Further Improvement of Surface Flux Estimation in the Unstable Surface Layer Based on Large‐Eddy Simulation Data

Abstract

The Monin-Obukhov similarity theory (MOST) is widely used for the surface turbulence flux-gradient relations in modeling and data analysis. Here we quantify multiscale turbulence processes by applying our newly developed analysis technique to large-eddy simulation data, and find that in the unstable surface layer, large convective eddies (with the scaling of boundary layer depth) and local free convection exist in addition to small eddies. An empirical MOST function (considering the last two processes only) is found to underestimate the surface friction velocity and heat flux both by about 30%. Much better results can be obtained using a function that explicitly considers all three processes. Generally, the nondimensional wind shear exhibits larger scatter and deviates more from the MOST than the temperature gradient. Based on these results, we propose the revised Sorbjan (1986, https://doi.org/10.1007/BF00120989) function (with coefficients determined from this study) for wind shear and MOST function for temperature gradient, for estimating surface fluxes in the unstable surface layer. The three-dimensional multiscale analysis method we develop in this study is of general nature and can be of interest for problems of three-dimensional multiscale process description in other disciplines.In this section, we will first present briefly the OPD technique and its extension to three dimensions (see Method S1 in the supporting information for details). The standard MOST and previous efforts to improve the flux-gradient relations for the unstable condition will be described as well. Finally, we will describe how the data used in this study are obtained via large-eddy simulation.

Connection Between Mass Flux Transport and Eddy Diffusivity in Convective Atmospheric Boundary Layers

“…On the other hand, increasing evidence about the impacts of large‐scale coherent eddies on the non‐local transport in the ASL (e.g., Khanna & Brasseur, 1997; Li et al., 2018; Liu et al., 2019; Salesky & Anderson, 2020; Zilitinkevich et al., 2006) highlights the need to extend parameterization based on MOS beyond the local transport. Furthermore, a recent study by Chor et al.…”

“…On the other hand, increasing evidence about the impacts of large-scale coherent eddies on the non-local transport in the ASL (e.g., Khanna & Brasseur, 1997;Li et al, 2018;Liu et al, 2019;Salesky & Anderson, 2020;Zilitinkevich et al, 2006) highlights the need to extend parameterization based on MOS beyond the local transport. Furthermore, a recent study by Chor et al (2020) has demonstrated the importance of both local (diffusive) and non-local (caused by organized turbulent motions) throughout the convective ABL using an objective diagnostic method.…”

The Effect of Surface Heating Heterogeneity on Boundary Layer Height and Its Dependence on Background Wind Speed

“…The dependence on heterogeneity scale is reduced by background wind flow. As found in our previous work (Liu et al, 2019), PBLH is a proper scaling parameter to account for large eddy effects on surface flux parameterization. Therefore, the findings regarding the effect of surface heterogeneity on PBLH in the current paper suggest that PBLH can act as a linkage between surface heterogeneity and surface turbulent fluxes.…”

Surface‐Layer Wind Shear and Momentum Transport From Clear‐Sky to Cloudy Weather Regimes Over Land

“…It also suggests, as shown by Liu et al. ( 2019 ) and Fodor et al. ( 2019 ), that empirical Monin‐Obukhov similarity functions do not explicitly include the effect of large scale up‐ and downdrafts associated with convective eddies (using the scaling of boundary layer depth) underestimate the momentum flux that is generated in the surface layer.…”

“…This would imply that for a similar wind gradient (large scale wind), more momentum flux is generated on those convective cloud days compared to clear sky days. It also suggests, as shown by Liu et al (2019) and Fodor et al (2019), that empirical Monin-Obukhov similarity functions do not explicitly include the effect of large scale up-and downdrafts associated with convective eddies (using the scaling of boundary layer depth) underestimate the momentum flux that is generated in the surface layer.…”