The role of the mean relative humidity on the dynamics of shallow cumuli LES Sensitivity experiments

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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)
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Large-eddy simulation -The BOMEX shallow cumulus
case
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LES sensitivity experiments - Set-up analogous to
Derbyshire's EUROCS deep convection case
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Results for cloud core mass fluxAll averages
taken over the 3rd hour
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Results for cloud core vertical velocity and
core fraction
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Mixed parcel diagram -Buoyancy reversal
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Buoyancy reversal - Dependency on mean vertical
gradients G
1.0
0.0
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? and fractional entrainment rate.Diagnosis
from LES results.
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Kain Fritsch parameterization- Assume flat PDF
for c e e0 ??2 (Bretherton et al. 2004)
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Bowen ratio, convective velocity scale w and
mass-flux
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Bowen ratio, convective velocity scale w and
mass-flux
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Conclusions

• Set I - Change relative humidity, but keep
  mean buoyance the same
• Cloud fraction changes gt
• Mass flux changes
• Set II - Change Bowen ratio, but keep surface
  buoyancy flux the same
• Cloud fraction changes gt
• Mass flux changes (also at cloud base)

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Results for cloud core fractional entrainment
and detrainment
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c from LES