Impact of Mesoscale Motions on Monin-Obukhov Similarity and Surface Energy Balance over a Homogeneous Surface

Abstract

Mesoscale motions are an important factor influencing the applicability of Monin-Obukhov similar theory (MOST) and the surface energy balance. The heterogeneous surface and the vegetation-covered surface also increase the complexity of these two problems. In order to understand the effect of mesoscale motions on turbulent fluxes more clearly, this study analyzes the impact of mesoscale motions on MOST and surface energy balance using the multiresolution decomposition method and the observational data over a homogeneous bare soil surface. Two conclusions are obtained: (1) When mesoscale motions are excluded, the scatter of similarity relations is evidently reduced; the observed values of the dimensionless velocity gradient and dimensionless temperature gradient in stable conditions become less than the values estimated using the universal functions, and the flux-variance relationship for temperature clearly deviates from the −1/3 power law. (2) When mesoscale motions are excluded, the number of energy balance closure outliers at night is dramatically reduced, and the energy balance ratio (EBR) is increased. However, when turbulent mixing is weak, even if mesoscale motions are excluded, the EBR is still relatively low, which is possibly due to the fact that the energy is mainly transported by mesoscale motions instead of turbulence.

Flux–Gradient Relationships Below 2 m Over a Flat Site in Complex Terrain

“…Nevertheless, many experiments have been made in locations where significant surface heterogeneity was present, such as land-water discontinuities (Grachev et al 2018) or the presence of natural and farming land (Huo et al 2015). The local circulations that these terrain discontinuities induce may add dispersion in the flux estimations when using the classical formulations (Guo and Zuo 2017). Other perturbing factors are the presence of relevant canopies (Novick and Katul 2020), or humidity advections (Haghighi and Or 2015), especially in the case of contrasting semi-arid and well-watered terrains (Li et al 2019).…”