1 / 49

Cloud Physics: Content

Cloud Physics: Content. Introduction - Importance and definition of clouds - size distribution of cloud particles - cloud classification - thermodynamics · humidity measures · saturation · stability Water clouds - homogeneous nucleation - heteorogeneus nucleation - diffusional growth

irving
Download Presentation

Cloud Physics: Content

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Cloud Physics: Content • Introduction- Importance and definition of clouds - size distribution of cloud particles- cloud classification- thermodynamics · humidity measures· saturation· stability • Water clouds - homogeneous nucleation- heteorogeneus nucleation- diffusional growth • Precipitation • Ice Phase • Measurement of cloud parameters • Modeling of clouds Generate saturation by updraft or lifting Microphysics: What degree of saturation is needed? How quickly and how efficient does a cloud rain? Investigate cloud parametrizations in COSMO model METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  2. Cloud Modelling Ch.5. Grid-Scale Clouds and Precipitation 5.1 General Aspects (all) 5.2 Bulk Approach NB 5.3 Cloud Condensation and Evaporation SD 5.4 Warm Rain Scheme TM 5.5 A One-Category Ice Scheme CT 5.6 A Two-Category Ice Scheme AH METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  3. Organisation 9 written exercises 1 presentation (30 November) 2 computer exercises METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  4. A look into the cloud How do drop size distributions look like? Which processes determine the drop size distribution? • formation of new droplets (nucleation) • growth by condensation and collision/coalescence • loss by breakup and sedimentation METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  5. Moments of droplet spectra N 0. moment droplet concentration [cm-3] Asfc2. moment surface area βe volume extinction coefficient ρw/LWC 3. moment liquid water content [g m-3] reff 3./2. moment effective radius [μm] z 6. moment radar reflectivity factor [mm6 m-3] METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  6. Homogeneous Nucleation formation of a pure water droplet by condensation of saturated air • air does not contain any particles (aerosols, pollutants) • random collisions of water molecules in gaseous phase form small water droplets (embryos) • the higher the vapor pressure the more molecules hit the droplet • after a certain size is reached embryos form a stable droplet M. Quante Difference of Gibb‘s free energy between both systems METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  7. Chemical Potential μ • was introduced by american all-round scientist Josiah Williard Gibbs 1839-1903 • is an analogue to electric potential and gravitational potential, utilizing the same idea of force fields as being the cause of things moving- to react with other substances (chemical reaction)- to transfer to a different state (phase transition)- to redistribute in space (diffusion). Look at an open one phase system of N components mk mass fraction of component k μ chemical potential μ describes the change of internal energy of the full systemby the local gain or loss of mass of the component k METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  8. Gibbs fundamental equations Saturation corresponds to the thermodynamic condition of equilibrium betweenthe different phase states of substances. For an open one phase system composed of N substances the thermodynamic functions can be written with the help of the chemical potential μ and the mass fraction m: Internal Energy U (S,V,mk) Enthalpy H (S,p,mk) free Energy F (T,V,mk) free Enthalpy G (T,p,mk) Helmholtz free Energy Gibbs free energy compare to specific formulation of the Clausius-Clapeyron equation; case of a one component system formulation for isothermal, isobaric change of state METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  9. Gibbs-Duhem-Relation • is the total differential of the chemical potential after Josiah Willard Gibbs and Pierre Duhem • describes the relationship between changes in chemical potential for components in a thermodynamical system provides a relationship between the intensive variables of the system. For a simple system with different components, there will be k+1 independent parameters or "degrees of freedom“ For isothermal and isobaric processes the Gibbs-Duhem-relation is reduced to METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  10. Homogeneous Nucleation How does system energy ΔE develops as a function of droplet radius r? decrease of Gibbs free energy due to phase change of molecule work required tochange surface V droplet volume [m3] nl number of water molecules in droplet [m-3] l,v chemical potential per molecule water (vapour) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  11. Curvature Effect The curvature effect is a barrier to droplet formation, because the tiniest droplets have the tightest curvature, which tends to destroy the droplet by net evaporation rather than allowing the droplets to grow by net condensation Work Δw [J], necessary to changesurface area A [m2] with surface tension  [N/m]- about 7.5·10-2 N/m- difficult to determine (pure water, cold temperatures,..) Overcome bonding force of molecules nach Seinfeld&Pandis, 1998 Rogers & Yau: c1 = -1.55 x 10-4 N m-1 K-1 for -20 to +20 °C c2 = 0.118 Nm-1 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  12. Krümmungseffekt

  13. Homogeneous Nucleation Necessary condition for droplet growth growth is energetically favoured ΔE e<es ΔE* e>es A surface [m2] V volume [m3] n number of water molecules [m-3] k Boltzmann constant e vapor pressure [Pa] es saturation vapor pressure (r= ) 0 r rc isobaric, isothermal conditions METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  14. Kelvin-Equation Describes the equilibrium vapor pressure over a curved surface of pure water. Known also as Thomson equation (Lord Kelvin) Given a certain saturation ratio S = es(r)/es the critical droplet radius rccan be calculated telling when a stable drop has formed r < rc drop evaporates r > rc drop grows by condensation Rv gas constant of water vapor w water density Lord Kelvin = William Thomson (1824-1907), professor for sciences, Glasgow METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  15. Critical Droplet Radius Drop which is formed by random collision of n molecules, is only stabile at the given supersaturation s METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  16. ok Heteorogeneous Nucleation in natural clouds rarely supersaturations of more than a few percent are detected (exemption ice clouds) homogeneous nucleation is mainly responsible for c condensation in the atmosphere • Aerosol causes heteorogeneous nucleation • hydrophobic (oil, gasoline,paraffin, waxes) particles repel water → higher supersaturation needed for drop nucleationg • neutral→ supersaturation as homogeneous nucleation at aerosol radius • hydroscopicsea salt, sulfuric, nitric acid particlesparticle attracts water, strongly affine particles swellat low relative humidity (haze) → low supersaturation for drop nucleation METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  17. Aerosol Characteristics • shape can be spherical, cristalline,feathery, agglomerate or irregular • particle size reach from 10-3μm (molecule cluster) to 10 μm (large salt crystals, combustion particles)PM10 – particle mass with d<10 µmPM2.5 – particle mass with d<2.5 µm • chemical composition highly variable(sulfate, carbon, salt, nitrogen, biological substances ..) Composition, size and shape determine optical properties Aerosol optical density @0.5 μm (AOD), single scattering albedo ωo, asymmetry factor g METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  18. VOC VOC VOC SOA Aerosol Sources • primary aerosol accounts to approx. 75 % 20 % wind driven, 40 % sea salt (spray), 10 % forest fires, 5 % industrial processes • secondary production in the atmosphere 25 %Formation in the atmosphere by condensation or disintegration of liquid and solid substances Numbers denote mass burden (Tg) & emission (Tg/a, in parenthesis) after Andreae and Rosenfeld 2008 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  19. Aerosol Input to the Atmosphere mineral aerosol sea salt industrial emissions biomass burning volcanic eruptions Wolkenphysik, Susanne Crewell, SS 2007

  20. Vertical Distribution of Aerosol Wolkenphysik, Susanne Crewell, SS 2007

  21. Aerosol Concentration over oceans 1000 cm-3 over continents 10000 cm-3 within cities 100000 cm-3 clean air: ca. 50-100 cm-3 strong decrease with height Aitken large giant nuclei 106 104 102 100 10-2 10-4 10-6 dN/d(log D) [cm-3] maritime aerosol continental aerosol urban aerosol 10-2 100 102 Diameter [μm] METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  22. Aerosol Cycle Importance: health (respiration), visibility, climate (direct/indirect), cloud formation, heteorogeneous reactions, long range transport of nutrients and pollutants Daniel Jacob, Atmospheric Chemistryhttp://acmg.seas.harvard.edu/publications/jacobbook/index.html natural aerosol mainly incoarse mode (sea salt and dust reff=2.5 μm) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  23. Precursor gas, e.g.sulfate oxidation SO2+OH  HOSO2 HOSO2+ O2  HO2+SO3 SO3+ H2O  H2SO4 (g) H2SO4 (g)  H2SO4 (s) homogeneous heteromolecularcondensation of sulferic acid-already at low relative humidityof 50-80 % by recuction ofsaturation pressure of H2O/H2SO4 mixture nucleationmode coarsemode accumula-tion mode METSWN, Susanne Crewell & Ulrich Löhnert, WS 2009/10

  24. Aerosol Distribution • Often parametrized as Junge-Distribution (after chemist Christian Junge) • Distribution of number of particles N(r); and also particle volume V(r) for r >0.1μm Typical U.S. aerosol size distributions by volume URBAN RURAL Bimodal distribution is composed of accumulated and coarse mode Wolkenphysik, Susanne Crewell, SS 2007

  25. Size and Surface Area Distribution Size distribution dN/d(logD) with logarithmic number axis Surface area distribution dS/d(logD) linear number axis 106 104 102 100 10-2 10-4 10-6 dN/d(log D) [cm-4] maritime aerosol continental aerosol urban aerosol 10-2 100 102 Diameter [μm] METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  26. Heteorogeneous Nucleation condensation nuclei are highly frequent in the atmosphere and cause droplet formation → cloud condensation nucleiCCN insolvable increase in drop size with sameaerosol: physics as homogeneous nucleation → curvature effect solvable physics and chemistry changeaerosol: → curvature effect → solution effect Vapor pressure for a solvent (such as water) due to the presence of dissolved material (such as salt) is reduced in proportion to the mol fraction of the solute (salt) Raoult'sches Gesetz METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  27. Solution Effect: Raoult‘sches Gesetz strongly diluted solution: n << no es saturation vapor pressure over pure water surface e‘s saturation vapor pressure over aquatic solution nonumber of water molecules n number of dissolved molecules n+no total number of molecules Francois Marie Raoult (1839-1901), professor for chemistry, Grenoble METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  28. Solution Effect Raoult Law strongly diluted solution: n << no b Result: solution droplet that is in equilibrium at much lower supersaturation than pure water droplet of the same size es‘equilibrium vapor pressure over solution nonumber of water molecules n number of dissoved molecules i van't Hoff Faktor, degree of ionisation ~2 (typical Na+Cl-) No Avogadro number 6.022 x 1023 mol-1 M mass of solute (s) or water (w) m molecular weight of solutant (s) or water (w) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  29. Combination of Curvature & Solution Effect r not too small Köhler, H., 1936, Trans. Far. Soc, 32, 1152-1161. Hilding Köhler, 1888-1982, professor for meteorology, Uppsala, Sweden METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  30. Köhler Equation From dS/dr = 0 folllows the critical radius with S* = S - 1 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  31. Condensation stabil Increase in supersaturation, e.g. by lifting instabil S* Droplet growth as less supersaturation is needed at larger radius r* swollen aerosol, haze activated particle= cloud droplet METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  32. Aerosol Swelling aerosol swelling (non activated) cloud droplets (activated) M. Quante Relative humidity of deliquescence (RHD): point at which a dry particle spontaneously takes up water to form a saturated solution METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  33. Aerosol Swelling aerosol swelling (non activated) cloud droplets (activated) Clouds and Haze M. Quante Relative humidity of deliquescence (RHD): point at which a dry particle spontaneously takes up water to form a saturated solution METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  34. Exercise to compute particle size distributions METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  35. Aerosol Concentration accumulation mode coarse mode Forschungsbericht 203 43 257/05 Umweltbundesamt-FB 000942, Birmili (IfT Leipzig) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2011/12

  36. Size and Surface Area Distribution Forschungsbericht 203 43 257/05 Umweltbundesamt-FB 000942, Birmili (IfT Leipzig) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  37. Particle Formation Birmili (IfT Leipzig) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  38. Expanded Köhler-Theory Weakly solvable component Solvable gas during growth dissolved material increases the capacity for activationExamples HNO3, HCL, NH3 M. Quante METSWN, Susanne Crewell & Ulrich Löhnert, WS 2011/12

  39. Köhler Equation for Polluted Air Kulmala et al., 1997 (Nature): Existence of stable cloud droplets without supersaturation:fog/smog in strongly polluted areas or at volcanic eruptions TraditionellMulti-phase and multi-component Köhler-Theorie Effect of highly dissolved condensed gas (salpetric acid) ro radius of weakly solvable aerosol nuclei xw Molar ratio of weakly solvable species (s) or gases (g) bs Raoult effect of solvable salt ba effect of highly solvable condended gas METSWN, Susanne Crewell & Ulrich Löhnert, WS 2011/12

  40. Köhler Equation in Polluted Air 1) clasical Köhler curve for dry 30 nm particle ammonium sulfate 2) additionally to 1) insolvable 500 nm large nuclei 3) additionally to 1) solvable 500 nm nucleus of CaSO4 4) additionally to 3) highly solvable gas; 1 ppb HNO3 1.003 1.0 0.998 nucleus is solved decay of salpeteric acid 0.1 1 10 radius in μm Attention: also surface tension of solution is changing METSWN, Susanne Crewell & Ulrich Löhnert, WS 2011/12

  41. Which aerosols act as CCN? Cloud Condensation Nuclei M. Quante METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  42. What is the difference between CCN and aerosol? Heintzenberg & Charlson, 2009: Clouds in the perturbed climate system METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  43. How many CCN available? polluted maritime max. observed supersaturationin strongly convective clouds0.5 % arctic the higher the supersaturation the more particles are activated Wallace & Hobbs, Fig. 6.5 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  44. Aerosol – cloud interaction Ramanathan et al., Science 2001

  45. What‘s that? http://visibleearth.nasa.gov/view_rec.php?id=5192 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  46. Ship Tracks NASA, 2002 Atlantic, France, Spain AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  47. Twomey - Mechanismus pollutedatmosphere 1. indirect aersol effectchange in microphysics known since more than 50 years droplet concentration polluted: up to 1000 cm-3 clean air: 50-100 cm-3 clean air Toon, Science 2003 2. indirect aersol effectreduction of precipitation efficiency METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  48. Aerosol Influence on Climate Heterogeneous condensation and cloud growth more aerosol particles are activated to cloud droplets more cloud droplets compete for the available water vapor smaller droplets → higher reflectivity → brighter clouds http://terra.nasa.gov/FactSheets/Aerosols/ METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  49. Proof of Indirect Effect N ~ 100 cm-3 W ~ 0.75 g m-3 re ~ 10.5 µm N ~ 40 cm-3 W ~ 0.30 g m-3 re ~ 11.2 µm from D. Rosenfeld • ship emissions enhance number of cloud condensation nuclei (CCN) • increased CCN concentration leads to increased number of droplets and reduces their average size • enhanced concentration and smaller particles reduce drizzle production • enhanced liquid water content (LWC) due to smaller loss (reduced drizzle) • clouds are optical thicker and brighter along ship tracks METSWN, Susanne Crewell & Ulrich Löhnert, WS 2011/12

More Related