Parameterization in models Introduction to cloud issues

1. Parameterization in modelsIntroduction to cloud issues tompkins@ictp.it

2. GCM Grid cell 40-400km Clouds in GCMs - What are the problems ? Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically)

3. ~500m ~100km Macroscale Issues of Parameterization VERTICAL COVERAGE Most models assume that this is 1 This can be a poor assumption with coarse vertical grids. Many climate models still use fewer than 30 vertical levels currently, some recent examples still use only 9 levels

4. ~500m ~100km Macroscale Issues of Parameterization HORIZONTAL COVERAGE, a

5. Macroscale Issues of Parameterization Vertical Overlap of cloud Important for Radiation and Microphysics Interaction ~500m ~100km

6. Macroscale Issues of Parameterization In cloud inhomogeneity in terms of cloud particle size and number ~500m ~100km

7. ~500m ~100km Macroscale Issues of Parameterization Just these issues can become very complex!!!

8. radiation convection turbulence microphysics dynamics Clouds in GCMs - What are the problems ? Clouds are the result of complex interactions between a large number of processes

9. Cloud top and base height Amount of condensate Cloud fraction and overlap In-cloud conden-sate distribution Cloud environment Cloud-radiation interaction Phase of condensate Cloud particle shape Cloud particle size Cloud macrophysics Cloud microphysics “External” influence Clouds in GCMs - What are the problems ? Many of these processes are only poorly understood - For example, the interaction with radiation

10. small ice Ice Mass Ice mass Medium ice Liquid Mass Ice number Large ice What do we want to represent? Complexity Cloud Mass “Single Moment” Schemes “Double Moment” Schemes “Spectral/Bin” Microphysics Most GCMs only have simple single-moment schemes

11. Clouds in GCMs - How ? Main variables: Cloud fraction, a - refers to horizontal cover since cloud fills vertical Cloud condensate mass (cloud water and/or ice), ql. Diagnostic approach Prognostic approach NOT DISTINCT - CAN HAVE MIXTURE OF APPROACHES

12. Cloud microphysical processes • We would like to include into our models: • Formation of clouds • Release of precipitation • Evaporation of both clouds and precipitation • Therefore we need to describe • the change of phase from water vapour to water droplets and ice crystals • the transformation of small cloud droplets/ice crystals to larger rain drops/ice particles • The advection and sedimentation/falling of these species • the evaporation/sublimation of cloud and precipitation size particles

13. Microphysics: Complex System! • Overview of • Warm Phase Microphysics T>273K • Mixed Phase Microphysics 250K<T<273K • Pure ice Microphysics T<250K

14. Droplet Classification

15. Important effects for particle activation Planar surface: Equilibrium when e=es and number of molecules impinging on surface equals rate of evaporation Surface molecule has fewer neighbours Curved surface: saturation vapour pressure increases with smaller drop size since surface molecules have fewer binding neighbours.

16. Nucleation of Water:Homogeneous Nucleation • Drop of pure water forms from vapour • Small drops require much higher super saturations • Kelvin’s formula for critical radius for initial droplet to be “survive” • strongly dependent on supersaturation • Would require several hundred percent supersaturation (not observed in the atmosphere).

17. Nucleation of Water:Heterogeneous Nucleation • Collection of water molecules on a foreign substance, RH > ~80% (Haze particles) (Note, not same when drying) • These (hydrophilic) solluble particles are called Cloud Condensation Nuclei (CCN) • CCNalways present in sufficient numbers in lower and middle troposphere

18. Important effects for particle activation Planar surface: Equilibrium when e=es and number of molecules impinging on surface equals rate of evaporation Surface molecule has fewer neighbours Curved surface: saturation vapour pressure increases with smaller drop size since surface molecules have fewer binding neighbours. Effect proportional to r-1 Dissolved substance reduces vapour pressure Presence of dissolved substance: saturation vapour pressure reduces with smaller drop size due to sollute molecules replacing solvent on drop surface (assuming esollute<ev) Effect proportional to r-3

19. “Curvature term” Small drop – high radius of curvature easier for molecule to escape “Solution term” Reduction in vapour pressure due to dissolved substance Heterogeneous Nucleation Haze particle in equilibrium e/es equilibrium

20. Parameterizing Nucleation and droplet growth • Nucleation: Since “Activation” occurs at supersaturations less than 1% most schemes assumes all supersaturation is immediately removed as liquid water • Note that this assumption means that models can just use one “prognostic” equation for the total water mass, the sum of vapour and liquid • Usually, the growth equation is not explicitly solved, and in single-moment schemes simple (diagnostic) assumptions are made concerning the droplet number concentration when needed (e.g. radiation). These often assume more CCN in polluted air over land.

21. Diffusion growth (water) • Nucleation small droplets • once droplet is activated, water vapour diffuses towards it = condensation • reverse process = evaporation • droplets that are formed by diffusion growth attain a typical size of 0.1 to 10 mm • rain drops are much larger than that • drizzle: 50 to 100 mm • rain: >100 mm • other processes must also act in precipitating clouds For r > 1 mm and neglecting diffusion of heat D=Diffusion coefficient, S=Supersaturation Note inverse radius dependency

22. Collision-Coalescence • Drops of different size move with different fall speeds - collision and fusion • large dropsgrow at the expense of small droplets • Collection efficiency low for small drops • process depends on width of droplet spectrum and is more efficient for broader spectra - Paradox • large drops can only be produced in clouds of large vertical extent – Aided by turbulence and entrainment • important process for low latitudes where deep clouds of high water content are present

23. Gp qlcrit ql Gp ql qlcrit Parameterizating “Autoconversion” of cloud drops to raindrops Autoconversion (Kessler, AMS monogram 1969) Sundqvist, QJRMS, 1978 what are the issues for data assimilation? “Non-local” collection P=Precipitation Flux

24. RH>100.6% “Activation” Diffusional Growth CCN ~10 microns Coalescence Different fall speeds Schematic of Warm Rain Processes Heterogeneous Nucleation RH>78% (Haze)

25. Ice Nucleation • Ice processes complex and poorly understood • Droplets do not freeze at 0oC! • Can also be split into Homogeneous and Heterogeneous processes • Processes depend on temperature and history of cloud • Homogeneous freezing of water droplet occurs between –35 and –40oC (often used assumption in microphysical schemes). • Frequent observation of ice at warmer temperatures indicates role for heterogeneous processes

26. Ice Nucleation • Spontaneous freezing of liquid droplets smaller than 5 mm requires temperature less than -40oC. • Observations of liquid in cloud are common at -20oC. • Ice crystals start to appear in appreciable numbers below around -15oC. • Heterogenous Nucleation responsible: Process less clear • Ice nuclei: Become active at various temperatures less than 0oC, many fewer • Observations: • < -20oC Ice free clouds are rare • > -5oC ice is unlikely • ice supersaturation ( > 10% ) observations are common Fletcher 1962

27. Heterogeneous Nucleation I will not discuss heterogeneous ice nucleation in great detail in this course due to lack of time and the fact that these processes are only just starting to be tackled in Large-scale models. See recent work of Ulrike Lohmann for more details aerosol Supercooled drop

28. Ice habits can be complex, depends on temperature: influences fall speeds and radiative properties Ice Habits http://www.its.caltech.edu/~atomic/snowcrystals/

29. The saturation water vapour pressure with respect to ice is smaller than with respect to water A cloud, which is saturated with respect to water is supersaturated with respect to ice ! Mixed Phase clouds: Bergeron Process (I)

30. Bergeron process (II) Ice particle enters water cloud Ice particles grow at the expense of water droplets Cloud is supersaturated with respect to ice Diffusion of water vapour onto ice particle Cloud will become sub-saturated with respect to water Water droplets evaporate to increase water vapour

31. Gp ql qlcrit Modification of Sundqvist to take Bergeron Process into account Sundqvist, QJRMS, 1978 Collection Bergeron Process Otherwise, most schemes have neglected ice processes, removing ice super-saturation “al la Warm rain” – See Lohman and Karcher JGR 2002(a,b) for first attempts to include ice microphysics in GCM

32. Aggregation • Ice crystals can aggregate together to form snow • Temperature dependent, process increases in efficiency as temperature exceeds –5C, when ice surface becomes sticky • Also a secondary peak between –10 and –16C when dendrite arms get entangled

33. Riming • If vapour exceeds the water saturation mixing ratio, water can condense on ice crystal, and then subsequently freeze to form “graupel”: Round ice crystals with higher densities and fall speeds than snow dendrites • Graupel and Hail are also formed by aggregating liquid water drops in mixed phased clouds (“riming”) • If the Latent heat of condensation and fusion keeps temperature close to 273K, then high density hail particle forms, since the liquid water “spreads out” before freezing. Generally referred to as “Hail” – The higher fall speed (up to 40 m/s) imply hail only forms in convection with strong updraughts able to support the particle long enough for growth http://www.its.caltech.edu/~atomic/snowcrystals/

34. Aggregation and Riming: Simple stratified picture

35. Ice Habits Ice habits can be complex: influences fall speeds and radiative properties Note shape/diameter distribution not monotonic with height, Turbulence! From Fleishauer et al 2002, JAS

36. Falling Precipitation • Need to know size distribution • For ice also affected by ice habit • Poses problem for numerics Courtesy: R Hogan, U. Reading From R Hogan www.met.rdg.ac.uk/radar

37. Pure ice Phase: Homogeneous Ice Nucleation • At cold temperature (e.g. upper troposphere) difference between liquid and ice saturation vapour pressures is large. • If air mass is lifted, and does not contain significant liquid particles or ice nuclei, high supersaturations with respect to ice can occur, reaching 160 to 170%. • Long lasting contrails are a signature of supersaturation Institute of Geography, University of Copenhagen

38. Heteorogeneous Nucleation • On the other hand, in air polluted by organic and mineral dust, supersaturation achieve perhaps 130%. • Research is ongoing to determine the nature of ice nuclei at these colder temperatures • This process probably more prevalent in NH where air is less ‘clean’ 22nd April 2003: Modis Image from http://modis.gsfc.nasa.gov

39. Schematic of parcel evolution, T<-38oC RHice 160% RH<160% Lifting/cooling 130% 100% aerosol Few IN Many IN No IN

40. Simple Parcel model calculationsMaximum humidity important for determining number of ice crystals nucleated From Ren and Mackenzie QJRMS 2005 Lift (cool) parcel Parcel containing aerosols Single aerosol type 4 major issues! e.g. Jensen 94, Spice et al. 97, Gierens 2003, Ren and Mackenzie 2005

41. Cooling rates-Vertical velocity • Even in “polluted” NH air, homogeneous nucleation can dominate in strong updraughts • Knowledge of local velocities is crucial • However important wind fluctuations occur on length-scales comparable to/smaller than grid length • Upscale cascade from turbulence • Gravity waves • Subgrid instabilities (cloud top instabilities)…… • It is clear that models are deficient in representing these • E.g: Lohmann shows inadequacy of ECMWF model, and how enhancement of turbulence activity can produce improved spectra From haag and Kaerchner aircraft ECMWF resolved motions only

42. Limb Sounder and Mozaic Data(Pictures courtesy of Klaus Gierens and Peter Spichtinger, DLR) • Observations (e.g. Mozaic aircraft and Microwave sounders) have revealed supersaturated states are common, also seen in radiosonde datasets 3000 km supersaturated segment observed ahead of front

43. Summary: Warm Cloud E.g: Stratocumulus Evaporation Condensation (Rain formation - Fall Speeds - Evaporation of rain)

44. Summary: Deep Convective Cloud • Heteorogeneous Nucleation of ice • Splintering/Bergeron Process • Melting of Snow and Graupel • Precipitation Falls Speeds • Evaporation in Sub-Cloud Layer

45. Summary: Cirrus cloud Homogeneous Nucleation (representation of supersaturation) Heterogeneous Nucleation (representation of nuclei type and concentration) Sedimentation of Ice crystals? Size distribution and formation of snow?

46. Cloud Schemes - A Brief History C C C C C

47. Simple Bulk Microphysics VAPOUR (prognostic) Evaporation Condensation CLOUD (prognostic) Evaporation Autoconversion RAIN (diagnostic) WHY?

48. Mostly treated as instant “No supersaturation assumption” “Threshold” linear or exponential terms with efficiency adjustments Microphysics - a “complex” GCM scheme Fowler et al., JCL, 1996 Similar complexity to many schemes in use in CRMs

49. a small a large GCM grid box precipitation not equal in each case since autoconversion is nonlinear Reminder: Autoconversion (Kessler, 1969) Cloud Cover: Why Important? In addition to the influence on radiation, the cloud cover is important for the representation of microphysics Imagine a cloud with a liquid condensate mass ql The incloud mass mixing ratio is ql/a Complex microphysics perhaps a wasted effort if assessment of a is poor