The Atmosphere: Part 4: Moist convection • Composition / Structure • Radiative transfer • Vertical and latitudinal heat transport • Atmospheric circulation • Climate modeling Suggested further reading: Hartmann, Global Physical Climatology (Academic Press, 1994)
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The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to
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The Atmosphere:Part 4: Moist convection
• Composition / Structure• Radiative transfer
• Vertical and latitudinal heat transport• Atmospheric circulation• Climate modeling
Suggested further reading:
Hartmann, Global Physical Climatology (Academic Press, 1994)
Radiative-convective equilibrium(unsaturated)
Better, but:
• surface still too cold
• tropopause still too warm
Moist convection
Above a thin boundary layer, most atmospheric convection involves phase change of water: condensation releases latent heat
When saturation occurs …..
• Heterogeneous Nucleation
• Supersaturations very small in atmosphere – condensation very fast
• Drop size distribution sensitive to sizedistribution of cloud condensation nuclei
Formation of precipitation(how to produce droplets big enough to fall?)
• Bergeron-Findeisen Process(rapid transfer of moisture from liquid to solid condensate)
• Stochastic coalescence (sensitive to drop size distributions)
• Strongly nonlinear function of cloud water concentration
• Time scale of precipitation formation ~10-30 minutes
— little support for overriding importance of ice nucleation in general
Formation of precipitation(how to produce droplets big enough to fall?)
• Bergeron-Findeisen Process(rapid transfer of moisture from liquid to solid condensate)
• Stochastic coalescence (sensitive to drop size distributions)
• Strongly nonlinear function of cloud water concentration
• Time scale of precipitation formation ~10-30 minutes
Moist variables and thermodynamics
e — vapor pressure of water [hPa]
es(T) — saturation vapor pressure of water [hPa]
q — specific humidity = (mass vapor)/(mass air) [g/kg]
qs — saturation specific humidity [g/kg]
U=q/qs — relative humidity [%]
Clausius-Clapeyron:
(assuming es<<p)
d lnesdT L
RT2
→ es exp − LRT
q ep ,
mvmair
0.622
Destabilization by condensation in saturated air
∂T∂z −Γ
s cp ln
ds cpdT Γdz dQT − L dq
T
dq −dqsp, T − p desT − pdesdT dT
dT Γdz − LcpT dq L
cpTpdesdT
dT
dTdz
−Γm
Γm Γ 1 − LcpTp
desTdT
−1
~If the parcel is saturated, q=qs,
Гm ranges from 3 K/km (moist surface tropical air) to 10 K/km (cold air, e.g. near tropopause); typical value 7 K/km.