Intra-storm Variation in Oxygen and Hydrogen Isotope Ratios of Precipitation from Dallas, TX: 2001-2003
Abstract
We report the results of precipitation sampling for δ D and δ 1^8O covering the years 2001-2003. Dallas, north central Texas, is located such that it samples moisture from the Gulf of Mexico and from fronts emanating from the polar region or the Pacific. Previous isotope hydrographs of White Rock Creek, Dallas, indicated departures from simple mixing along the local meteoric water line (MWL) consistent with variability in the slope and D excess of the precipitation in δ D - δ 1^8O space. In addition to sampling the precipitation, we measured amounts, surface temperature and water temperature for some samples. Over 1000 D-1^8O pairs have been measured with up to 40 samples of a single storm to get variability on the 10-20 minute scale. Key results are: 1) On surface temperature-isotope plot, the total variability exhibited by Dallas precipitation captures a large portion the global mean ranges as reported in the IAEA data set. 2) The isotopic ratios measured in intra-storm events exhibit very little correlation with surface temperature. 3) The maximum decrease in the δ 1^8O value within a single storm event is usually less than 10 per mil with no obvious large Rayleigh-type depletion events; with fraction of vapor remaining values of about 0.5. 4) This observation is confirmed by measurements of δ D in water vapor versus 1/concentration that show a similar magnitude depletion of the heavy isotope, i.e. about 8x the oxygen isotope effect. 5) Within single storms, distinct cloud bands also exhibit distinct isotope ranges with different MWL slopes and intercepts suggestive of advective transport of water vapor. 6) A significant subset of the water vapor data also shows a 70 per mil variation in δ D at nearly constant water vapor concentration also suggestive of advective transport of water associated with changes in wind direction. On a long term basis (here monthly means), there is a tendency for MWL slopes and deuterium excess to increase with decreasing surface temperature with MWL slopes >9 yielding D excesses as high as 20. The local meteoric water line for Dallas is δ D=7 δ 1^8O+8. Distribution diagrams for each of the 3 years have distinctive shapes with the wettest year exhibiting the narrowest distribution with Gulf moisture more dominant. The driest year produced the distribution with the largest spread in isotope ratios. These new results present a much more complex data set than what would be predicted by simple closed or open system models of precipitation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.C51B1039G
- Keywords:
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- 3344 Paleoclimatology;
- 1704 Atmospheric sciences;
- 1719 Hydrology;
- 1854 Precipitation (3354);
- 1040 Isotopic composition/chemistry