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Two contrasting flow regimes exist in the stable boundary layer (SBL), as evidenced from both observational and modeling studies. In general, numerical schemes such as those used in numerical weather prediction and climate models (NWPCs) reproduce a transition between SBL regimes. However, the characteristics of such a transition depend on the turbulence parameterizations and stability functions used to represent the eddy diffusivity in the models. The main goal of the present study is to detail how the two SBL regimes occur in single-column models (SCMs) by analyzing the SBL structure and its dependence on external parameters. Two different turbulence closure orders (first order and an E-l model) and two types of stability functions (short and long tail) are considered. The control exerted by the geostrophic wind and the surface cooling rate on the model SBL regimes is addressed. The model flow presents a three-layer structure: a fully turbulent, weakly stable layer (WSL) next to the surface; a very stable layer (VSL) above that; and a laminar layer above the other two and toward the domain top. It is shown that theWSLand VSL are related to both SBL regimes, respectively. Furthermore, the numerically simulated SBL presents the two-layer structure regardless of the turbulence parameterization order and stability function used. The models also reproduce other features reported in recent observational studies: an S-shaped dependence of the thermal gradient on the mean wind speed and an independence of the vertical gradient of friction velocity du* on the mean wind speed. [ABSTRACT FROM AUTHOR]

Observations of the vertical and temporal structure of the nocturnal boundary layer before and after a transition from the weakly to the very stable regime have been presented in Part I. Here, similar transitions are investigated using a one‐dimensional second‐order closure numerical model, with an energy budget solved at the surface. The transition is driven by a decreasing mean wind at the top of the domain, and simulations with different cloud covers and surface thermal properties are considered. The time of the transition depends on the wind speed at the top of the domain and on the "coupling strength" between the surface and the atmosphere, which is affected by the cloud cover and surface thermal properties. The vertical profiles and temporal evolutions of the terms of the budgets of turbulent kinetic energy (TKE), heat flux and temperature variance are presented. Of these, only TKE budget presents the same dominant terms in both regimes. Absolute heat flux in the model is proportional to the cube of the wind speed in the very stable regime. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Submeso motions add complexities to the structure of the stable boundary layer. Such motions include horizontal meandering and gravity waves, in particular when the large-scale flow is weak. The coexistence and interaction of such submeso motions is investigated through the analysis of data collected in Antarctica, in persistent conditions of strong atmospheric stratification. Detected horizontal meandering is frequently associated with temperature oscillations characterized by similar time scales (30 min) at all levels (2, 4.5 and 10 m). In contrast, dirty gravity waves superimposed on horizontal meandering are detected only at the highest level, characterized by time scales of a few minutes. The meandering produces an energy peak in the low-frequency spectral range, well fitted by a spectral model previously proposed for low wind speeds. The coexistence of horizontal and vertical oscillations is observed in the presence of large wind-direction shifts superimposed on the gradual flow meandering. Such shifts are often related to the variation of the mean flow dynamics, but also to intermittent events, localized in time, which do not produce a variation in the mean wind direction and that are associated with sharp decreases in wind speed and temperature. The noisy gravity waves coexisting with horizontal meandering persist only for a few cycles and produce bursts of turbulent mixing close to the ground, affecting the exchange processes between the surface and the stable boundary layer. The results confirm the importance of sharp wind-direction changes at low wind speed in the stable atmosphere and suggest a possible correlation between observed gravity waves and dynamical instabilities modulated by horizontal meandering. [ABSTRACT FROM AUTHOR]

Investigation of low-wind cases observed during the Urban Turbulent Project campaign (Torino, Italy) and at the Santa Maria meteorological station (Santa Maria, Brazil) provides insight into the wind-meandering phenomenon, i.e. large, non-turbulent oscillations of horizontal wind speed and temperature. Meandering and non-meandering cases are identified through analysis of the Eulerian autocorrelation functions of the horizontal wind-velocity components and temperature. When all three autocorrelation functions oscillate, meandering is present. As with weak turbulence, meandering shows no dependence on stability but is influenced by presence of buildings and depends on wind speed. We show that, while the standard deviation of the horizontal velocity is always large in low-wind conditions, the standard deviation of the vertical velocity shows very different behaviour in meandering and non-meandering conditions. In particular, the value of the ratio of the standard deviations of the vertical and horizontal velocities typifies the meandering condition. [ABSTRACT FROM AUTHOR]

• New method based on Eulerian auto-correlation functions to evaluate the coherent structures. • Estimation and parameterization of the impact of stability on the shear length scale. • Parametrization of the separation length scale between coherent vortices at the canopy top. Observations of the vertical structure of the turbulent flow in different stability regimes above and within the Amazon Forest at the Amazon Tall Tower Observatory (ATTO) site have been presented in Part I. Here, the influence of stability on the inception and development of coherent structures is investigated. According to the mixing-layer analogy the coherent vortices that dominate the turbulent flows at the canopy-atmosphere interface are generated by hydrodynamical instabilities triggered by an inflection in the vertical profile of the mean wind speed at or near the canopy top. The coherent motions time, T p e a k and separation length scale, Λ , depend on the shear length scale, L s , close to the canopy top. The present analysis studies the characteristics of L s , T p e a k and Λ in the five stability regimes defined in Part I. The behaviour of L s with stability is evaluated and parameterized. L s increases with decreasing stability, presenting two asymptotes for large unstable and stable stratification and a linear behaviour close to neutral stratification. Coherent structures and their timescale are detected with an original method based on the autocorrelation functions of 5-min subsets of turbulent quantities. The vertical time scale is larger in neutral conditions and decreases for both increasing and decreasing stability. At the canopy top the separation length scale presents a linear dependence on L s , whose slope is maximum in neutral conditions and decreases departing from neutrality. An original parameterization of the dependence of this slope on h / L , where h is the canopy height and L is the Obukhov length, is shown. Combining the parameterizations, the dependence of the separation length scale is finally presented. Λ has a maximum for slightly unstable conditions, linearly decreases towards neutral and weakly stable stratifications and then it tends to zero for very intense unstable and stable stratifications. [ABSTRACT FROM AUTHOR]

The parallelized large-eddy simulation model (PALM) was used to understand better the turbulent exchanges of a passive scalar above and within a forested region located in the central Amazon. Weak (2 ms − 1 ) and strong (6 ms − 1 ) wind conditions were simulated. A passive scalar source was introduced to the forest floor for both simulations. The simulations reproduced the main characteristics of the turbulent flow and of the passive scalar transport between the forest and the atmosphere. Noteworthily, strong and weak wind conditions presented different turbulence structures that drove different patterns of scalar exchange both within and above the forest. These results show how passive scalar concentration is influenced by the wind speed at the canopy top. Additionally, higher wind speeds are related to stronger sweep and ejection regimes, generating more intense plumes that are able to reduce the passive scalar concentration inside the forest canopy. This work was the first that used PALM to investigate scalar transport between the Amazon rainforest and the atmosphere. [ABSTRACT FROM AUTHOR]

This work investigates the characteristics of submeso motions and their influence on the dynamics of the atmospheric boundary layer in different stability conditions. Submeso motions generally refer to a complex mix of non-stationary stochastic processes of different nature on scales that separate the main turbulent eddies (few meters in stable conditions) and the smallest mesoscale motions (few kilometres). Submeso motions can take a variety of forms including gravity waves, density currents or drainage flows. During low-speed conditions submeso activities manifest themselves as horizontal meandering of the wind velocity vector. Meandering generally refers to large non-turbulent oscillations of the horizontal wind velocity components, associated to the oscillation of the wind direction and temperature. Although horizontal meandering is typically associated to stable stratifications of the boundary layer, wavelike oscillations associated to low turbulent activity have also been observed in diurnal conditions. Some insight in the understanding of the meandering dynamic has been achieved, but the causes of the meandering phenomenon and its complex interactions with turbulence are still poorly understood. While these modes may have a strong impact on the horizontal transport of pollutants in the presence of weak and intermittent turbulent mixing, their accurate parameterisation within weather and climate models remains an open question. In this work we study the role of radiative forcing on the generation of horizontal meandering in different stability conditions. To this purpose, measurements of wind velocity components, temperature, radiation, CO2 and water vapour concentrations carried out at two distinct sites (Santa Maria, Brazil; Ny-Ålesund, Svalbard, Arctic) were analysed. To detect meandering occurrences and their characteristic time‐scales Eulerian autocorrelation functions and spectral analysis of the wind components and temperature are employed. Horizontal meandering is identified by a clear negative lobe in the Eulerian autocorrelation functions corresponding to a peak located at low-frequency in the spectra of wind components and temperature. The performed analysis confirmed that horizontal modes represents a distinctive feature of the very stable boundary layer, characterised by a strong radiative cooling, low wind conditions and a weak and intermittent mechanical turbulent production. Horizontal meandering has been observed also during daytime in presence of low positive radiative forcing, often related to cloudy conditions and weak large scale flows. In all atmospheric conditions meandering appears strongly correlated to scalar oscillations, confirming its important role in the horizontal transport of pollutant in low-wind conditions. [ABSTRACT FROM AUTHOR]