stephan de roode (knmi/imau) simon axelsen (imau) pier siebesma (knmi)
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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 3150
500
1000
1500
2000
2500
3000
BOMEX
height [m]
virtual potential temperature θv [K]
300 305 310 3150
500
1000
1500
2000
2500
3000
BOMEXExp 1Exp 2Exp 3Exp 4
height [m]
virtual potential temperature θv [K]
LES sensitivity experiments - Set-up analogous to Derbyshire's EUROCS deep convection case
300 305 3100
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 [K]
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 fluxAll averages taken over the 3rd hour
0 0.005 0.01 0.015 0.02 0.0250
500
1000
1500BOMEX[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 0.04core fraction
Mixed parcel diagram -Buoyancy reversal
χ = menvmcld+menv
Buoyancy reversal - Dependency on mean vertical gradients
Γθ =∂θ∂z , Γq =∂q
∂z0.0
1.0
and fractional entrainment rate.Diagnosis from LES results.
0 0.001 0.002 0.003 0.0040
200
400
600
800
1000BOMEXExp1Exp2Exp3Exp4
fractional entrainment rate ε [m -1 ]
hεight abovε lou basε [m]
0 0.1 0.2 0.3 0.4 0.5 0.60
200
400
600
800
1000
BOMEXExp1Exp2Exp3Exp4
*
hεight abovε lou basε [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
0.004BomexExp1Exp2Exp3Exp4
fractional entrainment rate
ε [m-1]
iagnosε from LES
ritial mixing fration sθuarε *
2
orε 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]
hεight [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 6cloud core vertical velocity [m/s]
0 0.01 0.02 0.03cloud core mass flux [m/s]
Conclusions
• Set I - Change relative humidity, but keep mean buoyance the sameCloud fraction changes =>Mass flux changes
• Set II - Change Bowen ratio, but keep surface buoyancy flux the sameCloud 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
1000BOMEXExp1Exp2Exp3Exp4
fractional entrainment rate ε [m -1 ]
hεight abovε lou basε [m]
0 0.005 0.01 0.015fractional 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
*
hεight abovε lou basε [m]