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NextThe evening boundary layer: turbulence or no turbulence?
Bas van de Wiel, Ivo van Hooijdonk & Judith Donda in collaboration with:
Fred Bosveld, Peter Baas, Arnold Moene, Harm Jonker, Jielun Sun, Herman Clercx, e.a.
Scope
How much wind needed to keep turbulence going?
The very stable boundary layer: ‘cold’
The weakly stable boundary layer: ‘warm’
Concept: collapse driven by SEB
Idealistic case: Fresh snow (low heat capacity & conductance)
H=H0 H<H0
GHQdt
TdC n
sv
Here: Qn denoted as H0
Regime 1 Regime 2
Surface Energy Balance:
“Weather lab”: ensembles
• Clear nights: similar radiative forcing
• “Wind strength decides on regime”
• Classes 40m wind:-0.5-1.0 m/s-1.0-1.5 m/s-2.0-... etc.
-for given wind a flux maximum is found
vice versa
-for given demand (Qn-G) characteristic speed is found: Umin
After some calculations……
3/1
202
0min ))(ln(
4
27)(
zzz
c
GQgzU obsobs
p
nobs
The minimum wind speed for sustainable turbulence
Correction for soil heat
parameters Radiative Loss
Stability indicators
3*0
/
u
cHgz
L
z p
Scaling based on fluxes:
Scaling based on gradients:
Combined scaling:
Also combines knowledge flow AND boundary condition
LzzHu /];;[ *
RizTzU ]/;/[ 20 )/(
/
zU
zTgRi
min0 /];;/[ UUzHzU min
/
/
zU
zU
Or formally:
“Shear Capacity”];;/[ zGQzU n
Formal track
Step 3: analysis, prediction
-local scaling, gradient form
-model independent scaling:
-derivation from TKE-equation
min/
/
zU
zUSC
3/1
220
0min
1/
zc
HgzU
p
Conclusion
• Shear Capacity (U/Umin) compares transport
capacity flow to flux demand at surface
• Prediction idealized configurations & observed
reality
Details:Van Hooijdonk et al. (2014; J.A.S. Submitted)Donda et al. (submission June 2014)
Outlook
• Parameterisation Forecast models
• DNS/LES/RANS simulations
• Other climatologies: Fluxnet – data
What’s the use??
In practice (Louis 1979)
Physically preferable
-Non-physical curve aimed to enhance mixing in the very stable regime only
-But.....
-it causes too much mixing in the well-behaved, weakly stable case as well....!