Stepping Towards New Parameterizations for Non-Canonical Atmospheric Surface-Layer Conditions, a Dispersive-Flux Approach

While advances in computation are enabling finer grid resolutions in numerical weather prediction (NWP) models, representing land-atmosphere exchange processes as a lower boundary condition remains a challenge. This is partially a result of the fact that land-surface heterogeneity exists at all spatial scales and its variability does not "average" out with decreasing scales. Such variability need not rapidly blend away from the boundary thereby impacting the near-surface region of the atmosphere. This near-surface region is host to myriad of important meteorological processes such as fog, frost, dew, near surface convection, thermal inversions, subsidence, as well as intense momentum jets, which if not accounted for, can distort weather forecasts. Traditionally, momentum and energy fluxes linking the land surface to the flow in NWP models have been parameterized using atmospheric surface layer (ASL) similarity theory. There is ample evidence suggesting that this is acceptable for stationary and planar-homogeneous flows in the absence of subsidence (Stull, 1988; Kaimal & Finnigan, 1994). However, heterogeneity remains a ubiquitous feature eliciting appreciable deviations when using ASL similarity theory, especially in scalars such moisture and air temperature whose blending is less efficient when compared to momentum (Brutsaert, 1982; Li et al. 2015). Characterizing all aspects of land-surface heterogeneity such as land-cover, topography, transitions, stability and their effect on NWP is well outside the scope of a single project. However, fundamental aspects of such heterogeneity associated with temperature can still be studied and utilized to improve the lower-boundary representation of NWP. In particular, the focus of this project is to quantify the effect of surface thermal heterogeneity with scales ~O(1/10) the height of the atmospheric boundary layer and characterized by uniform roughness. Such near-canonical cases describe inhomogeneous scalar transport in an otherwise planar homogeneous flow when thermal stratification is weak or absent. In this work, we present a suite of large-eddy simulation cases that characterize the effect of surface thermal heterogeneities on the atmospheric flow using the concept of dispersive fluxes (Finnigan, 2000).

Results illustrate a regime in which the flow is mostly driven by the surface thermal heterogeneities, in which the contribution of the dispersive fluxes can account for more than 40% of the total sensible heat flux at a height of 100 meters, regardless of the spatial distribution of the thermal heterogeneities, and with a weak dependence on time averaging. Results also illustrate an alternative regime in which the effect of the surface thermal heterogeneities is quickly blended, and the dispersive fluxes provide, instead, a quantification of the flow spatial heterogeneities produced by coherent turbulent structures result of the surface shear stress. We believe that results from this research are a first step in developing new parameterizations appropriate for non-canonical ASL conditions.