Annual variation of precipitation δD and δ18O at Barrow, AK related to seasonal shifts in moisture source

Isotopic compositions of precipitation are replete with information about atmospheric circulation. Included in the signature are evaporation conditions, transport distance, condensation temperature and dew point, and sub-cloud re-evaporation - the meteorological conditions from the evaporation source to the precipitation site. In this work we present event-scale analysis of moisture source regions and their link to isotopic compositions for 64 storms in all seasons between 2009 and 2013 at Barrow, AK. Moisture source regions were identified using back trajectories in HYSPLIT, an airmass tracking program. The back trajectories' initial conditions - the condensation profiles at precipitation time - came from measured cloud profiles. Doppler vertical velocity and reflectivity measurements from a millimeter wavelength cloud radar were processed¹ to produce the best available representation of the actual distribution and density of condensation rate in the precipitating cloud. From the back trajectory results we calculated the moisture source dew point and an estimate of Rayleigh distillation by cooling along the trajectory. Rayleigh distillation was calculated as the difference between the moisture source dew point and the final condensation temperature. Finally, the moisture source results were partitioned into three categories to account for variations in transport and transport path. The categories - Local, Orographic and Bering Strait - emphasized the physical differences between the evolution of airmasses that travel over the Alaskan and Brooks Ranges, those that travel around them, and those that do not travel far from their source. Moisture source varies dramatically among seasons. Much of the winter moisture comes from the Aleutian Low or the Alaskan Gulf Coast and travels over the Alaskan and Brooks Ranges. Spring events have both local and distal moisture sources. Distal moisture comes from the Bering Strait and the western Subarctic Gyre. Summer and fall show predominantly Arctic moisture, from the Beaufort, Bering and Chukchi seas. These seasonal variations in moisture sources result from a combination of factors, including seasonal variations in atmospheric circulation - the control on storm tracks, sea surface temperature, and sea ice conditions. Seasonal variation allows us to assess the relative importance of moisture source conditions, transport distance, and transport path on isotopic composition. Rayleigh distillation was significant in explaining 15% of the variance across all events. When coupled with source dew point, 42% of variance could be explained. Distillation contributed most to explaining the isotopic composition of Bering Strait events, followed by orographic events. For local events, distillation was not a significant control on isotopic composition. For all categories, particularly local, dew point was successful in explaining up to 50% of variance. Characterizing the isotopic composition in relation to its moisture source and trajectory affords greater interpretation strength for future measurements. In particular, deviations from the demonstrated annual pattern may allow us to recognize the enhanced Arctic moisture signal resulting from sea ice decline. 1. Zhao, C and Garrett, T.J. 2008, J. Geophys. Res.