1. Nov. 9
Artificial Modification of Clouds and Precip
Thunderstorm Electrification
Cloud and Precipitation Chemistry
Additional problems due next Wednesday:
6.28, 6.34 (explain), 6.35, 6.36

2. Artificial Modification of Clouds, or, “Cloud Seeding”:
Why ?
improve conditions for aircraft
encourage rain (for agriculture)
Focus has been on cold clouds (T<0C)
Idea: ice concetrations limited by ice nuclei number
=> Increase IN

3. “dry ice” - frozen CO2, thus T= -78 C
Encourages homogeneous nucleation in its wake
1cm diameter => 1011 ice crystals!
Supercooled liquid =>small ice crystals=>evaporation
Used at airports (non-US, I think)
Note
stability

4. Artificial IN: silver iodide most common
Works best when cloudtops ~ -14 C (why?)
Who ? Israel, South Africa,Australia, US, more recently,
China, Thailand
11/4/05

5. Though done to enhance precip, after much $$$, unclear results
cloud seeding arguably most successful in mountaineous
Areas
Orographic lifiting produces large amounts of supercooled
Water, at predictable locations
Airflow patterns also important
But: regions with little rain, also have variable rain anyway

6. Under the right conditions (weak stable layer cap),
Can release further cloud buoyancy
Picture from either Caribbean or Florida. More recent
experiments in Texas show less dramatic cloud changes

7. Diffusing hurricanes through seeding:
STORMFURY: Willoughby et al., BAMS, 1985
Seeding moves all freezing to 0C level,lower level of latent
Heat release strengthens updrafts, intensifying hurricane,
Ultimately widening eyewall & reducing wind speed.
STORMFURY
Could not prove
hypothesis;
hurricane seeding
abandonned.

8. Urban island effects on precipitation
Plenty of IN
Aerosols found to decrease
Droplet spectra,
Leading to greater
Separation of charge
In mixed-phase regions,
Increased lightning
But
Also increased precipitation

9. Lightning: Why care ? damage to property, life, aerospace
indicator of convective dynamics, strength
Technological advances have recently fostered field

10. Updrafts separate charges+ + + -
graupell - - - - +
Graupel becomes negatively charged, lighter ice particles
Are positive. Thought to reflect relative ease of movement
Of positive ions in ice.

12. A ground-based lightning detection network detects
Cloud-to-ground lightning. Satellites detect all lightning.
In Tropics: all lightning/CG lightning ~ 10
In midlatitudes: all lightning/CG lightning ~ 2
Occasionally CG is positive. More violent. Mostly along
Rocky Mountains
satellite
Detection at ground

13. Lightning detection from space: FORTE, LIS on TRMM
LIS annual climatology
Most lightning over land; FL highest incidence in US

14. High FL lightning frequency related to ocean-land breezes
On both sides + lake breeze
Maximum in July, next is August

15. FPL commissioned FSU to characterize lightning for Miami-Dade

16. Lightning also of interest for hurricane studies

17. Cloud and Precipitation Chemistry
Clouds react to aerosols, but also modify them,
By type and number
Clouds transport gases and particles
scavenge aerosols
dissolve aerosols
foster aqueous-phase chemical reactions
scavenge through precipitation. Either diffusional,
Or inertial

18. Transport of aerosols
Possibly a large contributor to aerosol concs in marine
subtropics