Monin-Obukhov Similarity Scaling Over Contrasting Surfaces During the Morning and Evening Transition

Monin-Obukhov Similarity Theory (MOST) is the basis for nearly all flux-gradient relationships within the atmospheric surface layer. It is found to some degree in nearly all numerical weather models and is used extensively to organize and communicate mean and turbulent flow statistics. Two of the most limiting assumptions behind its derivation are that the surface is flat, homogeneous and that the flow is quasi-stationary. Sloping, rough terrain and morning and evening transitional periods are notable violations of these assumptions. Here, data are used from the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program to examine the applicability of MOST over rough, sloping surfaces as well as during transitional periods in the morning and evening.

MATERHORN is a multi-institution, multi-disciplinary project designed to improve weather predictability in mountainous terrain. Two field campaigns were conducted at Dugway Proving Ground in Utah's West Desert, USA, from 25 September – 21 October 2012 and from 1 May – 30 May 2013. We present results from three flux towers situated over broadly different surface types. Two of the sites were located on flat terrain and one was situated on 6° slope. One of the flat sites and the slope site can be characterized as desert steppe with sparse vegetation (~ 1m tall), the second flat site as playa (dry lake bed with smooth surface and no vegetation). At all three sites, momentum and sensible heat fluxes were measured at multiple heights (5 or 6 levels up to 28 m agl) using sonic anemometers and fine wire thermocouples, and arrays of 20-25 thermocouples measured temperatures between 1 cm and 3.2 m agl.

We find that MOST performs adequately for the flat sites under moderate stabilities. The universal functions, ϕ_h and ϕ_m, show an asymptotic behavior during transitional stabilities at all three sites but the behavior is much more pronounced at the slope site. Counter-gradient heat fluxes, where the heat flux vector points in the direction of increasing temperature, are observed during the morning and evening transition at all three sites. At the desert steppe sites the temperature gradient changes sign before the heat flux does while the exact opposite occurs at the plays site. Finally, the inclusion of slope effects in MOST is investigated.