The Megalopolitan Snowstorm of 11–12 February 1983: Isotopic Composition of the Snow

The stable isotopic composition (δD and δ18O) of precipitation and water vapor from the megalopolitan snowstorm of 11–12 February 1983 collected in the New York metropolitan area has been related to both synoptic scale and mesoscale structure of the storm. The δD increased from a very low value of −242.3%. at the beginning of the storm to a distinct maximum of −85.5% during and shortly after the time that thunder and lightning were reported, then decreased slowly to about −110% The storm-integrated δD as measured at Mohonk Lake, New York, was the second lowest value, δ170%o, for all storms monitored from 1977 to 1984. The extremely low δ values throughout most of the storm arose primarily from two effects acting together. A large proportion of condensation took place abnormally high in the troposphere at abnormally low temperatures. The low temperatures increased the amount of isotopic fractionation recurring during the condensation process while the high elevation resulted in source vapor that was extremely fractionated due to prior precipitation. With the exception of the time period surrounding the thunderstorms, a stratiform model accurately predicted the isotopic values of the precipitation using an ω profile varying sinusoidally with pressure in the cloud. During passage of the thunderstorms, however, stratiform solutions significantly underestimated the δ values using the sounding derived from the National Meteorological Center analysis. The discrepancies between model calculations and measured isotopic values of precipitation were able to provide a check on the meteorological analyses. The analysis was modified to incorporate the effects of a large-amplitude gravity wave, which passed through the region at this time. Solutions for both a penetrative convection model and one of slantwise convection in an almost symmetrically neutral environment matched observed isotopic precipitation values closely.