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.

Previous observational studies in the stable boundary layer diverge appreciably on the values of dimensionless ratios between turbulence-related quantities and on their stability dependence. In the present study, the hypothesis that such variability is caused by the influence of locally dependent nonturbulent processes, referred to as submeso, is tested and confirmed. This is done using six datasets collected at sites with different surface coverage. The time-scale dependence of wind components and temperature fluctuations is presented using the multiresolution decomposition, which allows the identification of the turbulence and submeso contributions to spectra and cospectra. In the submeso range, the spectra of turbulence kinetic energy range increases exponentially with time scale. The exponent decreases with the magnitude of the turbulent fluctuations at a similar manner at all sites. This fact is used to determine the smaller time scale with relevant influence of submeso processes and a ratio that quantifies the relative importance of such nonturbulent processes with respect to turbulence. Based on that, values for the local stability parameter that are unaffected by nonturbulent processes are found. It is shown that the dimensionless ratios do not usually converge to a given value as the time scale increases and that it is as a consequence of the locally dependent submeso influence. The ratios and their stability dependence are determined at the time scales with least influence of nonturbulent processes, but significant site-to-site variability persists. Combining all datasets, expressions for the dependence of the dimensionless ratios on the local stability parameter that minimize the role of the submeso contribution are proposed.

A model for the exchange between the surface and the atmosphere under stable conditions is proposed. It is based on the classical scheme first suggested by Blackadar and comprises prognostic equations for the wind components and air and ground temperature. The main difference from previous works consists in the fact that the turbulent intensity is determined by a prognostic equation for turbulent kinetic energy (TKE), rather than by using stability functions that arbitrarily relate it to atmospheric stability. Results show that the model reproduces the condition of connection and disconnection between the surface and upper levels. Furthermore, it leads to periodic turbulence bursts when one level within the stable boundary layer (SBL) is considered and the use of additional levels increasingly leads to more complex solutions, characterizing the occurrence of global intermittency. Such turbulence bursts occur in the disconnected state and cause large fluctuations of the variables near the surface. The boundary layer height plays a role in the sense that for the same geostrophic winds, connection is favored for shallower layers. Although playing a role in the intermittency characteristics, soil type is not determinant to their existence, as the bursts occur even for very high values of heat capacity. Vertical profiles for both the intermittent and connected state are analyzed and in general agree with observations. It is shown that, near the surface, weak turbulence bursts favor the exchange between the air and the cooler ground, leading to a local temperature decrease, while stronger events that mix the air deeper in the SBL cause an average warming tendency. An opposite pattern occurs at the upper SBL. Intermittency is favored over a range of low geostrophic winds and clear skies, in agreement with previous suggestions. The vertical structure of the intermittent events is analyzed, and it shown that they are generated at the surface by a local shear increase above a threshold, propagating upward through the turbulence transfer term in the TKE equation. It is proposed that such events constitute a natural characteristic of the disconnected SBL, which occurs along with low large-scale winds and clear skies.

Oscillations in the horizontal components of the wind velocity associated with oscillations in air temperature during low–wind speed episodes are ubiquitous in the stable boundary layer and are labeled as wind meandering. The meandering structure is recognizable by a clear negative lobe in the Eulerian autocorrelation functions of the horizontal wind velocity components and of the sonic temperature and by a corresponding peak at low frequency in the velocity components and temperature spectra. These distinctive features are used to isolate meandering occurrences and to study its properties in relation to the classical description of the planetary stable boundary layer. It is shown that the ratio of the variance of the wind velocity vertical component over the variance of the composite of the wind velocity horizontal components splits the frequency distribution of meandering and nonmeandering events and divides the nocturnal boundary layer in two different regimes characterized by different turbulent properties. The data comparison with a turbulence model based on Rotta return to isotropy showed that meandering and nonmeandering cases may have similar dynamics. This suggests that meandering may not be connected to a laminarization of the flow and shows that the Rotta scheme may still describe the energetic transfer between wind velocity components in the very stable boundary layer if the Rotta similarity constant c depends on the flux Richardson number. The data confirm a c value of 2.2 for Ri_f = 0 compatible with its conventional value. The analysis presented refers to one year of continuous measurements on 10 levels carried out at a coastal site in southeastern Brazil.

The nocturnal boundary‐layer regime transition from weakly stable (strong wind) to very stable (weak wind) is analyzed using 10 levels of turbulence observations made at a 140 m micrometeorological mast near the southeastern Brazilian coast. The combination of synoptic and local flow favors the systematic occurrence of such a transition, typically 5 to 7 h after sunset. The regime transition is marked by decreases in temperature, wind speed, turbulent kinetic energy (TKE) and absolute heat flux. The decrease in temperature is often abrupt and the inflection point in the temperature series marks the regime transition. Absolute heat flux peaks before the transition during the weakly stable period, while temperature variance peaks near the transition. Composites from 36 cases when the cooling rate exceeded 2 °C/h are used to describe the vertical structure of the stable boundary layer (SBL) in both regimes. For these abrupt transitions, dimensionless variables that relate thermal and mechanical properties of the flow are compared as indicators of the SBL regime, and the gradient Richardson number is found to be better for that purpose. The absolute heat flux is shown to be proportional to the cube of the wind speed only in the strong wind limit of the very stable regime. Simulations of similar transitions using a second‐order model are described in Part II.