Abstract
Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales.
Highlights
The stable oxygen and hydrogen isotope compositions of precipitation have long been used to examine the hydrologic cycle and biosphere
The atmospheric river observatory (ARO) is comprised of a site at Bodega Marine Laboratory (BML) in Bodega Bay, CA, USA (BBY; 15 m above mean sea level (MSL); 38.32◦ N, 123.07◦ W) and an inland site at Cazadero, CA, USA (CZC; 478 m MSL, 38.61◦ N, 123.22◦ W)
ThisStable quantity arisesindue to the kinetic during evaporation and relates to the relative isotopes precipitation can fractionation provide a wealth of insight into the processes driving (and humidity, with high d values associated with low relative humidity (RH)
Summary
The stable oxygen and hydrogen isotope compositions of precipitation have long been used to examine the hydrologic cycle and biosphere These compositions, typically expressed in delta notation (δ18 O and δ2 H), are a relative measure of 18 O/16 O and 2 H/1 H ratios with respect to Vienna standard mean ocean water (VSMOW) reported in parts per thousand (permil, h): e.g., δ18 O = ((18 O/16 O)sample /(18 O/16 O)VSMOW – 1) × 1000. The use of stable isotopes in single precipitation events, remains underdeveloped as the cost of analysis has been prohibitively high until recently [1,2] Among these single precipitation events, atmospheric rivers (ARs) present a unique opportunity to examine the processes driving some of Earth’s most powerful precipitation events. Atmospheric rivers, long and narrow regions in the lower atmosphere characterized by strong horizontal water vapor transport, are responsible for 90% of poleward vapor transport in the midlatitudes globally [3] and cause the majority of heavy rain and Atmosphere 2019, 10, 86; doi:10.3390/atmos10020086 www.mdpi.com/journal/atmosphere
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