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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.

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.

A 140-m micrometeorological tower provides detailed observations of the vertical structure of the mean and turbulent fields of meteorological variables of a coastal region in southeastern Brazil, and reveals the extent to which a nearby power plant affects the local atmospheric boundary layer.

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 the WSL and 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 δu* on the mean wind speed.

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The mean wind speed at which the stable boundary layer (SBL) experiences a turbulence regime transition (Vr) depends on other flow characteristics, such as its thermal stability. Here, Vr variability is examined both at a single site and across sites using three multiple‐level datasets: Santa Maria, Cooperative Atmosphere–Surface Exchange Study–1999 (CASES‐99), and Fluxes over Snow Surfaces (FLOSS II). A method to determine Vr is introduced and validated. It is taken as the mean wind speed at which the vertical gradient of the turbulence velocity scale switches sign. Emphasis is given to the control exerted on Vr by quantities that are external to the SBL, such as radiation, roughness, and soil properties. In each of the experiments, Vr increases with net radiative loss at the surface (Rn) at a rate that is site‐dependent. It also increases for smaller roughness lengths, as indicated by its wind direction dependence. No conclusive relationship has been found between Vr and downward longwave radiation observed at the surface. The across‐site comparison indicates that soil heat capacity influences the rate at which Vr increases with Rn.

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