Presentation on theme: "Numerical Weather Prediction Parametrization of Diabatic Processes Clouds (1) Cloud Microphysics Richard Forbes and Adrian Tompkins firstname.lastname@example.org Cloud."— Presentation transcript:
1Numerical Weather Prediction Parametrization of Diabatic Processes Clouds (1) Cloud Microphysics Richard Forbesand Adrian TompkinsCloud ParametrizationClouds 1
51. Water Cycle 2. Radiative Impacts 3. Dynamical Impacts The Importance of Clouds1. Water Cycle2. Radiative Impacts3. Dynamical ImpactsCloud ParametrizationClouds 1
6Clouds in GCMs - What are the problems ? radiationconvectionmicrophysicsturbulencedynamicsClouds are the result of complex interactions between a large number of processesCloud ParametrizationClouds 1
7Clouds in GCMs - What are the problems ? Many of these processes are only poorly understood - For example, the interaction with radiationCloud top and base heightCloud fraction and overlapAmount of condensateCloud-radiation interactionIn-cloud conden-sate distributionCloud environmentPhase of condensateCloud particle shapeCloud particle sizeCloud macrophysicsCloud microphysics“External” influenceCloud ParametrizationClouds 1
9Cloud Parametrization Issues: Which quantities to represent ? Cloud water dropletsRain dropsPristine ice crystalsAggregate snow flakesGraupel pelletsHailstonesNote for ice phase particles: Additional latent heat. Terminal fall speed of ice hydrometeors significantly less (lower density). Optical properties are different (important for radiation).
10Cloud Parametrization Issues: Complexity ? Small iceMedium iceLarge iceIce MassLiquid MassIce massIce numberCloudMass“Single Moment”Schemes“Double Moment”Schemes“Spectral/Bin”MicrophysicsMost GCMs only have simple single-moment schemes
11Cloud Parametrization Issues: Diagnostic or prognostic variables ? Cloud condensate mass (cloud water and/or ice), ql.Diagnostic approachPrognostic approachSourcesSinksAdvection + sedimentationCAN HAVE MIXTURE OF APPROACHESCloud ParametrizationClouds 1
15Cloud microphysical processes We would like to include into our models:Formation of cloudsRelease of precipitationEvaporation of both clouds and precipitationTherefore we need to describeNucleation of water droplets and ice crystals from water vapourDiffusional growth of cloud droplets (condensation) and ice crystals (deposition)Collection processes for cloud drops (collision-coalescence), ice crystals (aggregation) and ice and liquid (riming) leading precipitation sized particlesThe advection and sedimentation (falling) of particlesthe evaporation/sublimation/melting of cloud and precipitation size particlesCloud ParametrizationClouds 1
16Microphysics: A complex system! Overview ofWarm Phase Microphysics T > 273KMixed Phase Microphysics ~250K < T < 273KPure ice Microphysics T < ~250K
19Nucleation of cloud droplets: Important effects for particle activation Planar surface: Equilibrium when e=es and number of molecules impinging on surface equals rate of evaporationSurface molecule has fewer neighboursCurved surface: saturation vapour pressure increases with smaller drop size since surface molecules have fewer binding neighbours.
20Nucleation of cloud droplets: Homogeneous Nucleation Drop of pure water forms from vapourSmall drops require much higher super saturationsKelvin’s formula for critical radius for initial droplet to “survive”Strongly dependent on supersaturationWould require several hundred percent supersaturation (not observed in the atmosphere).Cloud ParametrizationClouds 1
21Nucleation of cloud droplets: Heterogeneous Nucleation Collection of water molecules on a foreign substance, RH > ~80% (Haze particles)These (hydrophilic) soluble particles are called Cloud Condensation Nuclei (CCN)CCN always present in sufficient numbers in lower and middle troposphereNucleation of droplets (i.e. from stable haze particle to unstable regime of diffusive growth) at very small supersaturations.
22Nucleation of cloud droplets: Important effects for particle activation Planar surface: Equilibrium when e=es and number of molecules impinging on surface equals rate of evaporationSurface molecule has fewer neighboursCurved surface: saturation vapour pressure increases with smaller drop size since surface molecules have fewer binding neighbours.Effect proportional to 1/r (curvature effect)Dissolved substance reduces vapour pressurePresence of dissolved substance: saturation vapour pressure reduces with smaller drop size due to solute molecules replacing solvent on drop surface (assuming esollute<ev)Effect proportional to 1/r3 (solution effect)
23Nucleation of cloud droplets: Heterogeneous Nucleation Haze particle in equilibrium“Curvature term”Small drop – high radius of curvature easier for molecule to escapeequilibriume/es“Solution term”Reduction in vapour pressure due to dissolved substance
24Diffusional growth of cloud water droplets Once droplet is activated, water vapour diffuses towards it = condensationReverse process = evaporationDroplets that are formed by diffusion growth attain a typical size of 0.1 to 10 mmRain drops are much largerdrizzle: 50 to 100 mmrain: >100 mmOther processes must also act in precipitating cloudsFor r > 1 mm and neglecting diffusion of heatD=Diffusion coefficient, S=SupersaturationNote inverse radius dependencyCloud ParametrizationClouds 1
25Collection Collision-coalescence of water drops Rain drop shape Chuang and Beard (1990)Drops of different size move with different fall speeds - collision and coalescenceLarge drops grow at the expense of small dropletsCollection efficiency low for small dropsProcess depends on width of droplet spectrum and is more efficient for broader spectra – paradoxLarge drops can only be produced in clouds of large vertical extent – Aided by turbulence (differential evaporation), giant CCNs ?Cloud ParametrizationClouds 1Cloud ParametrizationClouds 125
26Parameterizing nucleation and droplet growth Nucleation: Since “Activation” occurs at supersaturations less than 1%, most schemes assumes all supersaturation is immediately removed as liquid waterNote that this assumption means that models can just use one “prognostic” equation for the total water mass, the sum of vapour and liquidUsually, 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.
27Microphysical Parametrization “Autoconversion” of cloud drops to raindrops qlqlcritGpLinear function of ql (Kessler, 1969)Function of ql with additional term to avoid singular threshold and non-local precipitation term (Sundqvist 1978)Or, for example, for a more complex double-moment parametrization (Seifert and Beheng,2001), derived directly from the stochastic collection equation.KesslerSundqvistqlqlcritGpMicrophysics - ECMWF Seminar on Parametrization 1-4 SepCloud ParametrizationClouds 1Cloud ParametrizationClouds 127
28Schematic of Warm Rain Processes Coalescence~10 micronsRH>100.6%“Activation”DiffusionalGrowthHeterogeneous NucleationRH>78% (Haze)CCNDifferent fall speeds
33Ice Nucleation Droplets do not freeze at 0oC ! Ice nucleation processes can also be split into Homogeneous and Heterogeneous processes, but complex and not well understood.Homogeneous freezing of water droplets occurs below about -38oC,Homogeneous nucleation of ice crystals dependent on a critical relative humidity (fn of T, Koop et al. 2000).Frequent observation of ice between 0oC and colder temperatures indicates heterogeneous processes active.Number of activated ice nuclei increases with decreasing temperature.Observations:< -20oC Ice free clouds are rare> -5oC ice is unlikely (unless ice falling from above!)ice supersaturation ( > 10% ) observations are commonFletcher 1962
34Ice Nucleation: 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 starting to be tackled in Large-scale models. See recent work of Ulrike Lohmann for more detailsaerosolSupercooled drop
35Pure 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 supersaturationInstitute of Geography, University of Copenhagen
36Diffusional growth of ice crystals Deposition Equation for the rate of change of mass for an ice particle of diameter D due to deposition (diffusional growth), or evaporationDeposition rate depends primarily onthe supersaturation, sthe particle shape (habit), Cthe ventilation factor, FThe particular mode of growth (edge growth vs corner growth) is sensitive to the temperature and supersaturation
37Diffusional growth of ice crystals Ice Habits Ice habits can be complex, depend on temperature: influences fall speeds and radiative properties
39Homogeneous Freezing Temperature Diffusional growth of ice crystals Mixed Phase Clouds: Bergeron Process (I)Ice SaturationHomogeneous Freezing TemperatureWater SaturationThe saturation water vapour pressure with respect to ice is smaller than with respect to waterA cloud, which is saturated with respect to water is supersaturated with respect to ice !Cloud ParametrizationClouds 1
40Ice particles grow at the expense of water droplets Diffusional growth of ice crystals Mixed phase cloud Bergeron process (II)Ice particles grow at the expense of water dropletsIce particle enters water cloudCloud is supersaturated with respect to iceDiffusion of water vapour onto ice particleCloud will become sub-saturated with respect to waterWater droplets evaporate to increase water vapourCloud ParametrizationClouds 1
41Collection processes: Ice Crystal Aggregation Ice crystals can aggregate together to form “snow”“Sticking” efficiency increases as temperature exceeds –5CIrregular crystals are most commonly observed in the atmosphere (e.g. Korolev et al. 1999, Heymsfield 2003)CPIModel500 mmWestbrook et al. (2008)Field & Heymsfield ‘03T=-46oCLawson, JAS’99
42Parametrization of ice crystal diffusion growth and aggregation Some schemes represent ice processes very simply, removing ice super-saturation (as for warm rain process).Others, have a slightly more complex representation allowing ice supersaturation (e.g. current ECMWF scheme).Increasingly common are schemes which represent ice, supersaturation and the diffusional growth equation, and aggregation represented as an autoconversion to snow or parametrization of an evolving particle size distribution (e.g. Wilson and Ballard, 1999). See Lohmann and Karcher JGR 2002(a,b) for another example of including ice microphysics in a GCM.42Cloud ParametrizationClouds 1
43Collection processes: Riming – capture of water drops by ice Graupel formed by collecting liquid water drops in mixed phased clouds (“riming”), particulaly when at water saturation in strong updraughts (convection). Round ice crystals with higher densities and fall speeds than snow dendritesHail forms if particle temperature close to 273K, 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
47Parametrization of rimed ice particles Most GCMs with parametrized convection don’t explicitly represent graupel or hail (too small scale)In cloud resolving models, traditional split between ice, snow and graupel and hail but this split is rather artificial.Degree of riming can be light or heavy.Alternative approach:Morrison and Grabowski (2008) have three ice prognostics, ice number concentration, mixing ratio from deposition, mixing ratio from riming.Avoids artificial thresholds between different categories.47Cloud ParametrizationClouds 1
48Other microphysical processes Splintering, Shedding Evaporation, Melting Other processes include evaporation (reverse of condensation), ice sublimation (reverse of deposition) and melting.Shedding: Large rain drops break up – shedding to form smaller drops, places a limit on rain drop size.Splintering of ice crystals, Hallet-Mossop splintering through riming around -5°C. Leads to increased numbers of smaller crystals.
49Particle Size Distributions Mass DistributionDensitiesTerminal fall speedsFrom Fleishauer et al (2002, JAS)Field (2000), Field and Heymsfield (2003)Cloud ParametrizationClouds 1Cloud ParametrizationClouds 149
50Falling Precipitation Need to know size distributionFor ice also affected by ice habitPoses problem for numericsCourtesy: R Hogan, U. ReadingFrom R Hogan
51Microphysics at the Cloud Scale Typical time-height cross section of a front from the vertically pointing 94GHz radar at Chilbolton, UKIce Nucleation and diffusion growthAggregationWarm phaseMeltingInteraction with the dynamicsFall streaks, small scale dynamics drives microphysicsImage from Robin Hogan. Data from RCRU RAL.Cloud ParametrizationClouds 1Cloud ParametrizationClouds 151
55Summary: Warm Cloud E.g: Stratocumulus Evaporation Condensation (Rain formation - Fall Speeds - Evaporation of rain)
56Summary: Deep Convective Cloud Heteorogeneous Nucleation of iceSplintering/Bergeron ProcessMelting of Snow and GraupelPrecipitation Falls SpeedsEvaporation in Sub-Cloud Layer
57Summary: Cirrus cloud Homogeneous Nucleation (representation of supersaturation)Heterogeneous Nucleation(representation of nuclei type and concentration)Diffusional growth, AggregationSedimentation of ice crystals
59Microphysics - a “complex” GCM scheme Fowler et al., JCL, 1996Similar complexity to many schemes in use in CRMsCloud ParametrizationClouds 1
60Summary 1: A complex system! Molecular ScaleNucleation/activation, Diffusion/condensation/evaporationParticle ScaleCollection/collision-coalescence/aggegation, Shedding/splinteringParcel ScaleParticle Size DistributionsCloud ScaleHeterogeneityInteraction with the dynamics (latent heating)
61Summary 2: Simplifying assumptions Parametrization of cloud and precipitation microphysical processes:Need to simplify a complex systemAccuracy vs. complexity vs. computational efficiency trade offAppropriate for the application and no more complexity than can be constrained and understoodDynamical interactions (latent heating), radiative interactionsStill many uncertainties (particularly ice phase)Particular active area of research is aerosol-microphysics interactions.Microphysics often driven by small scale dynamics…..Next lecture: Cloud CoverSub-grid scale heterogeneityLinking the micro-scale to the macro-scale
62References Reference books for cloud and precipitation microphysics: Pruppacher. H. R. and J. D. Klett (1998). Microphysics of Clouds and Precipitation (2nd Ed). Kluwer Academic Publishers.Rogers, R. R. and M. K. Yau, (1989). A Short Course in Cloud Physics (3rd Ed.) Butterworth-Heinemann Publications.Mason, B. J., (1971). The Physics of Clouds. Oxford University Press.Hobbs, P. V., (1993). Aerosol-Cloud-Climate Interactions. Academic Press.Houze, Jr., R. A., (1994). Cloud Dynamics. Academic Press.Straka, J., (2009). Cloud and Precipitation Microphysics: Principles and Parameterizations. Cambridge University Press. Not yet released!