Cirrus Clouds

1. Cirrus Clouds MOD 10

2. Overview Cirrus clouds – definition and historical notes Picture gallery Frequency of cirrus clouds Generation mechanisms Properties of cirrus clouds Climate issues related to cirrus clouds Recommended book: Cirrus (Lynch et al., Eds.), Oxford UP, 2002

3. Cirrus clouds – definition • The highest clouds in the troposphere, therefore cold. • Sometimes reaching into the lowermost stratosphere. • Composed of ice, no liquid water. • There should be often many aqueous solution droplets at temperatures below the supercooling limit if liquid water. • These might play a role in the formation of irregular crystal habits (aggregates).

4. historical notes • Descartes and Mariotte concluded that Ci are made of ice, because of the 22° halo, that they could explain by ray tracing. • Classification and naming of these wispy fibrous clouds as “cirrus” by Luke Howard (1803). • That Ci are the highest clouds in the troposphere became clear in the 2nd half of the 19th century. • Theories on formation pathways of ice in the atmosphere became established in the 1930/40ies (Volmer, Krastanow, Wall). • In the 40es first ice crystals were collected in the UT by Helmut Weickmann.

5. Forecasting and WMO classification • Ci forecasting using physical rules (instead of weather sayings) began in the late 50ies/ early 60ies. The first Ci climatology was established in 1961 by H. Appleman. • WMO’s definition of Ci, Cc, Cs is based purely on morphology and visual appearance, not on physical properties. • Special kinds or forms of cirrus are sub-visual cirrus and condensation trails. These are not yet included in official classification.

6. some examples from “wolkengalerie.mpch-mainz.mpg.de”thanks to Stephan Borrmann Ci spissatus Ci spissatus means dense cirrus. Fibratus means fibrous appearance of Ci. Ice crystals are driven away from the main cloud and evaporate. Ci fibratus

7. Ci stratiformis undulatus Slow development of a cirrus cloud deck, with some wave action making the undulatus effect. Ci floccus Cirrus clouds with a rounded “head”

8. Ci uncinus A cirrus cloud with a mare’s tail is called cirrus uncinus. The mare’s tail is due to ice crystals falling in a sheared wind field. While falling into subsaturated air below,the ice crystals evaporate, set free latent heat, which can induce instable stratification, local convection and new formation of ice crystals above the shear layer.

9. Cs A rather uniform cirrus cloud deck Cc A rather clumpy cirrus cloud deck

10. Condensation trails (contrails) • special form of cirrus • form before natural Ci forms • persistent in ice-supersaturated air • spread and expand in a sheared wind field, • eventual transformation into natural looking cirrus • until ice crystals fall out and evaporate • or until subsidence terminates contrail lifetime • global coverage (1992) 0.1% (line-shaped contrails) • Contrail-cirrus can cover 10 times as much (Mannstein & Schumann, MZ, 2005) Contrails over central Europe on 4th May 1995 at 7:43 UTC based on NOAA12 AVHRR

11. Contrail formation needs about 1/3 sec after about 2 min contrails start to break up into waves and rings, then ice crystals start to evaporate

12. In slightly supersaturated air the ice in the rings vanishes (because of adiabatic heating in the downward travelling vortices), however a faint spoor of an ice curtain extending from the flight altitude down to the location of the vortices (secondary wake) stays persistent.

13. In more supersaturated air also the ice in the bursting vortices survives, a strong contrail appears that can undergo contrail-to-cirrus transformation.

14. The bursting vortices may produce such mammae when supersaturation is high enough. This can also be modelled, Gerz et al. 1997)

15. Frequency of cirrus clouds • Some notions: • Frequency of occurrence F • Amount when present A • Fractional coverage C

16. satellite view of cirrus occurrence Satellite data generally yield frequency of occurrence, i.e. F = #(data with cirrus)/#(all data). Result depends on sensor sensitivity. GOES/VAS: 25-30% (over con. US) HIRS: 43% (globally) SAGE II: 50-70% (including sub-visual Ci) Warren et al (ground based): 75% over indonesia. Whereas the human eye can detect Ci clouds down to =0.03, some satellite sensors view down to =10-4.

17. HIRS high cloud amounts (all clouds with 0.1) January July >14 km >10 km >6 km from Don Wylie’s web page

18. Global distribution of sub-visible cirrus in the UTLS Wang et al, JGR, 1996

19. Global distribution of ice supersaturated regions MLS data, Spichtinger et al., QJRMS, 2003

20. Climatology of persistent contrails,potential contrail coverage • If aircraft would fly • always and everywhere, • the coverage of the sky • with contrails would • equal the potential • contrail coverage. • Potential coverage depends on • T, RH, • properties of the propellant used (Kerosene, LH2) • aircraft performance Sausen et al., TAC, 1998

21. Climatology of persistent contrails, current climate The actual coverage of persistent (line-shaped, i.e. young) contrails depends on the actual air traffic. The main aviation flight paths show up here. Sausen et al., TAC, 1998

22. Dynamic cirrus generation mechanisms • Cumulonimbus convection  anvil cirrus • Baroclinic fronts and lows • Orographic lifting All these mechanisms imply upward vertical motion. (There may be some ice formation in downward motion, via contact nucleation and via efflorescence nucleation).

23. relation of cirrus to vertical wind Eleftheratos and Zerefos, 2005

24. Generation mechanisms reflected in satellite imagery The basic generation mechanisms are reflected in the satellite climatologies: • ITCZ maxima: Mostly anvil cirrus. Tropical convection most important source of water vapour in the UT; keeps Ci alive even far away from and long after the convective event.

25. Stormtracks • Stormtracks: fronts and lows, jet stream Jet stream related Ci occurs in the NE flowing parts of the jet, SE of the jet core. Very little Ci NW of the jet core, no Ci in the trough.

28. Orographic cirrus • Maximum over Himalaya: orographic cirrus. • Orographic cirrus induced by flow over a mountain ridge • strong vertical motions in the UT • favourite subject for in-situ observations • stay at the same place for some period while air is flowing through • onset of ice formation is relatively clearly defined • quasi-Lagrangian experiments (i.e. flight along the air trajectories) • Orographic cirrus clouds are at the wave crests of the air flow. The aerosol particles form ice at upward motion and evaporate at the downslope, potentially several times. • study cloud processing affecting the aerosol, in particular its ice formation ability

29. Formation of contrails Thermodynamic contrail formation criterion: Schmidt (1941), Appleman (1953), reviewed and extended by Schumann (1996). In contrast to natural cirrus clouds, contrails can already form at lower ambient humidity, in principle already under totally dry conditions (RHi=0%).

30. The Schmidt-Appleman criterion Water saturation must be reached during the mixing process.

31. The Schmidt-Appleman criterion for LH2 contrails • EIH2O/Q is 2.55 times larger for LH2 than for Kerosene • contrail formation at higher T, i.e. at lower altitude. • LH2 burning does not produce particles • LH2 contrails optically thinner than Kerosene contrails.

32. persistent contrails and indirect effect Only under ice-supersaturated conditions contrails can survive the initial jet- (20 s) and vortex-phases (ca. 2 min), to disperse then and spread by diffusion and wind shear into the atmosphere until it can only hardly be distinguished from natural cirrus clouds. It is conceivable, but not yet proven, that also the aerosol emitted from jet engines can lead to cirrus formation later on, even when no contrail was formed initially.

33. Properties of cirrus clouds Adapted from Lynch (2002), after Dowling and Radke (1990)

34. Crystal habits Nakaya created the first systematic classification scheme for snowflakes. Classification of falling snow into 41 individual morphological types. The most complex classification scheme is an extension of Nakaya's table, published by Magono and Lee in 1966. There are 80 separate morphological types in their classification scheme. http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm K. G. Libbrecht Caltech

36. Predominant crystal habit depends in a complex way on temperature and supersaturation during the history of crystal growth. Other factors may also play a role. http://www.dri.edu/Projects/replica/magono/snow.html

37. Climate issues related to cirrus clouds • Cold ice clouds trap IR radiation, but also reflect solar radiation. • Balance between greenhouse and albedo effects determines net impact on climate system. • For Ci, this can be + or -. • Sign depends on micro- and macrophysical properties, which in turn depend on generation mechanism (and many other circumstances). • The predominant sign depends on geographic location. • Different cooling/heating effects at TOA, UT, and surface. • This could imply, in a changing climate, dynamical feedbacks from cirrus changes.

38. Climatological radiative effects Global radiative impact (W/m2) of various high cloud types on the energy budget at three altitudes Adapted from Chen et al. (2000). Ranges are: Thin (0.02<<3.55), Moderate (3.55 <<22.63), Thick (>22.63).

39. Climatological radiative effects, cont’d (1) • Climate change can induce • changes in predominant cloud types • changes of mean (+- variance) cloud properties •  cloud-climate feedback • variations of cloud fraction are another kind of feedback

40. Climatological radiative effects, cont’d (2) • Different cloud types affect different components of the radiative flux in different layers of the atmosphere: • High level clouds most effectively change OLR, because they are much colder than the lower troposphere and the surface. • For the same reason their effect on the downward LW radiation at the surface is small. • Optically thin clouds have always a small effect by themselves, but it must be weighted with their average amount.

41. Other roles of cirrus clouds in the atmosphere • heterogeneous chemistry ice surfaces. Contribution to ozone destruction in the UTLS. • important factor in the dehydration of air entering the tropical lower stratosphere.