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.

中文翻译：

---

地表热不均匀性和大气边界层：色散通量的相关性

虽然越来越多的计算能力使数值天气预测模型能够获得更精细的网格分辨率，但表示陆地 - 大气交换过程仍然具有挑战性。这部分是由于地表异质性存在于所有空间尺度这一事实，其变异性不一定随着规模的减小而“平均化”。此处介绍的工作使用大涡模拟和色散通量的概念来量化热不均匀（尺度约为大气边界层高度的 10%）但均匀粗糙的表面的影响。这些接近标准的情况描述了在广泛的不稳定大气流动范围内的非均匀标量传输。结果表明存在平均流主要由表面热不均匀性驱动的状态。在这种情况下，色散通量的贡献可以占地面以上 100 m 处总感热通量的 40% 以上，在地表附近约占 5-10%。该结果与热异质性的空间分布无关，并且微弱地依赖于用于定义色散通量的平均时间。此外，存在一种替代方案，其中表面热异质性的影响迅速混合，并且色散通量与在等效均匀表面上获得的那些相匹配。结果进一步说明了与传统湍流共谱标度不同的色散感热通量的新共谱标度的存在。