ordinary differential) form using a similarity variable, and 3) addition of predicted flux profiles. The model agrees well with temperature, wind, and flux profiles in several comparisons: 1) LES results (Beare et al., 2006) and field data for a weakly stable boundary layer (h/L = 1.6), 2) field observations for a very stable boundary layer (h/L = 42) (Mahrt and Vickers, 2006), and 3) heat flux profiles in the evening transition boundary layer (Caughey et al., 1979), where h and L are the SBL height and Monin-Obukhov length. It is found that the profile shapes, particularly for temperature and heat flux, are sensitive to time and stability.

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