1 / 21

Warm cloud microstructures

Warm cloud microstructures. Liquid water content (LWC): amount of water per unit volume of air Droplet concentration: # droplets per unit volume of air Droplet size distribution/spectrum: droplet concentration vs. size interval. Liquid water content & entrainment.

reece
Download Presentation

Warm cloud microstructures

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. Warm cloud microstructures • Liquid water content (LWC): amount of water per unit volume of air • Droplet concentration: # droplets per unit volume of air • Droplet size distribution/spectrum: droplet concentration vs. size interval

  2. Liquid water content & entrainment • Liquid water content (LWC) correlated with updraft speed; large intra-cloud variability • Actual LWC << adiabatic (skew-T-predicted) LWC due to entrainment of unsaturated ambient air

  3. Liquid water content & entrainment • Cloud water evaporates into (subsaturated) entrained air  cools, sinks • Parcels can descend several km, even within updrafts (penetrative downdrafts) • Causes patchy LWC distributions and broadens DSDs

  4. Marine vs. continental warm clouds • CCNs more concentrated over land (soil particles, forest fires, pollution)  LWC distributed over more droplets • Thus, smaller mean droplet sizes and narrower drop size distributions (DSDs) in continental clouds • Marine clouds can be shallower and still precipitate due to larger mean droplet size

  5. Cold cloud microphysics

  6. Ice nucleation • Useful analogies between warm/cold microphysics • For supercooled (i.e., T < 0) droplet to freeze, ice embryo must be large enough that growth decreases system energy • Both homogeneous and heterogeneous nucleation mechanisms (latter requires less extreme environment)

  7. Ice nucleation (cont.) • Homogeneous nucleation – chance aggregation of water molecules to form ice embryo exceeding critical size (T < -40) • Heterogeneous nucleation – water molecules collect on freezing nucleus within droplet (can occur at much warmer T) • Contact nucleation – external particle contacts droplet (may occur at still higher T) • Deposition – vapor changes directly to ice on suitable particles

  8. Ice nucleation (cont.) • Particles with ice-like molecular structure and that are water-insoluble tend to be more effective ice nuclei (e.g., certain clays, organic materials) • Occurs at higher T if air supersatured relative to water rather than to ice only (since this allows condensation-freezing) • Ice nuclei concentration increases exponentially as T decreases

  9. Ice multiplication • Observed ice particle concentration often exceeds predicted ice nuclei concentration • Ice crystal breakup • Supercooled droplets freezing in isolation • Freezing of droplets onto ice particle (riming) – numerous ice splinters shed by droplets encountered by falling particle • Last mechanism probably most important, but still doesn’t explain explosive growth in ice particle concentration observed in some clouds (more research needed)

  10. Growth by deposition • Analogous to droplet growth by condensation, except nonspherical shape must be accounted for (elecrostatic analogy) • Supersaturationw.r.t. ice much greater than w.r.t. water (10-20 % vs. 0-1 %) • Thus, ice particles grow much faster from vapor than do droplets • Growth maximized ~-14 C - difference between saturation vapor pressures of water vs. ice maximized

  11. Ice crystal habits • Basic habits determined by T during vapor deposition (plates  columns  plates  columns as T decreases) • All essentially hexagonal, but axis ratio varies greatly • Basic shapes embellished when air nearly saturated (or supersaturated) relative to water

  12. Growth by riming (accretion) • Ice particles collide with supercooled droplets • Graupel –original habit indiscernible • If hailstone collects supercooled water rapidly, latent heat release can prevent some of collected water from freezing – “wet growth” (light, bubble-free layers in stone) • Hailstone lobes – enhanced collection efficiencies for droplets

  13. Growth by aggregation • Ice particle collisions much more likely when terminal fall speeds different • Collision frequency enhanced by riming since fall speeds of rimed particles more sensitive to dimensions, amount of riming • Adhesion frequency determined by habit (e.g., higher for dendrites than plates) and T

  14. Growth to precipitation size • Growth by deposition alone too slow to produce large raindrops • Depositional growth proceeded by riming and aggregational growth, which both increase with size • Bright band – melting ice particles have higher radar reflectivity; upon melting completely, terminal fall speeds increase, reducing concentrations below

  15. Related Topics

  16. Cloud modification • Warm cloud seeding with hygroscopic nuclei • Fog mitigation: seeded droplets grow at expense of fog droplets and fall out • Rain initiation: inject water droplets or nuclei into cloud base; condensational growth occurs within updraft, then collision-coalescence as droplets descend • Cold cloud modification • Likely more efficient since ice particles can grow very rapidly in presence of supercooled droplets • Precip initiation: dry ice induces homogeneous nucleation, raising ice nuclei concentration toward optimal level • Dissipation of supercooled clouds/fog: overseed with dry ice or silver idodide, glaciating the cloud  ice crystals become small and supersaturation relative to ice low  crystals evaporate

  17. Cloud modification (cont.) • Hail suppression • Artificial nuclei should decrease average size of ice particles by increasing competition for supercooled water • Overseeding could cause nucleation of most supercooled droplets, reducing growth by riming • Cloud modification has had mixed success

  18. Thunderstorm electrification • Graupel or hailstones (rimers) become negatively charged by, and positively charge, cloud particles (precise mechanism unknown) • Positive charge carried aloft by updrafts • Electric field intensifies until dielectric strength of air exceeded  lightning

  19. Cloud-to-Ground Lightning • 90 % of ground flashes negatively charged • Stepped leader – discharge originating between main negatively charged region and positively charged cloud base • Travels groundward in discrete steps • Induces (+) charge on ground (repels electrons) , triggering discharge that moves upward • Once two discharges connect, electrons flow to ground and visible lightning stroke propagates upward to cloud • See book for subsequent details

  20. Cloud-to-Ground Lightning (cont.) • Understand what’s going on in these figures!

  21. Thunder • Return stroke heats air to > 30,000 K • Pressure in channel increases to 10-100 atm • Induces supersonic shock wave in addition to sound wave (thunder) • At distances > 25 km, thunder generally refracted above earth’s surface (inaudible)

More Related