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)
Supersaturation w.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 atm
Induces supersonic shock wave in addition to sound wave (thunder)
At distances > 25 km, thunder generally refracted above earth’s surface (inaudible)