Surface Thermal Heterogeneities and the Atmospheric Boundary Layer: The Relevance of Dispersive Fluxes

• Published in: Boundary-layer meteorology 2020-06, Vol.175 (3), p.369-395
• Main Authors: Margairaz, Fabien, Pardyjak, Eric R., Calaf, Marc
• Format: Article
• Language: English
• Subjects: Analysis; Atmospheric boundary layer; Atmospheric flows; Atmospheric models; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Boundary layers; Computational fluid dynamics; Computer applications; Computer simulation; Dispersion; Earth and Environmental Science; Earth Sciences; Enthalpy; Heat exchange; Heat flux; Heat transfer; Heterogeneity; Large eddy simulation; Large eddy simulations; Meteorology; Numerical prediction; Oceanic eddies; Planetary boundary layer; Prediction models; Research Article; Scaling; Sensible heat; Sensible heat flux; Sensible heat transfer; Spatial distribution; Surface boundary layer; Surface layers; Time dependence; Turbulence; Weather; Weather forecasting
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-020-00509-w

While the increasing availability of computational power is enabling finer grid resolutions in numerical-weather-prediction models, representing land–atmosphere exchange processes remains challenging. This partially results from the fact that land-surface heterogeneity exists at all spatial scales, and its variability does not necessarily ‘average’ out with decreasing size. The work presented here uses large-eddy simulations and the concept of dispersive fluxes to quantify the effects of a surface that is thermally inhomogeneous (with scales that are approximately 10% of the height of the atmospheric boundary layer), but uniformly rough. These near-canonical cases describe inhomogeneous scalar transport over a broad range of unstable atmospheric flows. Results illustrate the existence of a regime where the mean flow is mostly driven by the surface thermal heterogeneities. In this regime, the contribution of the dispersive fluxes can account for more than 40% of the total sensible heat flux at 100 m above the ground and about 5–10% near the surface. This result is independent of the spatial distribution of the thermal heterogeneities and weakly dependent on the averaging time used to define the dispersive fluxes. Additionally, an alternative regime exists where the effects of the surface thermal heterogeneities are quickly blended and the dispersive fluxes match those obtained over an equivalent homogeneous surface. Results further illustrate the existence of a new cospectral scaling for the dispersive sensible heat fluxes that differs from the traditional turbulence cospectral scaling. We believe that these results might elucidate pathways for developing new parametrizations for the non-canonical atmospheric surface layer.