Warm cloud microstructures

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)