Observations of supercooled liquid water over Tasmania and the Southern Ocean

The glaciogenic cloud seeding program over Tasmania is unique for both its longevity and repeated claims of positive results. Smith et al. (1979) reported a 30% increase in the first research trial (1964-1971), Ryan & King (1997) reported a 37% increase in a second research trial (1979-1983) and Morrison et al. (2009) reported a 5-14% increase in the monthly rainfall during seeded periods between 1960-2005 over a catchment of 2500 km2.

Limited in-situ observations have reported large concentrations of supercooled liquid water, commonly at or above 0.3 g/kg for 5-minute flight legs at -8° C. However, a complete investigation of any aerosol-cloud-precipitation interaction/evolution involving in-situ and remote observations remains to be undertaken. The physical changes arising from cloud seeding have not been observed and documented, preventing a more complete confidence in the statistical observations.

A-Train observations (CloudSat, Calipso as well as radiometer observations from MODIS) present a new opportunity to explore the phase and temperature of these clouds and potentially shed insight into the structure of more extensive Southern Ocean cloud fields. A MODIS climatology of cloud-top phase finds that extensive fields of supercooled liquid water (SLW) extend over the Southern Ocean and are commonly present in regions of winter-time orographic precipitation across southeast Australia. These satellite observations are consistent with the in-situ observations of high concentrations of SLW relative to glaciogenic cloud seeding programs over the Western United States. The cloud climatology is being extended to A-Train observations. Field observations from the 2010 season will also be presented in which efforts are being made to seed upwind of an A-Train overpass.