Oslo 1

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Published on January 2, 2008

Author: Altoro

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Slide1:  Theory and basic knowledge: The fundamental differences between continental and maritime clouds, and the aerosol control of these differences (3 hours): The determination of cloud base drop concentrations as a function of CCN concentrations, updraft velocity and CCN size distribution. The evolution of cloud drop size distribution with height in the clouds. The formation of warm rain and aerosol control of it. Supercooled clouds and ice precipitation processes and aerosol control of it. The lifecycle of convective clouds, and the fundamental difference between "continental" aerosol rich clouds and "maritime" aerosol lean clouds. The reversal of air pollution effects by sea salt aerosols. 2. Demonstration of pollution impacts on clouds and precipitation from model and satellite observations (1 hour). 3. Demonstration of salt aerosols reversing the pollution impacts on clouds and precipitation from satellite observations (1 hour). 4. Cloud model simulations of the aerosol effects. (1 hour). 5. Public seminar : From ship tracks to pyro-clouds: Present day major climate changes by aerosols suppressing precipitation. Slide2:  Theory and basic knowledge: The fundamental differences between continental and maritime clouds, and the aerosol control of these differences (3 hours): The determination of cloud base drop concentrations as a function of CCN concentrations, updraft velocity and CCN size distribution. The evolution of cloud drop size distribution with height in the clouds. The formation of warm rain and aerosol control of it. Supercooled clouds and ice precipitation processes and aerosol control of it. The lifecycle of convective clouds, and the fundamental difference between "continental" aerosol rich clouds and "maritime" aerosol lean clouds. The reversal of air pollution effects by sea salt aerosols. 2. Demonstration of pollution impacts on clouds and precipitation from model and satellite observations (1 hour). 3. Demonstration of salt aerosols reversing the pollution impacts on clouds and precipitation from satellite observations (1 hour). 4. Cloud model simulations of the aerosol effects. (1 hour). 5. Public seminar : From ship tracks to pyro-clouds: Present day major climate changes by aerosols suppressing precipitation. Slide3:  Relative sizes of cloud droplets and raindrops; r is the radius in micrometers, n the number per liter of air, and v the terminal fall speed in centimeters per second. The circumference of the circles are drawn approximately to scale, but the black dot representing a typical CCN is twenty-five times larger than it should be relative to the other circles. Adapted from Adv. in Geophys. 5, 244 (1958). Slide4:  MR3421 - Cloud Physics - Spring 1999 Formation of Cloud Droplets (Read Chapter 6 - Rogers & Yau) Why do cloud droplets form almost immediately upon reaching supersaturation? s is the surface tension (energy/area or force/length) Smaller drops require higher es for equilibrium Slide5:  Statistical thermodynamic calculations show that S must be 300-600% for one homogeneous nucleation event per cm3 per second in the natural atmosphere. Since S rarely exceeds 1-2%, homogeneous nucleation is never consistently achieved. Slide6:  The relative humidity and supersaturation (both with respect to a plane surface of pure water) with which pure water droplets are in (unstable) equilibrium at 5ºC. Slide7:  Nucleation (phase transition across a free energy barrier) of droplets requires a particle (condensation nucleus). Hygroscopic nuclei are soluble in water and decrease es(r) significantly. hygroscopic hydrophobic http://terra.nasa.gov/FactSheets/Clouds/ r es(r) + - + + + + + + + + + + + + - - - - - - - - - - - - With non-water molecules on the surface, the equilibrium (equal transfer across the interface) occurs at lower pressure M = mass of solute C = 3imv/4prLms Slide8:  Curvature effect Increased r, decreases equilibrium/saturation vapor pressure over the drop (fewer molecules required outside the drop at equilibrium) To attain this new equilibrium, vapor molecules will want to enter the drop at a higher rate than they leave (growth) But this positive feedback can’t get started at typical atmospheric saturation ratios. Adding solute, decreases equilibrium vapor pressure over the drop since fewer liquid molecules are available to escape (fewer molecules required outside at equilibrium ) Solution effect To attain this new equilibrium, vapor molecules will want enter the drop at a higher rate than they leave (growth) (note: saturation vapor pressure is the vapor pressure required for equilibrium) (Why does this process require supersaturation?) Slide9:  Now for a solution droplet (compared to a pure water plane surface) the equilibrium vapor pressure is increased due to curvature effects and decreased due to solution effects: Which term dominates below 100% RH? Why does the Köhler curve approach 1.0 for large r? Growth does not continue without bound since drops start to compete for vapor Slide11:  From Seinfeld and Pandis Slide12:  At some supersaturation… From Seinfeld and Pandis OD model of CCN activation:  OD model of CCN activation source proportional to vertical velocity sink depends on CCN size distribution and chemical composition surface tension depends on particle size solute term depends on salt solubility OD model of CCN activation:  S = Smax OD model of CCN activation Guibert et al. 2003 Slide15:  Activity Spectrum = number of activated particles at some supersaturation S and below maritime: C=30-300 cm-3; k=0.3-1.0 continental C=300-3000 cm-3; k=0.2-2.0 Slide16:  Supersaturation is controlled by updraft velocity so... Marine: C=150 k=0.6 Continental: C=1500 k=1.1 Slide17:  Marine: C=150 k=0.6 Continental: C=1500 k=1.1 And the maximum supersaturation becomes… Slide18:  Theory and basic knowledge: The fundamental differences between continental and maritime clouds, and the aerosol control of these differences (3 hours): The determination of cloud base drop concentrations as a function of CCN concentrations, updraft velocity and CCN size distribution. The evolution of cloud drop size distribution with height in the clouds. The formation of warm rain and aerosol control of it. Supercooled clouds and ice precipitation processes and aerosol control of it. The lifecycle of convective clouds, and the fundamental difference between "continental" aerosol rich clouds and "maritime" aerosol lean clouds. The reversal of air pollution effects by sea salt aerosols. 2. Demonstration of pollution impacts on clouds and precipitation from model and satellite observations (1 hour). 3. Demonstration of salt aerosols reversing the pollution impacts on clouds and precipitation from satellite observations (1 hour). 4. Cloud model simulations of the aerosol effects. (1 hour). 5. Public seminar : From ship tracks to pyro-clouds: Present day major climate changes by aerosols suppressing precipitation. Slide19:  NaCl nuclei with Nc = (650cm-3) s 0.7 Why is maximum supersaturation higher for 2m/s updraft velocity? Why is final droplet concentration greater for 2m/s updraft velocity? Why is average radius greater for 0.5m/s updraft velocity? Why is final deviation of radius greater for 0.5m/s updraft velocity? Why is LWC greater for 0.5m/s updraft velocity? Slide20:  Relative sizes of cloud droplets and raindrops; r is the radius in micrometers, n the number per liter of air, and v the terminal fall speed in centimeters per second. The circumference of the circles are drawn approximately to scale, but the black dot representing a typical CCN is twenty-five times larger than it should be relative to the other circles. Adapted from Adv. in Geophys. 5, 244 (1958). Slide21:  Can we grow raindrops by diffusional growth alone? Let cloud base mixing ratio = 10 g vapor / kg air 1 kg air occupies ~ 1 m3 or 106 cm3 = 10-5 g cm-3 Assume only 100 drops cm-3 nucleate at cloud base Lets allow the cloud condense all the vapor onto the drops Each drop will contain 10-5/100 = 10-7 g or cm3 of water. 10-7=4/3pR3 R = 28 mm Assume continental aerosols with 1000 drops cm-3 Then R = 13 mm But a minimum size raindrop is 100 mm Slide22:  Relative sizes of cloud droplets and raindrops; r is the radius in micrometers, n the number per liter of air, and v the terminal fall speed in centimeters per second. The circumference of the circles are drawn approximately to scale, but the black dot representing a typical CCN is twenty-five times larger than it should be relative to the other circles. Adapted from Adv. in Geophys. 5, 244 (1958). Slide25:  x Collision Efficiency x, the separation between the drop centers, or impact parameter, has a maximum value of R+r R r Why do small r/R have low efficiencies? Why does efficiency decrease beyond r/R~0.6? How could efficiency exceed 1.0 (near r/R~1)? Slide26:  Growth of all drops in the distribution - stochastic coalescence gl(r) [log-increment mass density function] vs r rf = radius of the mean droplet mass rg = radius of the mean mass of mass density function Slide27:  Observations: in (a) S1 is depleted and S2 increases in (b) S2 gains by S1 self-combinations (autoconversions) are small in (c) S2 gains are larger when S1 drops add directly to S2 drops (accretion) in (d) S2 self-combinations extend the large size tail of the distribution Slide28:  Theory and basic knowledge: The fundamental differences between continental and maritime clouds, and the aerosol control of these differences (3 hours): The determination of cloud base drop concentrations as a function of CCN concentrations, updraft velocity and CCN size distribution. The evolution of cloud drop size distribution with height in the clouds. The formation of warm rain and aerosol control of it. Supercooled clouds and ice precipitation processes and aerosol control of it. The lifecycle of convective clouds, and the fundamental difference between "continental" aerosol rich clouds and "maritime" aerosol lean clouds. The reversal of air pollution effects by sea salt aerosols. 2. Demonstration of pollution impacts on clouds and precipitation from model and satellite observations (1 hour). 3. Demonstration of salt aerosols reversing the pollution impacts on clouds and precipitation from satellite observations (1 hour). 4. Cloud model simulations of the aerosol effects. (1 hour). 5. Public seminar : From ship tracks to pyro-clouds: Present day major climate changes by aerosols suppressing precipitation.

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