Assessment of Monin-Obukhov scaling over small slopes

Abstract

 In this note we combine two recent findings related to sloped stably-stratified boundary layers (SBL). One of them contrasts Monin-Obukhov height, L, and the low-level jet (LLJ) height, zj, induced by simple katabatic flows. Another result connects turbulent Prandtl number, Pr, to gradient Richardson number, Ri. In this way, one finds the ratio of the two heights (L/z_j) as a single value function of the underlying slope and Ri, thus giving a criterion where L may not be the most relevant near-surface layer scaling parameter for turbulent processes. For (L/z_j)) > 1 it is the LLJ which determines most of near-surface turbulent properties. This has significant consequences on properties of various near-surface fluxes as treated in NWP, air-chemistry and wind-energy exploring models because most of them deploy L in one or another way for describing the lower boundary condition. This note shows that for ever finer horizontal resolution in our models, L should accommodate slope effects on the near-surface turbulent fluxes.