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Organisation

Organisation. 9 written exercises 1 presentation (30 November) 2 computer exercises. Cloud Physics - Content. Introduction water clouds - nucleation of cloud droplets - droplet growth - growth of droplet populations ice phase - nucleation - growth mechanism - habits

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Organisation

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

  2. Cloud Physics - Content • Introduction • water clouds- nucleation of cloud droplets- droplet growth- growth of droplet populations • ice phase- nucleation- growth mechanism- habits • precipitation- warm and cold rain- radar rmeteorology- thunderstorms • measurements of cloud parameters • modeling of clouds- spectral models- cloud parametrizations in NWP and climate models METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  3. Repetition Ice Clouds Heterogeneous Freezing • How do ice crystals form? • And at which temperatures? Homogeneous Freezing METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  4. Repetition Ice Clouds • What are ice nuclei and how frequent are they? Example: Kaolinit (Mineral, Al4[(OH)8|Si4O10] )ice nuclei below -10° C → droplet freezing ice nuclei below - 20° C (and no saturation to water) → deposition nucleation Wikipedia silver iodide lead iodide Kaolinit South. Hem.(Expansion) South. Hem. (Mixing) North. Hem. (Expansion) Antarctica Fig. 6.30 Wallace & Hobbs Fig. 6.31 Wallace & Hobbs METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  5. Ice multiplication – Hallett-Mossop Process • The process (splintering following riming) was first reproduced in the laboratory by Hallett and Mossop (1974) • Ice production only occurs at between -3°C and -8°C (with a pronounced peak in the middle of the range) and in the presence of both large (>24 µm) and small droplets. http://bio-ice.forumotion.com/t19-what-really-is-the-hallett-mossop-process METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  6. Examples of Ice Clouds • Lee clouds • frontal cirrus • contrails • tropical cirrus • "subvisible cirrus" METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  7. Lee Clouds Orographic wave disturbances can lead to updrafts of a few meter per second This can cause lifting of up to a kilometer (corresponding to 10 K) producing sufficient supersaturation. In comparison frontal cirrus shields caused by lifting at cold or warm fronts are connected to velocities in the order of a few cm/s. METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  8. Frontal Cirrus frequent in mid latitudes Seifert and Crewell, 2008 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  9. Contrails Boucher, 2011 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  10. Aircraft Emissions Source: Stefan Borrmann 10-65 g/kg CO 3-6 g/kg NOx CnHm 3160 g/kg CO2 0.01-0.03g/kg soot O2 1230 g/kg H2O N, S 1 g/kg SO2 Emission indices in gramm emission per kg kerosine METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  11. H SO 2 4 Contrail Formation Isobaric mixing of hot exhaust gases with cold dry environment H2SO4 H2O n H O . 2 Ruß ± OH, H2O X Ion-cluster ice crystals S SO 2 H2SO4 H2O H2SO4 H2O n H O . 2 0.01 s 0.1 s 1s Age of exhaust plume Source: Stefan Borrmann METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  12. Contrails • isobaric mixing of hot exhaust gases with cold dry environment • nucleation at supersaturation in respect to liquid water – soot particles, volatile exhaust particles and background aerosol serve as condensation nuclei • diffusional growth of activated particles • homogeneous or heterogeneous freezing of drops • fast growth of ice crystals • evaporation of crystals in non-saturated environment:- in stratosphere within seconds- HNO3 can delay evaporation • model studies indicate studies show that contrails can also be formed without soot as sufficient background aerosol is available leading to larger ice crystals METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  13. Subvisible Cirrus • low optical thickness (VIS) from 0.03 to 0.05; low concentration (~25 l-1) • nearly transparent in VIS but not in the longwave reducing outgoing longwave radiation by a few Wm-2 • clouds are so thin that they cannot be detected by conventional instruments but by satellites • are probably formed by outflow of cumulus nimbus, ceasing contrails or slow large scale lifting • moisture at upper troposphere/lower stratosphere (UTLS) is important criteria but not known sufficiently well METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  14. Thin and subvisible Cirrus Kübbeler et al., 2011, ACP METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  15. IV Precipitation Formation Cloud droplets are rather small (ca. 10 μm) and occur in large populations (up to 1000 per cm3). Populations are rather stabile and show little interaction mostly general growth by diffusion Precipitation is formed then populations become instable precipitation particles are so large that they don‘t completely evaporate when falling • direct collision and coalescence of drops • interaction of water drops and ice crystals ice crstals growth at the expense of water drops warm rain cold rain growth processes lead to precipitation formation starting from condensation nuclei within 20 min METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  16. Coagulation Growth 16 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  17. Break-up of Drops Fig. 6.26 Wallace & Hobbs

  18. Temporal Development of DSD equilibrium is established after some time METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  19. Formation of Cold Rain Franklin 1789, Wagner 1911, Bergeron 1933, Findeisen 1938 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  20. Formation of Cold Rain Freezing + fast diffusional growth Cirrus shields, Amboss - 40 ° C - 15 ° C Collision + coalescence updraft zone 0 ° C melting layer base of cumulonimbus clouds rain out, or graupel/hail METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  21. Stratiform Precipitation at Mid-Latitudes melting layer fall streaks Radar time-height section Wallace & Hobbs Fig. 6.45 Deposition growth 1 mm ice plate falls through with LWC=0.5 gm-3 → spherical graupel r ~ 0.5 mm within a few minutes corresponding to vertical velocity of about 1m/s METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  22. Forms of Solid Precipitation Wallace & Hobbs Tab. 6.2 hexagonal plates stellar crystals hollow/solid columns needles dendrites capped columns irregular crystals graupel ice pellets hail METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  23. Growth of Ice Crystals • Water vapor deposition • growth rate of ice crystal depends on temperature andhumidity: optimum growth at -15°C. • Accretion • growth of ice crystals by riming with supercooled droplets • contact freezing leads to rimed particles • optimum in saturated layers of 0 to -10°C (Staudenmaier, 1999). • extreme riming leads to Graupel or snow pellets. • Aggregation • collision and coalescence of ice crystals • agglomeration is maximum close to 0°C. • ice crystals with dendritic shape can mechanistically can get caught and generate large aggregates METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  24. Diffusional Growth Normalized growth rate of ice crystals at two different pressures as function of temperature (Rogers und Yau, 1989) growth rate max ~ -15° Example: - 5 ° C → plate growthswithin 30 min to ~7μg (r=0,5mm) drizzle drops of about r=0.13 mm corresponding to falls speed of0.3 m/s Diffusional growth produces only weak precipitation METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  25. Growth by Riming • Mixed phase cloud • contact freezing of supercooled drops • Graupel • original form of crystal can‘t be seen anymore • e) sphericalf) conical • Hail • extreme form of riming • max D=13.8 cm and 0.7 kg • fast freezing causes liquid inclusions lightly rimed needle rimed column rimed plate rimed stellar spherical gaupel conicall gaupel Wallace & Hobbs Fig. 6.41 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  26. Formation of Hail • hailhas a minimumdiameterof 5 mm • forhailformationstrong updrafts(upto 50 m/s) and high amountsofsupercooledwaterareneeded • iceparticlescirculateoverthefullverticalrangeofthecumuluscloud multiple times - asthehail falls, itmaymelttovaryingdegreesandbepickedupagainandvcarried high intotheatmospheretore-freeze. • hailstonescontain a kernelwithsurroundingonion-likelayersand intransparent layers • alternatingglassyhailstonesaccumulatewater in thelower warm partofthecloudwhichfreezes in theuppercoldpart (wetand dry growthalternate) . • eachcycleadds an icelayerwetgrowth -> liquid waterspreadsacrosstumblinghailstonesandslowlyfreezesairbubblescanescaperesulting in a layerofclearicedry growth -> airbubblesare "frozen" in place, leavingcloudyice METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  27. Hail Formation METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  28. Growth by Aggregation Collection efficiency = Collision efficiency x Coalescence efficiency • Collision efficiency probably larger than for water droplets because of the larger fall speeds • Collision of ice particle with supercooled droplets has a coalescence efficiency of about 1 • Coalescence efficiency between ice particles is higher at higher temperatures and for dendrites (get stuck) • Observations: significant aggregation only at temperatures > -10°C • Diffusional growth for ice isfaster than for water light precipitation (formation without aggregation) rimed needles rimed columns rimed frozen drops dendrites Wallace & Hobbs Fig. 6.44 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  29. Growth by Aggregation • Agglomoration of ice crystals to form snow particles(get stuck, contact freezing) • collision rate depends on fall velocityGraupel (aggregates of frozen droplets) falls quickly depending strongly on diameter D (in cm) with D encircling the particle • snowflakes and rimed structures fall with about ~1 m/sFor D the diameter of melted particles Dendrites k~160 n=0.3Columns and plates k~234 n=0.3 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  30. Terminal Velocity Avramov, A., and J. Y. Harrington, 2010 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  31. Mass-Size Relation Depending on shape the relation between mass and size varies D – maximum linear dimension of crystal in cmM – crystal mass in g important for mass growth,remote sensing, microphysical modelling METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  32. Relation Mass-Size Avramov, A., and J. Y. Harrington, 2010 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  33. Snow Flakes Snow Rain Snow Snow a b Roger&Yau 2000 2 Sekhon&Sriwastava 1780 2.21 D (mm) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  34. Artificial Precipitation • of high interest in dry regions • first tests in November 1946 (Langmuir) • selection of a cloud with high amount of supercooled water • cloud seeding with ice-freezing nuclei, e.g. dry ice (carbon acid) or silver iodide (AgJ) • non-satisfing results - no clear statistics possible • also used for hail protection (Rosenheim, Stuttgart), because clouds with early onset of precipitation can not build large number of supercooled drops A Y-shaped path cut into a layer of super-cooled cloud by seeding with dry ice Wallace & Hobbs Fig. 6.47 METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  35. Modification of Clouds and Precipitation • injection of large hydroscopic nuclei in warm clouds to stimulate collision induced growth to rain drops • injection of artificial ice nuclei into cold clouds (most likely not containing many nuclei) to stimulate precipitation formation via ice phase difficult • injection of high concentrations of artificial ice nuclei into cold clouds → drastical reduction of supercooled clouds → suppression of riming and aggregation precipitation suppression (in particular hail) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  36. Success or Pure Chance? + 10 min + 19 min + 48 min Wallace & Hobbs Fig. 6.48 Icing causes latent heat release and supports buoyancy lifting above level of free convection METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  37. Artifical Clouds Paper plant contrails industries release heat, water vapor & cloud active aerosol (CCN & IN) METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  38. Exercise Precipitation Processes METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  39. Characteristic Scales of Precipitation Ascent/lifting is caused by O(1000 km) O(100 km) O(10 km) Synoptic Processes Orographic lifting Buoyancy/ secondary circulations Stratiforme precipitation (30 %) Convective precipitation (70 %) Humidity content O(1000 km) O(10 – 100 km) Advection Modification by evapotranspiration and secondary circulation QNV Bonn 12/03 Karlsruhe METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  40. Characteristic Scales of Precipitation Free Troposphere CAPE Boundary Layer turbulent mixing mountain flow Orography Vegetation Surface Heat andhumidity influence Mesoscale circulation Wolkenphysik, Susanne Crewell, SS 2007

  41. Secondary Circulations S. Raasch and G. Harbusch, 2001:An Analysis of Secondary Circulations and their Effects Caused by Small-Scale Surface Inhomogeneities Using LES. Boundary-Layer Meteorol., 101, 31-59. METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  42. Precipitation Process • spatial coverage, intensity and life cycle of a precipitation event is mainly determined by - vertical velocity and- available moisture • precipitation formation is prefered when - large variation of drop sizes - large vertical extent of clouds- strong updrafts exist Stratiform precipitation: extended, continuous precipitation connected with largescale ascent due to frontal or orographic lifting or horizontal convergence Convective precipitation: local showers connected with cumulus scale convection in unstable air mass In reality fluent transition between stratiform and convective METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  43. Formation of Cold Rain • If parts of the water cloud are above the freezing level precipitation formation can be initiated • ice grows at the cost of supercooled water • ice crystals fall faster than cloud droplets • growth by riming (water droplets are caught by collisions) • growth by aggregation (ice paricles connect via contact freezing or entangling) • Iceparticlesfromthemiddleortheupperpartofthecloudcan • reachthegroundasiceparticles • meltandreachthegroundas rain • melt, reachthegroundas rain andrefreezethere • melt, refreezeandreachtheground in form ofgraupel METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  44. Precipitation Types METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  45. Freezing Rain Supercooled drops reach surface which is colder than 0°C METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  46. Precipitation Efficiency • Relation between water vapor flux into cloud andsurface precipitation • What causes high efficiency? • effectiveness of collision-coalescence process • high humidity convergence • reduced evaporation • warm surface layerhigh humidity uptake, effective development of warm rain • enhanced residence time in cloud large height range leads to lowervelocities and better efficiency of warm and cold precipitation process • low Lifting Condensation Level (LCL)low evaporation below cloud • vertical wind shear METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  47. Thunderstorms Single cell thunderstorms Multi cell thunderstorms Super cell thunderstorms • (conditionailly) unstableairmasses • substantial boundarylayermoisture • lowlevelconvergence (orliftingtoreleasetheinstability) • strong updrafts, heavy rain, lightning, hail Life cycle and intensity increases from single to super cells. Single cells hardly produce tornados – super cells frequently METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  48. Stratification Convective available potential energy Convective inhibition Level of equilibrium LFC environmental lapse rate METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  49. Stratification Level of equilibrium LFC environmental lapse rate METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

  50. Classical Diurnal Cycle METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13

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