Determination of Ice-Phase Water Capture Temperatures Using Isotopic Composition and Habits of Ice Crystals Relevance to Snowpack Augmentation

Abstract

The oxygen 18/oxygen 16 (18O/16O) and deuterium/hydrogen (D/H) ratios of snowmelt have been used for estimating the weighted mean temperatures in clouds where ice-phase water capture has occurred during the precipitation-forming process. The isotopic measurements were combined with ice crystal replication and microphotographic observations of primary ice crystal habits and degrees of riming. Measurements from two complete winter seasons have enabled the development of climatological databases of these ice-phase water capture temperatures for the central 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 departures (δ18O, δD) of the oxygen and hydrogen isotopic ratios in these crystals from the standard mean ocean water values. It was found that precipitation falling from orographic winter storms collects most of its water substance in the lower 1?2 km of the supercooled clouds. In the Sierra Nevada, average ice-phase water capture occurred around ?10°C. In the Snowy Mountains the primary capture region appears to have an average temperature of ?5°C with a secondary region centered on ?12°C. Such databases may be useful when designing cloud-seeding projects in regions where snow is the principal form of precipitation.