The Isotopic Composition of Precipitation from Two Extratropical Cyclones

Abstract

Precipitation samples were collected at stations in the Eastern United States for two extratropical cyclones during the Genesis of Atlantic Lows Experiment (GALE) of 1986 and analyzed for their δ18O values. They represent the first synoptic-scale datasets of isotopic values. Measured isotope ratios are explained in terms of physical principles and meteorological processes. They are shown to be related to vertical profiles of ? cloud-top temperatures, evaporation beneath cloud base, isotope equilibration, and water vapor sources for the precipitation. Measured isotope ratios are then compared to values obtained from simple models of convective and stratiform precipitation. Both storms are shown to exhibit a consistent pattern of isotope ratios, with lowest δ18O values occurring in the stratiform precipitation well within the cold air, and highest values associated with the convective precipitation of the warm sector. A pronounced-amount effect, in which δ18O values decrease as rainfall totals increase, is also identified at many stations. The isotopic datasets from these storms may prove useful in deriving physical calibrations for climatological relationships between mean annual surface temperature or precipitation amount and the δ18O value of its precipitation for both present and past climate patterns.