Large‐eddy simulation of very stable boundary layers. Part I: Modeling methodology

Abstract

The large‐eddy simulation of very stable boundary layers (SBLs) remains challenging due to the large anisotropy of turbulent motions resulting from buoyancy suppression effects, and to the general reduction of its energetic scales. Here, we develop a novel large‐eddy simulation model setup to enable the modeling of the very SBL, avoiding the use of the Monin–Obukhov similarity theory at its lower boundary condition, as it breaks down in these conditions in agreement with previous studies. The new approach requires very fine grids in the vertical direction, in the order of a few centimeters. A series of Ekman‐layer‐type boundary layers under weak‐wind conditions (unormalg=1$$ {u}_{\mathrm{g}}=1 $$ and 2 m·$$ \cdotp $$sprefix−1$$ {}^{-1} $$), and strong surface cooling rates (1 and 3 K·$$ \cdotp $$hrprefix−1$$ {}^{-1} $$) at high latitude is studied using isotropic grids of 0.05 and 0.10 m resolution. The low‐order statistics show some sensitivity to the grid resolution; for example, both the SBL height and the low‐level jet position decrease with increasing grid resolution. In general, some differences are spotted regarding the shape of the wind speed and temperature vertical profiles, compared with weakly to moderately stratified conditions, suggesting that the structure and characteristics of the SBL evolve with increasing stratification. Moreover, the wind turning is significant from the surface, where the surface wind angle relative to the geostrophic wind is about 50°$$ {}^{\circ} $$. Lastly, two regimes within very stable conditions can be qualitatively identified primarily based on the gradient Richardson number (Ri$$ Ri $$) profiles: In the less stable regime, Ri$$ Ri $$ increases almost linearly with height and a quasi‐steady state is achieved; however, in the most stable regime, Ri$$ Ri $$ sharply increases near the surface and remains constant throughout the SBL while increasing with time up to Riprefix≈0.4$$ Ri\approx 0.4 $$. Thus, a quasi‐steady state is not achieved; and although turbulence does not collapse, it remains weak.

Large‐eddy simulation of very stable boundary layers. Part II: Length scales and anisotropy in stratified atmospheric turbulence

“…The simulations correspond to an Ekman‐layer‐type boundary layer under weak‐wind conditions ($$$ {u}_{\mathrm{g}}=1 $$$ and 2 m$$$ \cdotp $$$s$$$ {}^{-2} $$$) and strong surface cooling rates ($$$ {C}_{\mathrm{r}}=1 $$$ and 3 K$$$ \cdotp $$$hr$$$ {}^{-1} $$$) at high latitudes. Very high grid resolutions were used in a uniform and isotropic grid: $$$ \Delta =0.05 $$$ and 0.10 m. Further details on the LES modeling methodology, and on the temporal evolution of the first‐ and second‐order moments of these simulation can be found in Part I (Chinita et al ., 2022). The main conclusions of the present study are as follows:The Ozmidov and Corrsin scales decrease with increasing stability.…”

“…Our goal is to investigate the turbulence's character and structure of strongly SBLs under weak‐wind conditions using the set of LES runs of an Ekman‐layer‐type boundary layer described in Chinita et al . (2022) (hereafter referred as Part I). Strongly stratified turbulent flows have been studied in several previous studies (e.g., Brethouwer et al ., 2007; Chung and Matheou, 2012; Khani, 2018; MacDonald et al ., 2020; Maffioli et al ., 2016; Shih et al ., 2005).…”

On Predicting Offshore Hub Height Wind Speed and Wind Power Density in the Northeast US Coast Using High-Resolution WRF Model Configurations during Anticyclones Coinciding with Wind Drought

“…Our project is a partnership with Eversource-Orsted focusing on the South Fork Wind Farm (SFWF) in the Northeast U.S. coast. The relatively high model resolution is chosen due to our collaboration with the research group that has developed the University of Connecticut's large-eddy simulation model to depict wind turbines at a very fine scale [30], using the WRF output as the initial and boundary conditions to the LES model. For the domain selection, we considered available offshore observations covering the northeast U.S. coastline, especially near the South Fork Wind Farm (Figure 1).…”

Numerical insights into turbulent penetrative convection over localized heat sources

“…The penetrative convection describes the vertical heat flux transport in an initially stably thermally stratified environment, [1][2][3][4][5][6][7][8][9]. This situation is typical for the onset of the early morning heating of the earths surface caused by incoming solar radiation.…”