Numerical Simulations of the Lagrangian Averaged Navier-Stokes (Lans-α) Equations for Forced Homogeneous Isotropic Turbulence

The modeling capabilities of the Lagrangian Averaged Navier-Stokes-α equations (LANS-α) is investigated in statistically stationary three-dimensional homogeneous and isotropic turbulence. The predictive abilities of the LANS-α equations are analyzed by comparison with DNS data. Two different forcing techniques were implemented to model the energetics of the energy containing scales. The resolved flow is examined by comparison of the energy spectra of the LANS-α and the DNS computations; furthermore, the correlation between the vorticity and the eigenvectors of the rate of the resolved strain tensor is studied. We find that the LANS-α equations captures the gross features of the flow while the wave activity below a given scale α is filtered by the non- linear dispersion

Quantifying microburst wind and turbulence enhancement in canyons

Thunderstorms in arid and semi-arid regions like the U.S. intermountain west are often associated with dry microbursts. These microbursts are caused by evaporating precipitation in dry environments causing strong and difficult-to-observe cold outflow boundaries associated with rapid changes in wind and turbulence. A particularly dangerous situation can occur when microburst outflow winds are terrain-channeled into and within canyons during ongoing wildfire events, creating complex tactical challenges for firefighters and emergency managers. Given the dangers to firefighters by unpredictable microbursts and outflow boundaries within canyons, this paper quantifies the canyon-enhancement of wind and turbulence from microburst outflow boundaries using idealized large eddy simulations from the Weather Research and Forecasting (WRF) model. A series of simulations were conducted with the center of microburst downdrafts were placed 1.3 and 3.3 km upwind of a series of canyon types differing in length and slope angle.&nbsp;These canyon simulations are compared to microburst outflow boundary characteristics in flat terrain deriving topographic multiplier and differences in&nbsp;horizontal winds (wsp), upward vertical velocity (w), and turbulence kinetic energy (TKE).&nbsp;The increase in these variables is larger when the microburst is closer to the canyon and for steeper canyon walls generating increase in&nbsp;wsp&nbsp;by 4.5-6.6 m s-1,&nbsp;w&nbsp;by 1.9-8.4 m s-1,&nbsp;TKE&nbsp;by 1.4-6.6 m2&nbsp;s-2.&nbsp;&nbsp;The topographic multiplier for horizontal winds is 0.1-0.2 times higher within the long-distance canyons compared to the short-distance canyons. &nbsp; Branko Kosović, National Center for Atmospheric Research, Boulder, Colorado Luchetti, Nicholas, National Weather Service, Raleigh, North Carolina Friedrich, Katja, Atmospheric and Oceanic Sciences (ATOC), University of Colorado Boulder, Boulder, Colorado</p

WRF-LES Simulation of the Boundary Layer Turbulent Processes during the BLLAST Campaign

A real case long-term nested large eddy simulation (LES) of 25-day duration is performed using the WRF-LES modelling system, with a maximum horizontal grid resolution of 111 m, in order to explore the ability of the model to reproduce the turbulence magnitudes within the first tens of metres of the boundary layer. Sonic anemometer measurements from a 60-m tower installed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign are used for verification, which is focused on the turbulent magnitudes in order to assess the success and limitations in resolving turbulent flow characteristics. The mesoscale and LES simulations reproduce the wind speed and direction fairly well, but only LES is able to reproduce the energy of eddies with lifetimes shorter than a few hours. The turbulent kinetic energy in LES simulation is generally underestimated during the daytime, mainly due to a vertical velocity standard deviation that is too low. The turbulent heat flux is misrepresented in the model, probably due to the inaccuracy of the sub-grid scheme

Coupled mesoscale-LES modeling of a diurnal cycle during the CWEX-13 field campaign: From weather to boundary-layer eddies

Multiscale modeling of a diurnal cycle of real‐world conditions is presented for the first time, validated using data from the CWEX‐13 field experiment. Dynamical downscaling from synoptic‐scale down to resolved three‐dimensional eddies in the atmospheric boundary layer (ABL) was performed, spanning 4 orders of magnitude in horizontal grid resolution: from 111 km down to 8.2 m (30 m) in stable (convective) conditions. Computationally efficient mesoscale‐to‐microscale transition was made possible by the generalized cell perturbation method with time‐varying parameters derived from mesoscale forcing conditions, which substantially reduced the fetch to achieve fully developed turbulence. In addition, careful design of the simulations was made to inhibit the presence of under‐resolved convection at convection‐resolving mesoscale resolution and to ensure proper turbulence representation in stably‐stratified conditions. Comparison to in situ wind‐profiling lidar and near‐surface sonic anemometer measurements demonstrated the ability to reproduce the ABL structure throughout the entire diurnal cycle with a high degree of fidelity. The multiscale simulations exhibit realistic atmospheric features such as convective rolls and global intermittency. Also, the diurnal evolution of turbulence was accurately simulated, with probability density functions of resolved turbulent velocity fluctuations nearly identical to the lidar measurements. Explicit representation of turbulence in the stably‐stratified ABL was found to provide the right balance with larger scales, resulting in the development of intra‐hour variability as observed by the wind lidar; this variability was not captured by the mesoscale model. Moreover, multiscale simulations improved mean ABL characteristics such as horizontal velocity, vertical wind shear, and turbulence. Multiscale modeling of a real‐world diurnal cycle is presented for the first time, enabled by the generalized cell perturbation method The multiscale simulations exhibited realistic atmospheric features not captured by the mesoscale model such as convective rolls, global intermittency, and intra‐hour variability Diurnal evolution of turbulence was accurately simulated, with probability density functions of resolved turbulent velocity fluctuations nearly identical to CWEX‐13 lidar measurement

A Large-Eddy Simulation Study of the Influence of Subsidence on the Stably Stratified Atmospheric Boundary Layer

The influence of the large-scale subsidence rate, S, on the stably stratified atmospheric boundary layer (ABL) over the Arctic Ocean snow/ice pack during clear-sky, winter conditions is investigated using a large-eddy simulation model. Simulations of two 24-h periods are conducted while varying S between 0, 0.001 and 0.002 ms−1, and the resulting quasi-equilibrium ABL structures and evolutions are examined. Simulations conducted with S = 0 yield a boundary layer that is deeper, more strongly mixed and cools more rapidly than the observations. Simulations conducted with S &gt; 0 yield improved agreement with the observations in the ABL height, potential temperature gradients and bulk heating rates. We also demonstrate that S &gt; 0 limits the continuous growth of the ABL observed during quasi-steady conditions, leading to the formation of a nearly steady ABL of approximately uniform depth and temperature. Subsidence reduces the magnitudes of the stresses, as well as the implied eddy-diffusivity coefficients for momentum and heat, while increasing the vertical heat fluxes considerably. Subsidence is also observed to increases the Richardson number to values in excess of unity well below the ABL top

A Large-Eddy Simulation Study of the Influence of Subsidence on the Stably Stratified Atmospheric Boundary Layer

The influence of the large-scale subsidence rate, S, on the stably stratified atmospheric boundary layer (ABL) over the Arctic Ocean snow/ice pack during clear-sky, winter conditions is investigated using a large-eddy simulation model. Simulations of two 24-h periods are conducted while varying S between 0, 0.001 and 0.002 ms−1, and the resulting quasi-equilibrium ABL structures and evolutions are examined. Simulations conducted with S = 0 yield a boundary layer that is deeper, more strongly mixed and cools more rapidly than the observations. Simulations conducted with S &gt; 0 yield improved agreement with the observations in the ABL height, potential temperature gradients and bulk heating rates. We also demonstrate that S &gt; 0 limits the continuous growth of the ABL observed during quasi-steady conditions, leading to the formation of a nearly steady ABL of approximately uniform depth and temperature. Subsidence reduces the magnitudes of the stresses, as well as the implied eddy-diffusivity coefficients for momentum and heat, while increasing the vertical heat fluxes considerably. Subsidence is also observed to increases the Richardson number to values in excess of unity well below the ABL top