CLOUDS. Forschungszentrum Jülich, Germany

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1 CLOUDS Cornelius Schiller Forschungszentrum Jülich, Germany

2 Outline Introduction Role of clouds in the climate system Cloud types Cloud life cycle Cloud formation and precipitation Cooling Warm Clouds Cold clouds Precipitation Special aspects from recent studies IWC of cirrus clouds Super-Supersaturation Contrails

Cooling Warm Clouds Cold clouds Precipitation Special aspects

3 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 (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, many individual publications + Meteorology standard textbooks

Stephens (eds.), Cirrus, Oxford Univ. Press., 2002 IPCC, Climate Change 2007 The Physical Science Basis, Cambridge Univ. Press., 2007 Peter, T.

4 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 provide a medium for (heterogeneous) chemical reactions Clouds affect significantly vertical transport and redistribution of species in the atmosphere

Clouds provide a medium for (heterogeneous) chemical reactions Clouds

6 More CCN more but smaller drops (cloud albedo/twomey effect) higher reflectivity & longer lifetime less sun on Earth s surface cooling

7 Aerosols Clouds Climate considered in IPCC RF

8 Hydrological cycle total water on Earth: km 3 oceans 97.4 % polar ice 1.9 % ground water 0.5 % soil 0.01 % biosphere % atmosphere % atmospheric H 2 O 4% - 1 ppmv total atmospheric H 2 O 25 mm annual precipitation 800 mm H 2 O exchange rate days

9 Chemical reactions in clouds: washout

10 Chemical reactions in clouds: surface reactions stratospheric ozone source gas reservoirs reactive CFC HCl, ClONO 2 Cl 2, Cl, ClO, (ClO) 2

11 Corti et al. Cloud impact on vertical transport

12 -80 C Cloud types

13 Low Clouds Stratus (St) Cumulus Congestus Cumulus (Cu)

14 Medium-high Clouds Altostratus (As) Altocumulus (Ac)

15 High Clouds Cirrostratus (Cs) Subvisible cirrus (SVC/UTTC) Cirrus (Ci) Cirrocumulus (Cc) + contrails

16 Cumulonimbus (Cb) anvil: source of cirrus

17 Polar Stratospheric Clouds (mother-of-pearl; nacreaous) Noctilucent Clouds

19 Liu & Zipser, TRMM cloud occurence > 14 km

20 Precipitation staircase Prerequisites for cloud formation: water low T supersaturation Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN)

22 Cloud formation and precipitation Cooling Warm Clouds Cold clouds Precipitation

23 Cooling Isobaric cooling Adiabatic cooling

25 Frontal cloud formation

26 Convective cloud formation

27 Orographic cloud formation also up to cirrus / PSC altitudes

28 Adiabatic cooling latent heat release H v by condensation moist adiabatic lapse rate: < Γ lifting condensation level or dt/dt = -Γ w dry adiabatic lapse rate

30 Cumulus formation depends on H 2 O content and stability of atmosphere 2: unstable atmosphere updraft stopped high up cumulus congestus 1: stable atmosphere updraft stopped early cumulus humilis LCL { T causes updraft

31 Stability in the atmosphere unstable strong T-gradient stable weak T-gradient or inversion

32 Warm Clouds

33 Equilibrium between phases: Clausius Clapeyron equation vapour/water H v (T) specific heat (water evap.) M w molecular weight supercooled water

35 Equilibrium of water droplet vs flat surface Kelvin equation p w > p for equilibrium of droplet, air needs to be supersaturated

36 Vapour pressure over an aqueous solution: Köhler equation activation of particles to drops particles < critical size < drops Supersaturation of several 100% required in particle-free air cloud condensation nuclei (CCN) required Higher critical supersaturation is needed for less particle solubility (bad water uptake) smaller particles curvature term solute effect

37 Activation of aerosol particles to drops Good CCN: large, high water soluble fraction, i.e. salts Bad CCN: small, high insoluble fraction, i.e. soot, dust or high organic fraction

38 Aerosol Composition Water soluble inorganic Water soluble organic Insoluble McFiggins et al. (2006), ACP

39 Cold (ice) clouds

40 Supercooled water, mixed phase, ice clouds

41 Clausius Clapeyron equation vapour/ice H s molar enthalpy for ice sublimation Integration ln p H2O = -A/T + C [Marti & Mauersberger, GRL 1993] p = 40 hpa [H 2 O] = 1 ppmv T frost 1 K water/ice H m enthalpy for melting

42 p-t phase diagram for water metastable equilibrium supercooled water/vapour T=0 C, p=6.1 hpa

43 Bergeron-Findeisen process T < 0 C, p sat,w > p sat,i supercooled droplets cannot coexist in equilibrium with ice crystals

44 Ice Nuclei (IN)

45 Homogeneous / heterogeneous ice nucleation determind by IN composition and supersaturation bad IN: soluble solutions organics good IN: soot mineral dust

46 A hom. freez. of solution droplets B deliquescence + hom. freez. C hom/het freez. + secondary phase cryst. (immersion freezing) D het. freez. of solution droplets E deposition nucl. on insoluble/anhydrous particle F contact freezing nucleation

47 Ice saturation at low T: Homogeneous nucleation Koop et al., 2000 AIDA experiments

48 Ice saturation at low T: Heterogeneous nucleation

49 Supersaturation in the atmosphere inside clouds outside clouds Ovarlez et al.

50 Precipitation (some) drops need to grow to precipitable size mechanisms: water vapour condensation droplet coalescence ice processes

51 Diffusional growth of drops

52 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. 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.

53 Ice processes

54 Example: Microphysics in a Cb cloud

56 Clouds 3 Special aspects from recent studies IWC of cirrus clouds Super-supersaturation (part 1, part 2 by TP) Contrails and contrail cirrus

57 Ice Water Content (IWC) of cirrus Measurement: total water gas phase water T/K Schiller et al., 2008 > 5 aircraft campaigns tropical cirrus midlatitude cirrus polar cirrus

58 Detection limits 1st method total water measured gas phase 2nd method total water saturation (from pt measurement)

59 FISH/FLASH vs FSSP IWC

60 Mean IWC and frequency distribution

61 IWC and cooling / convection

62 Utrathin tropical cirrus (UTTC) final step of dehydration of stratospheric air Peter et al., 2002

63 Stabilisation of UTTC Vertical motion of a particle Growth/evaporation of particles

65 Peter et al., 2007 The High Supersaturation Puzzle altitude [km] ice saturation ratio S nucleation threshold 0 time minutes to hours

66 FLASH/OJSTER: supersaturation climatology green: mid latitudes blue: high latitudes red/yellow: tropics Aircraft Geophysica Falcon Lear Jet Krämer et al., 2009

67 Supersaturation: frequency distribution and T-dependence clear sky 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 inside clouds Krämer et al., 2009

68 Saturation ratio with respect to ice: partial pressure of water vapor pressure of ice S > 1 S = 1 S < 1 ice particles grow ice particles are in equilibrium with the gas phase ice particles evaporate

69 How does water vapor condense on ice particles? Sedimentation How is supersaturation maintained? p vap (T ) = A e B/T

70 Sedimentation

71 Steady-state S in an upwelling air parcel (w) = 0

72 Ice crystal number densities - vertical velocity w - frost point at which air starts - aerosol properties varied Mean ice crystal radii Ice water content Kärcher & Lohmann, 2002

73 Growth time of ice particles (up to 80% of equilibrium radius)

74 Supersaturation and N ice inside clouds (persisitent) supersaturation consistent with low N ice supersaturation puzzle freezing suppression puzzle homogen. freezing and low u z? heterogen. freezing? freezing supression by organics? FSSP measurements from 20 flights by S. Borrmann and M. dereus

75 Contrails / contrail cirrus

76 Contrails and RHi RHi > 100% persistent contrails RHi < 100% lifetime of minutes embedded in (sub-)visible cirrus? generating new cirrus?

77 Krämer, CONCERT 2008

78 Schumann, 2002

79 Particles in a fresh contrail contrail During CONCERT large ice crystals (100µm) observed in short-lived contrail entrainment from surrounding cirrus

Chapter 6: Cloud Development and Forms

Chapter 6: Cloud Development and Forms (from The Blue Planet ) Why Clouds Form Static Stability Cloud Types Why Clouds Form? Clouds form when air rises and becomes saturated in response to adiabatic cooling.