Stable Isotope Evidence for Moisture Sources in Central Texas and Polar Regions

Hydrogen and oxygen isotopes incorporated in the water molecules as tracers could provide valuable information about moisture sources for precipitation. They could also be utilized as tracers for the mixing process of different water bodies, for example, the snow-ice-sea water interfaces in polar regions. In Texas, I have systematically investigated the evolution of daily isotopic signatures from precipitation during 2017-2019, and intra-event isotopic evolution for two severe convective storm events. In polar regions, I have conducted isotopic analyses of numerous sea ice cores combined with snow/seawater samples from both the Arctic and the Antarctic. This study resulted in several conclusions:

1)The controls on the isotopic signature of precipitation in central Texas were investigated. Neither “amount effect” nor “temperature effect” was found for the temporal variations of the oxygen isotope of the daily precipitation in San Antonio; however, moisture source is the controlling factor on daily precipitation isotopic signatures, which generally agrees with the HYSLPIT model results.

2)Two severe convective storm events were studied: Hurricane Harvey of August 25-27, 2017, and hailstorm of April 12, 2016. The stable isotope compositions of precipitation for Hurricane Harvey changed remarkably before, during, and after Hurricane Harvey, which was due to changes of moisture sources from terrestrial to oceanic sources and sub-cloud dynamic processes. The isotopic analysis of the hailstone subsamples, together with their crystal structures, are examined here to provide information on the vertical profile of temperature of the clouds in which the hailstones formed.

3)The sea ice mass balance has different responses to local atmospheric circulations in polar regions: in the Arctic, melt ponds form in topographic depressions during the summertime; in the Antarctic, overall thinner ice cover and higher snow accumulation rates result in the widespread occurrence of snow-ice formation. In the Arctic, water balance calculations utilizing two isotopic tracers (oxygen isotope and deuterium excess) suggest that besides the melt of snow cover, the precipitation input in the melt season may also play a role in the evolution of melt ponds. In the Antarctic, we utilized an updated oxygen isotope mixing model to determine the snow-ice contribution for sea ice water balance. The updated mixing models developed here may become more valuable in a future scenario of increasing polar precipitation.