Surface Heterogeneity Studies with LES
Objectives
-
Develop advanced SubGrid-Scale (SGS) models with the ability
to dynamically account for the effects of heterogeneity.
-
Characterize how land surface heterogeneity propagates
and affects Atmospheric Boundary Layer (ABL) fluxes through the use of multiscale analysis.
-
Develop new parameterizations for mesoscale fluxes through analysis of simulated velocity and temperature fields.
-
Study how natural surface hetereogeneity interacts with ABL turbulence by
combining LES with high resolution remotely sensed surface conditions.
Accomplishment and On-going Work
-
We have developed new dynamic SGS models which specify the required model
coefficients at every position in the flow and every simulation time step based
on resolved flow field (Stoll and Porté-Agel, 2005). These models use Lagrangian
averaging in combination with the scale-dependent dynamic SGS models developed by
Porté-Agel et al (2000) and Porté-Agel (2004) to specify the Smagorinsky coefficient (Cs)
for the SGS stress and the SGS Schmidt (Scsgs) number without any ad hoc tuning.
These models are found to adjust in a consistent and systematic fashion to changes
in surface properties (Figure 1). In the case of Scsgs, our results show that
the common assumption taken in both heterogeneous and homogeneous boundary layers that Scsgs= constant is questionable.
 
Figure 1: Spatial distribution of the average (in time and in the spanwise direction)
SGS model parameters Cs and Cs2Scsgs-1 due to: a sudden change in surface roughness at x/H=p (left panel) and
due to a step change in the surface flux of a passive scalar at x/H=p (right panel).
-
Multiscale analysis of simulated velocity fields from LES have shown similar scaling relations,
for scale-to-scale correlations between surface shear stress and velocity, to the windtunnel results of
Venugopal et al. (2003) shown in Figure 2. The LES results (from Venugopal et al., 2004) show that large
scale surface heterogeneities result in break down of scaling (Figure 3 - left hand side LHS) but that small scale heterogeneity behaves
similar to the homogeneous case (Figure 3 - right hand side RHS).
Figure 2: Plot of maximum correlation (MC) at each scale
vs kz for different sensor heights. Three distinct regions of contribution to the
correlation are identified: the inertial sub-range, where the contribution of
correlation is virtually negligible; the production sub-range (-1 spectral slope),
where the correlation increases as a log-law with a slope of -1/2; and, the range
in which scales are larger than the boundary-layer height, where there is no
increase in correlation. The collapse of the curves indicates a possible universality
in the dependence of correlation on scale.
 
Figure 3:Plot of maximum correlation (MC) at each scale
vs kz for different simulation heights for two different cases of heterogeneous
ABLs with abrupt changes in surface roughness. The dashed line represents the
log-law found by Venugopal et al. (2003) in the production subrange (see Figure 2). LHS) the domain is broken into two
patches and RHS) the domain broken into 20 patches.
-
Using a combination of Remotely sensed surface conditions and LES with state-of-the-art SGS models,
the dynamic interaction between natural land surface properties and ABL fluxes of momentum, heat and water
vapor can be studied. An example of remotely sensed conditions from the Southern Great Plains 1997 SGP97
(hydrolab.arsusda.gov/sgp97/) field campaign with the instantaneous surface shear stress and surface
heat flux obtained from LES are given in Figure 4. This information is essential to improving the flux
parameterizations used in regional scale weather models.
Figure 4: Remotely sensed land surface properties from SGP97 and the dynamically calculated surface fluxes from LES: (a) aerodynamic surface roughntess, (b) surface temperature, (c) instantaneous surface shear stress and (d) instantaneous surface head flux.
|
Papers and Conference Proceedings
-
Stoll R, and F. Porté-Agel (2005), Dynamic subgrid models for momentum and scalar fluxes in large-eddy simulations of atmospheric boundary layers over heterogeneous terrain. submitted to Water Resources Research.
-
Venugopal V, Stoll R, Porté-Agel F, and E. Foufoula-Georgiou (2004), Scale-Effects of Surface Heterogeneity on Atmospheric Boundary Layer Turbulence. European Geosciences Union 1st General Assembly. abstract EGU04-A-06537.
-
Porté-Agel F (2004) A Scale-Dependent Dynamic Model for Scalar Transport in Large-Eddy Simulations of the Atmospheric Boundary Layer. Boundary-Layer Meteorology. 112 81-105. available here
-
Venugopal V, Porté-Agel F, Foufoula-Georgiou E., and M. Carper (2003), Multiscale Interactions Between Surface Shear Stress and Velocity in Turbulent Boundary Layers. Journal of Geophysical Research 108(D19) 4613-4622. available here
-
Porté-Agel F., Meneveau C, and MB Parlange (2000), A scale-dependent dynamic model for large-eddy simulation: application to a neutral atmospheric boundary layer. Journal of Fluid Mechanics 415: 261-84. available here
|
