Different aspects of the stable boundary-layer structure are contrasted between the very stable and the weakly stable regimes from a new point of view. This study finds a limit wind speed, referred to as the crossover threshold, when the average vertical gradient of the turbulent kinetic energy switches sign at all observational levels. When the wind speed exceeds this transition, the entire stable boundary layer becomes vertically fully coupled. Consequently the very stable boundary layer in this study is considered as a decoupled regime, while the weakly stable state is referred to as a coupled regime. It is shown that the vertical profiles of other quantities, such as friction velocity, heat flux and thermal gradients are strikingly different between the two coupling states.Decomposition of turbulent kinetic energy and heat flux into temporal scales indicates overlapping of non-turbulent sub-mesoscale flow with turbulence in the decoupled case, while there is a clearer scale distinction between the two types of motions when coupling takes place. The turbulent kinetic energy budget is dominated by dissipation and shear production in both coupling states. However, the relative importance of the buoyant destruction term is shown to be appreciably larger in the decoupled regime. In the heat flux budget equation, buoyant destruction is larger in magnitude than production by the thermal gradient in the decoupled case, but not when there is full coupling. These results indicate that the surface heat flux plays a major role in controlling the stable boundary-layer state, as previously proposed. For the entire dataset, the frequency distributions of turbulence quantities near the surface are shown to be bimodal. The two modes are associated with the two coupling states, each well described by independent log-normal distributions.

The determination of nocturnal surface fluxes in low wind conditions is a major problem for micrometeorological studies. The eddy correlation technique, extensively used in field measurements, becomes inappropriate if not enough turbulent activity exists. At the same time, the phenomenon of turbulence intermittency is responsible for the existence of localized events of short duration within which a large fraction of the total nighttime scalar exchange occurs. The scalar flux within a certain intermittent event varies considerably depending on the window used for the flux calculation. In many cases, events with very different time durations occur in the same night, and therefore, the proper determination of the surface flux would require averaging within data windows of different sizes for each event. In this work, the surface exchanges of temperature, moisture and carbon dioxide are analysed at a micrometeorological tower in southern Brazil. Intermittent turbulence is a common occurrence at the location. The analysis shows that the fluxes vary with turbulence intensity and the estimation technique. A variable-window size method for flux estimation is suggested and shown to produce an increase in the magnitude of the nocturnal surface fluxes.

Average heat and momentum fluxes observed by a network of surface stations during the Hudson Valley Ambient Meteorology Study (HVAMS) were found as functions of a spatially representative bulk Richardson number Ri br . Preferential sites were identified for the occurrence of strong turbulence under mesoscale stability conditions common to all stations. Locally sensed turbulence intermittency depends on the mesoscale flow stability. Nearly continuous turbulence with few long-lived intermittent events occurs when Ri br , Ri cr , the critical gradient Richardson number. Less-continuous mixing associated with a larger number of events occurs when Ri cr , Ri br , 5, with the weakest turbulence and fewer events observed for Ri br Ri cr . It was found that the need to allow for extra mixing above the conventional critical bulk Richardson number in numerical weather prediction models is primarily a consequence of spatial averaging in a heterogeneous landscape and is secondarily the result of turbulence above Ri cr at locations with ''nonideal fetch.''

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