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 size
distribution 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 lne s
 L2
dT
RT
→ e s  exp − L
RT
q   pe ,
m
  m v  0. 622
air
Destabilization by condensation in saturated air
s  c p ln
ds  c p dT  Γdz 
dQ
L dq
−
T
T
If the parcel is saturated, q=qs,
dq  −dq s p, T ~ − p de s T  − p de s dT
dT
∂T
∂z
 −Γ
de s
dT  Γdz  − L dq  L
dT
cp T
c p Tp dT
dT  −Γ m
dz
Γm
des T
 Γ 1 − L
c p Tp dT
−1
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.
Destabilization by condensation in saturated air
s  c p ln
ds  c p dT  Γdz 
dQ
L dq
−
T
T

 de s dT
adiabatic
process:
dqMoist
 −dq
s p, T  − p de s T  − p
dT
∂T
∂z
 −Γ
dQm  cp dT
 g dz L
 L de
dqs  0
L
dT  Γdz  −
dq 
dT
c
T
Tp
c
dT
p
p
Qm  cp T  gz  Lq
dT  −Γ m
moist static
dz energy is conserved
expect uniform Q in convectively
adjusted state
des T −1
Γ m  Γ 1 − L
c p Tp
dT
Moist radiative-convective equilibrium
(Manabe & Strickler 1964)
close to typical
observed midlatitude
profile
Moist radiative-convective equilibrium
Roles of various absorbers
Where does convection occur?
Net outgoing longwave radiation (DJF)
(measured from space: Wm-2)
convective clouds not
common in desert
belts: radiation from
warm low levels
less deep extratropical convective and nonconvective clouds
tropical deep convection: cold
cloud tops
Where does convection occur?
Climatological sea surface temperature
Deep convection over equatorial continents and warmest water
Calculated rad-con equilibrium T
vs. observed T
near-equatorial lapse rate maintained near
neutral stability by moist convection
Calculated rad-con equilibrium T
vs. observed T
pole-to-equator temperature contrast too
big in equilibrium state (especially in
winter)
Zonally averaged net radiation
Diurnally-averaged
radiation
IR
solar
Local radiative equilibrium
at all latitudes
Zonally averaged net radiation
Diurnally-averaged
radiation
Observed radiative budget
Implied energy transport:
requires fluid motions to
effect the implied heat
transport
Roles of atmosphere and ocean
net
ocean
atmosphere
Trenberth & Caron (2001)
Radiative effects of clouds
Low clouds cool:
• increase albedo
• radiate at near-surface T
High clouds warm:
• mostly thin — little effect on albedo
• radiate at low T — weakens IR cooling
Aerosols
Sea salt and dust — most mass but
few in number, so less important
Sulfate — small but large in number.
Biogenic (via DMS) and humaninduced (via SO2)
Volcanic aerosols in the stratosphere
Aerosols: direct effect
Aerosols: indirect effect