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

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

“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

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

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

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

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.

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

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

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.

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

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

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

FPL commissioned FSU to characterize lightning for Miami-Dade

Lightning also of interest for hurricane studies

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

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