Surface Thermal Heterogeneities and the Atmospheric Boundary Layer: The Thermal Heterogeneity Parameter
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Surface Thermal Heterogeneities and the Atmospheric Boundary Layer: The Thermal Heterogeneity Parameter Fabien Margairaz1
· Eric R. Pardyjak1 · Marc Calaf1
Received: 21 August 2019 / Accepted: 29 June 2020 © Springer Nature B.V. 2020
Abstract Representing land–atmosphere exchange processes at the ground surface of numericalweather-prediction models remains a challenge in spite of the recent advances in computing. Previous studies investigating the effects of spatial surface heterogeneities have been viewed from a turbulence perspective, mostly assuming the existence of a blending length scale above which surface-induced perturbations are modelled using an ad hoc bulk surface parameter representing a pseudo-equivalent surface condition. While these types of approaches can generate reasonable results, they fail to account for the long-lasting spatial perturbations that modify the mean flow. In this work, the interactions between the characteristic scales of surface thermal heterogeneities and the mean resolved fluid dynamics are investigated for a broad range of unstable atmospheric conditions. Thermal dispersive fluxes, which naturally appear as a means to account for persistent-in-time advection fluxes generated by unresolved spatial heterogeneities, provide a quantification of the interaction between surface thermal heterogeneities and the atmospheric boundary-layer mean flow. Hence, they also provide a deterministic approach for including the effect of unresolved processes on the mean flow. We introduce a new non-dimensional number (i.e., the heterogeneity parameter) that can be used to identify the flow conditions and surface configurations in which heterogeneity effects become important. The heterogeneity parameter can be used to distinguish cases with high and low dispersive-flux contributions based on the mean flow and characteristics of the thermal heterogeneities. These results suggest that under weak geostrophic forcing, surface heterogeneity effects should be accounted for in numerical-weather-prediction models. Keywords Convective boundary layer · Dispersive flux · Heterogeneity · Large-eddy simulation · Sensible heat flux
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Marc Calaf [email protected] Fabien Margairaz [email protected]
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Department of Mechanical Engineering, University of Utah, Salt Lake City, UT, USA
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F. Margairaz et al.
1 Introduction Land-surface thermal heterogeneities are ubiquitous in nature, and their effect on atmospheric flows is neglected, or at best, coarsely parametrized in numerical-weather-prediction (NWP) models by more or less accurate corrections to the traditional Monin–Obukhov similaritybased atmospheric surface-layer (ASL) parametrizations. While under certain atmospheric forcing conditions, such as strong geostrophic forcing, it has been shown that perturbations induced by surface thermal heterogeneities are quickly blended and have negligible impact on the flow, in other instances the effect of surface thermal heterogeneities should be considered and hence properly represented in NWP
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