Library Item
Isotopic Variations of Atmospheric Water Vapor on Synoptic Scales in Hanover, NH
Description:
High-resolution water vapor isotopic measurements have recently been made possible using laser-based technologies, which open up new opportunities for studying atmospheric processes and atmosphere-land interactions. In this work, continuous high-resolution (90 second interval) oxygen (δ18Ov) and hydrogen (δDv) isotopic ratios in near-surface vapor in Hanover, NH were measured using a Los Gatos Research DLT-100 vapor isotopic analyzer. Ancillary precipitation samples were also collected for isotopic measurements. Isotopic variations in water vapor can be identified on three characteristic time scales: (1) seasonal, (2) synoptic, and (3) diel (24-hour) timescales. This discussion focuses on synoptic scale variation. Both δ18Ov and δDv, and standard weather variables, like temperature, dew point, and pressure, shift systematically with the passage of weather fronts; in fact, large inter-storm variation in both isotopic and meteorological characteristics enables the differentiation between transient air masses. A consistent drop – as high as 22‰ and 175‰ in δ18Ov and δDv, respectively – is observed with the passage of cold fronts, which occur over an hours-long interval. In contrast, fewer than half of the documented warm front passages are associated with measurable changes in vapor isotopic ratios, although shallow, steady increases in vapor δ values and weather variables were expected with the arrival of relatively warmer and moister air during warm front passage. We speculate that isotopic changes associated with the slow, days-long warm front passage may be confounded by diel processes and other, shorter-termed transient weather events. No signal was observed with the passage of occluded fronts. Finally, vapor δ values showed no response to precipitation events on a synoptic timescale, likely due to consistently high relative humidity (>85%) in the region at the time of rainfall. On a synoptic timescale, variations in δD and δ18O vapor values correlate most strongly with variations in the near-ground dew point. In explanation, a cold air mass with a low dew point has generally had more rainout than a typical warm air mass. A Rayleigh distillation model is used to explain the correlation between vapor δ values and the dew point of the air mass. The air mass is assumed to originate in the northern Subtropical High, and produce rain as it cools adiabatically. This model explains the data reasonably well. During winter months, the model’s agreement with observations improves if it incorporates snow formation. The isotope records accumulated across this study identified several distinct transient weather events that went unrecorded by NOAA and other weather services. This demonstrates the potential for using isotopes to augment meteorological data in identifying and characterizing synoptic scale structures and processes. Understanding the vapor data on the synoptic timescale is also important for retrieving information on other time scales, e.g., interannual, seasonal and diel variations, from vapor isotope time series.
