Shallow Cu ensemble characteristics

1
Shallow Cu ensemble characteristics

• Cumuli have a uniform base marking the top of a
  turbulent subcloud mixed layer.
• Individual cumuli are complex mixtures of
  updrafts and downdrafts that vigorously entrain
  surrounding air.
• Entrainment evaporates and cools the cloudy
  updrafts.

Warner 1955
2
LES view of shallow Cu ensemble (BOMEX Siebesma
and Cuijpers 1995)Dx 100 m, Dz 40 m, Lx Ly
6.4 km
(dq/dt)adv - 1 g/kg-d Wmin -0.65 cm/s Qr -2
K/d
3
Conditional sampling of LES clouds (GCSS BOMEX
case, Siebesma et al. 2003)
Overshooting Cu
Clouds are barely buoyant. Even cores (buoyant
cloud) are diluted, not very buoyant.
Bottom-heavy cloud population All LES models
closely agree.
4
Humidity and humidity-based entrainment
Core bulk lateral entrainment rate e 1-2
km-1 inferred from e dqt,co/dz /(qt,co -
qt). Cu cores entrain vigorously
5
Vertical fluxes
Cu mass flux
momentum flux
wind
buoyancy flux
moisture flux
6
Updraft velocity and liquid water content
7
Air parcel circuits in a trade cumulus layer
Bretherton 1997