Scalar Dispersion in the Weakly Stable Boundary Layer: Effects of Surface Temperature Heterogeneity

Previous studies investigated the effects of surface-temperature heterogeneity (HET) in the stable boundary layer (SBL) on the turbulence, vertical mixing, and transport using large eddy simulations (LESs) (e.g., Stoll and Porte-Agel, 2009; Mironov and Sullivan, 2010). These effects were demonstrated and highlighted by comparison with similar temperature and turbulence properties for a surface-temperature homogeneous (HOM) SBL. The homogeneous case was generated earlier for the GABLS-1 weakly stable boundary layer (WSBL) as part of an LES-model intercomparison study (Beare et al., 2006). The LESs in the two SBLs were run for the same domain size [(400 m)^3], number of grid points (200), geostrophic wind speed (8 m/s), and initial thermal stratification. In the HET SBL, the surface temperature difference was obtained using spanwise homogeneous surface temperature strips alternating between two temperature values, with the same horizontal-mean surface temperature in both cases, i.e., the SBLs were cooled at the same rate.

In this paper, we use the LES fields from the SBLs to drive a Lagrangian particle dispersion model (LPDM) and investigate the differences in dispersion characteristics in the two SBLs. In an LPDM, thousands of source-released particles are tracked to obtain the concentration field and plume spatial statistics, e.g., mean height, vertical dispersion, etc. For the HET, the particles are released from a large array of sources within the cold and hot strips to create an ensemble of releases from which the mean, root-mean-square, etc. scalar concentrations are found. The differences in the concentration statistics especially the large scalar variance near the surface and SBL top in the HET relative to those in the HOM are discussed in terms of the differences in the surface temperature conditions and turbulence fields. In addition, we address the role of secondary circulations in the HET on the scalar variance differences, the weakening of the mean temperature gradient over the bulk of the HET SBL, and other characteristics.