the role of the mean relative humidity on the dynamics of shallow cumuli - les sensitivity...
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The role of the mean relative humidity on the dynamics of shallow cumuli - LES Sensitivity experiments
Stephan de Roode (KNMI/IMAU)Simon Axelsen (IMAU)Pier Siebesma (KNMI)
Large-eddy simulation -
The BOMEX shallow cumulus case
BOMEX Exp 1,2,3,4
Km/s 0.008 0.008
(g/kg)m/s 0.052 0.052
w'θ'z=0
w'qt'z=0
300 305 310 315
0
500
1000
1500
2000
2500
3000
BOMEX
height [m]
virtual potential temperature θ
v [ ]K
300 305 310 315
0
500
1000
1500
2000
2500
3000
BOMEXExp 1Exp 2Exp 3Exp 4
height [m]
virtual potential temperature θ
v [ ]K
LES sensitivity experiments - Set-up analogous to
Derbyshire's EUROCS deep convection case
300 305 310
0
500
1000
1500
2000
2500
3000
BOMEXExp 1Exp 2Exp 3Exp 4
height [m]
potential temperature θ [ ]K
θv = θ+δθ( ) 1+0.61q+δq( )[ ]
4 8 12 16
BOMEXExp 1Exp 2Exp 3Exp 4
total specific humidity q [g/kg]
θ
q g/kg
BOMEX
Exp 1 0.04 -0.2
Exp 2 0.07 -0.4
Exp 3 -0.07 0.4
Exp 4 -0.13 0.7
Results for cloud core : mass flux
All averages taken over the 3rd hour
0 0.005 0.01 0.015 0.02 0.0250
500
1000
1500
BOMEX[m/s]Exp1Exp2Exp3Exp4
core mass-flux [m/s]
height above cloud base [m]
Results for cloud core :
vertical velocity and core fraction
0 1 2 3 4 5 60
500
1000
1500
BOMEXExp1Exp2Exp3Exp4
core vertical velocity [m/s]
height above cloud base [m]
0 0.01 0.02 0.03 0.04
core fraction
Mixed parcel diagram -
Buoyancy reversal
χ =menv
mcld+menv
Buoyancy reversal -
Dependency on mean vertical gradients
Γθ =∂θ∂z
, Γq =∂q∂z
0.0
1.0
and fractional entrainment rate.
Diagnosis from LES results.
0 0.001 0.002 0.003 0.0040
200
400
600
800
1000
BOMEXExp1Exp2Exp3Exp4
fractional entrainment rate ε [m-1 ]
[ ]height above cloud base m
0 0.1 0.2 0.3 0.4 0.5 0.60
200
400
600
800
1000
BOMEXExp1Exp2Exp3Exp4
*
[ ]height above cloud base m
Kain & Fritsch parameterization- Assume flat PDF
for
ε = ε0 2 (Bretherton et al. 2004)
0 0.1 0.2 0.3 0.40
0.001
0.002
0.003
0.004
BomexExp1Exp2Exp3Exp4
fractional entrainment rate
ε [m-1]
diagnosed from LES
critical mixing fraction squared *
2
core sampling
Bowen ratio, convective velocity scale w* and mass-
flux
w'θ'0x 1000 [Km/s]
w'q'v0
x 10-5 [m/s]
BOMEX 8.0 5.2
Exp 1 9.9 4.2
Exp 2 13.6 2.1
Exp 3 6.1 6.2
Exp 4 4.2 7.3
-0.005 0 0.005 0.01 0.015 0.020
500
1000
1500
2000
BOMEXExp1Exp2Exp3Exp4
w'θv [ / ]' mK s
[ ]height m
Bowen ratio, convective velocity scale w* and mass-
flux
0 0.01 0.02 0.03 0.040
500
1000
1500
BOMEXExp1Exp2Exp3Exp4
cloud core fraction
height above cloud base [m]
0 2 4 6
cloud core vertical velocity [m/s]
0 0.01 0.02 0.03
cloud core mass flux [m/s]
Conclusions
• Set I - Change relative humidity, but keep mean buoyance the same
Cloud fraction changes =>
Mass flux changes
• Set II - Change Bowen ratio, but keep surface buoyancy flux the same
Cloud fraction changes =>
Mass flux changes (also at cloud base)
Results for cloud core :
fractional entrainment and detrainment
0 0.001 0.002 0.003 0.0040
200
400
600
800
1000
BOMEXExp1Exp2Exp3Exp4
fractional entrainment rate ε [m-1 ]
[ ]height above cloud base m
0 0.005 0.01 0.015
fractional detrainment rate [m-1 ]
* from LES
0 0.1 0.2 0.3 0.4 0.5 0.60
200
400
600
800
1000
BOMEXExp1Exp2Exp3Exp4
*
[ ]height above cloud base m