49 research outputs found
Stable Atmospheric Boundary Layers and Diurnal Cycles â Challenges for Weather and Climate Models
The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.SB acknowledges the financial support received from the
NationalScience Foundation by way of Grant AGS-1122315
Global Intermittency and Collapsing Turbulence in the Stratified Planetary Boundary Layer
Direct numerical simulation of the turbulent Ekman layer over a smooth wall is used to investigate bulk properties of a planetary boundary layer under stable stratification. Our simplified configuration depends on two non-dimensional parameters: a Richardson number characterizing the stratification and a Reynolds number characterizing the turbulence scale separation. This simplified configuration is sufficient to reproduce global intermittency, a turbulence collapse, and the decoupling of the surface from the outer region of the boundary layer. Global intermittency appears even in the absence of local perturbations at the surface; the only requirement is that large-scale structures several times wider than the boundary-layer height have enough space to develop. Analysis of the mean velocity, turbulence kinetic energy, and external intermittency is used to investigate the large-scale structures and corresponding differences between stably stratified Ekman flow and channel flow. Both configurations show a similar transition to the turbulence collapse, overshoot of turbulence kinetic energy, and spectral properties. Differences in the outer region resulting from the rotation of the system lead, however, to the generation of enstrophy in the non-turbulent patches of the Ekman flow. The coefficient of the stability correction function from Monin-Obukhov similarity theory is estimated as (Formula presented.) in agreement with atmospheric observations, theoretical considerations, and results from stably stratified channel flows. Our results demonstrate the applicability of this set-up to atmospheric problems despite the intermediate Reynolds number achieved in our simulations. © 2014 The Author(s)
Analysis of Vertical Turbulent Heat Flux Limit in Stable Conditions with a Local Equilibrium, Turbulence Closure Model
On the Velocity Gradient in Stably Stratified Sheared Flows. Part 1: Asymptotic Analysis and Applications
The Critical Richardson Number and Limits of Applicability of Local Similarity Theory in the Stable Boundary Layer
Measurements of atmospheric turbulence made over the Arctic pack ice during
the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to
determine the limits of applicability of Monin-Obukhov similarity theory (in
the local scaling formulation) in the stable atmospheric boundary layer. Based
on the spectral analysis of wind velocity and air temperature fluctuations, it
is shown that, when both of the gradient Richardson number, Ri, and the flux
Richardson number, Rf, exceed a 'critical value' of about 0.20 - 0.25, the
inertial subrange associated with the Richardson-Kolmogorov cascade dies out
and vertical turbulent fluxes become small. Some small-scale turbulence
survives even in this supercritical regime, but this is non-Kolmogorov
turbulence, and it decays rapidly with further increasing stability. Similarity
theory is based on the turbulent fluxes in the high-frequency part of the
spectra that are associated with energy-containing/flux-carrying eddies.
Spectral densities in this high-frequency band diminish as the
Richardson-Kolmogorov energy cascade weakens; therefore, the applicability of
local Monin-Obukhov similarity theory in stable conditions is limited by the
inequalities Ri < Ri_cr and Rf < Rf_cr. However, it is found that Rf_cr = 0.20
- 0.25 is a primary threshold for applicability. Applying this prerequisite
shows that the data follow classical Monin-Obukhov local z-less predictions
after the irrelevant cases (turbulence without the Richardson-Kolmogorov
cascade) have been filtered out.Comment: Boundary-Layer Meteorology (Manuscript submitted: 16 February 2012;
Accepted: 10 September 2012