Statistics of Surface-Layer Turbulence Over Terrain with Metre-Scale Heterogeneity

Abstract

Refuge has patchy vegetation in sandy soil. During midday and at night, the surface sources and sinks for heat and moisture may thus be different. Although the Sevilleta is broad and level, its metre-scale heterogeneity could therefore violate an assumption on which Monin-Obukhov similarity theory (MOST) relies. To test the applicability of MOST in such a setting, we measured the standard deviations of vertical (σw) and longitudinal velocity (σu), temperature (σt), and humidity (σq), the temperature-humidity covariance (¯tq), and the temperature skewness (St). Dividing the former five quantities by the appropriate flux scales (u*, *, and q*) yielded the nondimensional statistics σw/u*, σu/u*, σt/|t*|, σq/|q*|, and ¯tq/t*q*. σw/u*, σt/|t*|, and St have magnitudes and variations with stability similar to those reported in the literature and, thus, seem to obey MOST. Though σu/u* is often presumed not to obey MOST, our σu/u* data also agree with MOST scaling arguments. While σq/|q*| has the same dependence on stability as σt/|t*|, its magnitude is 28% larger. When we ignore ¯tq/t*q* values measured during sunrise and sunset transitions – when MOST is not expected to apply – this statistic has essentially the same magnitude and stability dependence as (σt/t*)2. In a flow that truly obeys MOST, (σt/t*)2, (σq/q*)2, and ¯tq/t*q* should all have the same functional form. That (σq/q*)2 differs from the other two suggests that the Sevilleta has an interesting surface not compatible with MOST. The sources of humidity reflect the patchiness while, despite the patchiness, the sources of heat seem uniformly distributed.