The Coupling Strength of the Stable Boundary Layer - Impacts on the Nocturnal Energy Budget Determination

Nocturnal observations from a large variety of sites, such as Oklahoma Mesonet weather stations and AmeriFlux flux stations, consistently show a linear increase of the mean wind speed (V) with the net radiative loss (|Rn|) for conditions when such a radiative loss is large enough. The rate of increase is variable among the sites, and in most of them the linear relationship observed for large |R_n| can be extrapolated to near the origin of the V x R_n diagram. We show a detailed analysis of mean and turbulent atmospheric variables under such conditions that support the idea that this linear increase approaches the separation between the two nocturnal turbulence regimes of weakly and very stable boundary layer. Based in these results, the concept of "coupling strength" is introduced as the net radiative loss that can be sustained by unit of mean wind speed in the nocturnal boundary layer. Therefore, the minimum wind speed that supports sustainable turbulence (V_r) increases linearly with the net radiative loss at a rate that decreases with increasing coupling strength.

It is shown that sensible and latent turbulent heat fluxes are dependent on the wind speed but not on net radiation for V<V_r and on net radiation, but not on wind speed for V>V_r. The implications for energy budget closure are discussed, and long-term datasets are used to investigate how the coupling strength evolves in response to long-term climate variability. Implications on average nocturnal fluxes of sensible and latent heat are addressed.