the atmosphere: lecture 4: moist convection · • bergeron-findeisen process (rapid transfer of...
TRANSCRIPT
![Page 1: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/1.jpg)
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)
![Page 2: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/2.jpg)
Radiative-convective equilibrium(unsaturated)
Better, but:
• surface still too cold
• tropopause still too warm
![Page 3: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/3.jpg)
Moist convection
Above a thin boundary layer, most atmospheric convection involves phase change of water: condensation releases latent heat
![Page 4: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/4.jpg)
When saturation occurs …..
• Heterogeneous Nucleation
• Supersaturations very small in atmosphere – condensation very fast
• Drop size distribution sensitive to sizedistribution of cloud condensation nuclei
![Page 5: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/5.jpg)
![Page 6: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/6.jpg)
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
![Page 7: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/7.jpg)
— little support for overriding importance of ice nucleation in general
![Page 8: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/8.jpg)
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
![Page 9: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/9.jpg)
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
![Page 10: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/10.jpg)
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.
![Page 11: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/11.jpg)
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
Moist adiabatic process:
dQm cpdT g dz L dq 0
Qm cpT gz Lq
moist static energy is conserved
expect uniform Q in convectively adjusted state
![Page 12: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/12.jpg)
Moist radiative-convective equilibrium(Manabe & Strickler 1964)
close to typical observed midlatitudeprofile
![Page 13: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/13.jpg)
Moist radiative-convective equilibriumRoles of various absorbers
![Page 14: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/14.jpg)
Where does convection occur?
tropical deep convection: cold cloud tops
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 non-convective clouds
![Page 15: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/15.jpg)
Where does convection occur?
Climatological sea surface temperature
Deep convection over equatorial continents and warmest water
![Page 16: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/16.jpg)
Calculated rad-con equilibrium Tvs. observed T
near-equatorial lapse rate maintained near neutral stability by moist convection
![Page 17: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/17.jpg)
Calculated rad-con equilibrium Tvs. observed T
pole-to-equator temperature contrast too big in equilibrium state (especially in winter)
![Page 18: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/18.jpg)
Zonally averaged net radiation
Diurnally-averaged radiation
IR
solar
Local radiative equilibrium at all latitudes
![Page 19: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/19.jpg)
Zonally averaged net radiation
Diurnally-averaged radiation
Observed radiative budget
Implied energy transport: requires fluid motions to effect the implied heat transport
![Page 20: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/20.jpg)
Roles of atmosphere and ocean
net
ocean
atmosphere
Trenberth & Caron (2001)
![Page 21: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/21.jpg)
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
![Page 22: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/22.jpg)
Aerosols
Sea salt and dust — most mass but few in number, so less important
Sulfate — small but large in number. Biogenic (via DMS) and human-induced (via SO2)
![Page 23: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/23.jpg)
Volcanic aerosols in the stratosphere
![Page 24: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/24.jpg)
Aerosols: direct effect
![Page 25: The Atmosphere: Lecture 4: Moist convection · • Bergeron-Findeisen Process (rapid transfer of moisture from liquid to solid condensate) • Stochastic coalescence (sensitive to](https://reader033.vdocuments.net/reader033/viewer/2022052815/60a1abb3a1fd92062e7f3d37/html5/thumbnails/25.jpg)
Aerosols: indirect effect