DETERMINATION OF ICE-PHASE WATER CAPTURE TEMPERATURES USING ISOTOPIC COMPOSITION AND HABITS OF ICE CRYSTALS - RELEVANCE TO SNOWPACK AUGMENTATION

The oxygen 18/oxygen 16 (O-18/O-16) and deuterium/hydrogen (D/H) ratio s of snowmelt have been used for estimating the weighted mean temperat ures in clouds where ice-phase water capture has occurred during the p recipitation-forming process. The isotopic measurements were combined with ice crystal replication and microphotographic observations of pri mary ice crystal habits and degrees of riming. Measurements from two c omplete winter seasons have enabled the development of climatological databases of these ice-phase water capture temperatures for the centra l Sierra Nevada and the Snowy Mountains of Australia. The results are based on the linear relationships between the temperature of formation in the clouds of ice crystals grown by vapor deposition and the depar tures (deltaO-18, deltaD) of the oxygen and hydrogen isotopic ratios i n these crystals from the standard mean ocean water values. It was fou nd that precipitation falling from orographic winter storms collects m ost of its water substance in the lower 1-2 km of the supercooled clou ds. In the Sierra Nevada, average ice-phase water capture occurred aro und -10-degrees-C. In the Snowy Mountains the primary capture region a ppears to have an average temperature of -5-degrees-C with a secondary region centered on -12-degrees-C. Such databases may be useful when d esigning cloud-seeding projects in regions where snow is the principal form of precipitation.