clouds · role of clouds in the climate system • clouds are a major factor in the earth‘s...
TRANSCRIPT
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OutlineIntroduction• Role of clouds in the climate system• Cloud types• Cloud life cycle
Cloud formation and precipitation• Cooling• Cooling• Warm Clouds• Cold clouds• Precipitation
Special aspects from recent studies• IWC of cirrus clouds• Super-Supersaturation• Contrails
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Literature
• Seinfeld, J. H. and S. H. Pandis, Atmospheric Chemistry and Physics, Wiley Interscience, 1997
• Pruppacher, H. R., and J. D. Klett, Microphysics of Clouds and Precipitation, D. Reidel Publishing Company, 1978
• Lynch, D. K., K. Sassen, D. O‘C. Starr, G. Stephens • Lynch, D. K., K. Sassen, D. O‘C. Starr, G. Stephens (eds.), Cirrus, Oxford Univ. Press., 2002
• IPCC, Climate Change 2007 – The Physical Science Basis, Cambridge Univ. Press., 2007
• Peter, T. et al., When dry air is too humid, Science, 2005• + many individual publications• + Meteorology standard textbooks
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Role of clouds in the climate system
• Clouds are a major factor in the Earth‘s radiation budget
• Clouds are a key step in the hydrological cycle• Clouds are a key step in the hydrological cycle
• Clouds provide a medium for (heterogeneous) chemical reactions
• Clouds affect significantly vertical transport and redistribution of species in the atmosphere
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More CCN→ more but smaller drops (cloud albedo/Twomey effect)→ higher reflectivity & longer lifetime→ less sun on Earth’s surface→ cooling
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Aerosols – Clouds – Climate
considered in IPCC RF
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Hydrological cycle
total water on Earth: 1.4·109 km3
oceans 97.4 %polar ice 1.9 %ground water 0.5 %soil 0.01 %soil 0.01 %biosphere 0.003 %atmosphere 0.001 %
atmospheric H2O 4% - 1 ppmv
total atmospheric H2O 25 mmannual precipitation 800 mmH2O exchange rate 10-11 days
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Chemical reactions in clouds: washout
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Chemical reactions in clouds: surface reactions
stratospheric ozone
source gas reservoirs reactiveCFC HCl, ClONO2 Cl2, Cl, ClO, (ClO)2
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Cloud impact on vertical transport
Corti et al.
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Cloud types
-80°C
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Low Clouds Stratus (St)
Cumulus (Cu)Cumulus Congestus
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Medium-high Clouds
Altostratus (As)
Altocumulus (Ac)
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Cirrostratus (Cs)
High Clouds
Subvisible cirrus (SVC/UTTC)
Cirrocumulus (Cc)Cirrus (Ci)
+ contrails
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anvil: source of cirrus
Cumulonimbus (Cb)
anvil: source of cirrus
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Polar Stratospheric Clouds (mother-of-pearl; nacrea ous)Noctilucent Clouds
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Liu & Zipser, TRMM cloud occurence > 14 km
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Precipitation staircase
Prerequisites for cloud formation:• water• low T• supersaturation• Cloud Condensation Nuclei (CCN)
or Ice Nuclei (IN)
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Cloud formation and precipitation
• Cooling• Warm Clouds• Cold clouds• Precipitation• Precipitation
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Cooling
• Isobaric cooling• Adiabatic cooling
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Frontal cloud formation
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Convective cloud formation
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Orographic cloud formation
also up to cirrus / PSC altitudes
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lifting condensation level
latent heat release ∆Hv by condensationmoist adiabatic lapse rate:
< Γ
Adiabatic cooling
dry adiabatic lapse rate
or dT/dt = -Γ·w
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Cumulus formation
depends on H2O content and stability of atmosphere
1: stable atmosphere
2: unstable atmosphereupdraft stopped high upcumulus congestus
LCL
∆T causes updraft
{
1: stable atmosphereupdraft stopped earlycumulus humilis
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Stability in the atmosphere
unstable stablestrong T-gradient weak T-gradient or inversion
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Warm Clouds
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Equilibrium between phases:Clausius Clapeyron equation
vapour/water
∆Hv(T) specific heat (water evap.)Mw molecular weight supercooled water
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Equilibrium of water droplet vs flat surface
Kelvin equation
pw > p° → for equilibrium of droplet, air needs to be supersaturated
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Supersaturation of several 100% required in particle-free air →cloud condensation nuclei (CCN) required
Vapour pressure over an aqueous solution: Köhler eq uation
activation of particles to drops particles < critical size < drops
Higher critical supersaturationis needed for• less particle solubility(bad water uptake)
• smaller particles
curvature term solute effect
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Activation of aerosol particles to drops
Good CCN: large, high water soluble fraction, i.e. saltsBad CCN: small, high insoluble fraction, i.e. soot, dust or high organic fraction
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Aerosol Composition
Water soluble inorganicWater soluble organicInsoluble
McFiggins et al. (2006), ACP
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Cold (ice) clouds
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Supercooled water, mixed phase, ice clouds
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vapour/ice
∆Hs molar enthalpy for ice sublimation
Clausius Clapeyron equation
∆Hs molar enthalpy for ice sublimation
Integration ln pH2O = -A/T + C[Marti & Mauersberger, GRL 1993]
∆[H2O] = 1 ppmv ⇒ ∆Tfrost ≈ 1 K
water/ice
∆Hm enthalpy for melting
p = 40 hPa
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p-T phase diagram for water
metastable equilibriumsupercooled water/vapour T=0°C, p=6.1 hPa
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Bergeron-Findeisen process
T < 0°C, p sat,w > psat,isupercooled droplets cannot coexist in equilibrium with ice crystals
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Ice Nuclei (IN)
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Homogeneous / heterogeneous ice nucleation
determind by IN composition and supersaturation
bad IN: soluble solutions good IN: sootorganics mineral dust
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A hom. freez. of solution droplets D het. freez. of solution dropletsB deliquescence + hom. freez. E deposition nucl. on insoluble/anhydrous particleC hom/het freez. + secondary phase cryst. F contact freezing nucleation
(immersion freezing)
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Ice saturation at low T: Homogeneous nucleation
Koop et al., 2000 AIDA experiments
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Ice saturation at low T: Heterogeneous nucleation
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Supersaturation in the atmosphere
inside clouds outside clouds
Ovarlez et al.
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Precipitation
(some) drops need to grow to precipitable size
mechanisms:• water vapour condensation• droplet coalescence• droplet coalescence• ice processes
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Diffusional growth of drops
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Droplet coalescence
falling (large) drops collect smaller drops in fall path (Mt 25,29)
For everyone who has will be given more, and he will have an abundance. Whoever does not have, even what he has will be taken from him. taken from him.
Denn wer hat, dem wird gegeben, und er wird im Überfluss haben; wer aber nicht hat, dem wird auch noch weggenommen, was er hat.
Car à celui qui a, on donnera, et il aura encore davantage; mais à celui qui n'a pas, on ôtera même ce qu'il a.
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Ice processes
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Example: Microphysics in a Cb cloud
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Clouds 3Special aspects from recent studies
• IWC of cirrus clouds• Super-supersaturation (part 1, part 2 by TP)• Contrails and contrail cirrus
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Ice Water Content (IWC) of cirrus
Measurement: total water – gas phase water
T/K
> 5 aircraft campaignstropical cirrus midlatitude cirrus polar cirrus
Schiller et al., 2008
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Detection limits
1st methodtotal water –measured gas phase
2nd methodtotal water – saturation(from pT measurement)
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FISH/FLASH vs FSSP IWC
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Mean IWC and frequency distribution
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IWC and cooling / convection
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Utrathin tropical cirrus (UTTC)final step of dehydration of stratospheric air
Peter et al., 2002
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Stabilisation of UTTC
Vertical motion of a particle
Growth/evaporation of particles
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The High Supersaturation Puzzleal
titud
e [k
m]
14
13
12
14
13
12
Peter et al., 2007
2.2
1.6
10 time minutes to hours
altit
ude
[km
]ic
e sa
tura
tion
ratio
S
12
14
13
12
nucleation
threshold
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FLASH/OJSTER: supersaturation climatology
green: mid latitudesblue: high latitudesred/yellow: tropics
AircraftGeophysicaFalconLear Jet
Krämer et al., 2009
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Supersaturation: frequency distribution and T-depen dence
reprocessed data (28 flights)
• only few data > homogeneous f.t.• no data > water saturation• inside cloud RHi peaks at 100%• broader distribution at T < 205 K
clear sky
inside clouds
Krämer et al., 2009
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Saturation ratio with respect to ice:
partial pressure of water
vapor pressure of ice
S > 1 � ice particles grow
S= 1 � ice particles are in equilibrium with the gas phase
S < 1 � ice particles evaporate
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How does water vapor condense on ice particles?
Sedimentation
How is supersaturation maintained?
pvap(T ) = A e –B/T
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Sedimentation
…
…
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Steady-state S in an upwelling air parcel (w)
= 0
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Ice crystal number densities- vertical velocity w- frost point at which air starts- aerosol properties varied
Kärcher & Lohmann, 2002
Mean ice crystal radiiIce water content
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Growth time of ice particles
(up to 80% of equilibrium radius)
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Supersaturation and N ice
(persisitent) supersaturation consistent with low Nice
supersaturation puzzle → freezing suppression puzzle
inside clouds
FSSP measurements from 20 flights by S. Borrmann and M. deReus
homogen. freezing and low uz?
heterogen. freezing?
freezing supression by organics?
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Contrails / contrail cirrus
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RHi > 100%persistent contrails
Contrails and RHi
RHi < 100%lifetime of minutes
embedded in (sub-)visible cirrus?generating new cirrus?
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Krämer, CONCERT 2008
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Schumann, 2002
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Particles in a fresh contrail
During CONCERT large ice crystals (100µm) observed in short-lived contrail→ entrainment from surrounding cirrus
contrail