Contrasting structures between the decoupled and coupled states of the stable boundary layer

Abstract

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.

Heat Flux in the Strong-Wind Nocturnal Boundary Layer

“…This threshold has been examined in a number of other recent studies (e.g. Van de Wiel et al 2012;Acevedo et al 2015). This threshold V appears to some degree in all of the datasets considered here (Fig.…”

Early Warning Signals for Regime Transition in the Stable Boundary Layer: A Model Study

“…Several observational studies indeed show a threshold wind speed for the existence of continuous turbulence (e.g. King 1994;Sun et al 2012;Acevedo et al 2015). More recent studies provide compelling observational evidence for the division into two regimes based on MSHF theory (Hooijdonk et al 2015;Monahan et al 2015;Bonin et al 2015).…”

“…Mahrt 1998;Fernando and Weil 2010;Monahan et al 2011;Sun et al 2012;Mahrt 2014;Bonin et al 2015;Acevedo et al 2015;Hooijdonk et al 2015). The weakly stable boundary layer (WSBL) generally occurs when winds are strong or when clouds are present.…”

The Influence of the Intermittent Behavior of the Nocturnal Atmospheric Flow on the Prediction of the Diurnal Temperature Range: A Simplified Model Analysis

“…The relative magnitude of those two terms from the turbulent kinetic energy budget equation ultimately leads to two distinct regimes in stable boundary layer (SBL) [6][7][8][9]. A decoupled regime is characterized by light winds and weak turbulence that can alternate with periods with more intense turbulence, in a phenomena known as global intermittency, or just intermittency [6].…”

“…During the same night, the flow can switch regimes, and those changes can occur intermittently or in an organized way and can lead to temperature variability as large as 10 K [11]. Recent studies [8][9][10] have identified that to a large extent the SBL regime is solely determined by mean wind speed. Anyhow, atmospheric models do not usually represent well 2 Advances in Meteorology the SBL variability of regimes, and it is pointed out as one of the causes of the unsuccessful prediction of the DTR behavior [12].…”