# Effect of the land surface thermal patchiness on the Atmospheric Boundary Layer through a quantification of the dispersive fluxes

## TL;DR

This study uses large-eddy simulations to quantify how land surface thermal patchiness influences the atmospheric boundary layer, revealing significant dispersive flux contributions and proposing new parameterizations for complex surface conditions.

## Contribution

It introduces a novel quantification of dispersive fluxes due to land surface thermal heterogeneity at small scales using large-eddy simulations.

## Key findings

- Dispersive fluxes can contribute over 40% of sensible heat flux at 0.1z_i.
- Surface thermal heterogeneity effects depend on a new non-dimensional parameter.
- Two regimes identified: flow driven by heterogeneity and rapid blending leading to homogeneous fluxes.

## Abstract

While advances in computation are enabling finer grid resolutions in numerical weather prediction models, representing land-atmosphere exchange processes as a lower boundary condition remains a challenge. This partially results of the fact that land-surface heterogeneity exists at all spatial scales and its variability does not `average' out with decreasing scales. The work here presented uses large-eddy simulations and the concept of dispersive fluxes to quantify the effect of surface thermal heterogeneity with scales $\sim$ 1/10th 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. 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 about 0.1$z_i$, with a value of 5 to 10\% near the surface, 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 match those obtained over an equivalent thermally homogeneous surface. This character seems to be governed by a new non-dimensional parameter representing the ratio of the large-scale advection effects to the convective turbulence. We believe that results from this research are a first step in developing new parameterizations appropriate for non-canonical atmospheric surface layer conditions.