Observations and modeling of water vapor isotopes from Mauna Kea, Hawaii

Understanding the processes that control the humidity of the arid regions of the subtropical free troposphere is essential for evaluating the water vapor feedback and its potential for change as the climate warms. Here we present deuterium isotope results for water vapor samples collected from the crest of Mauna Kea, Hawaii (4200 m). Four samples were collected in July 2006 at dewpoints ranging from -7C to -27C. The deuterium isotope content of the water vapor samples correlated with dewpoint and ranged from -238 per mil to -321 per mil. Preliminary modeling results with the NCAR tracer transport model MATCH suggest that the water vapor mixing ratio at this locale was set at last-saturation points around 400 hPa and between 20 and 40 degrees north latitude, substantially higher than and poleward of Mauna Kea. Isotopic modeling suggests that the observed deuterium composition of the water vapor on the crest of Mauna Kea can be understood, to first order, in terms of large scale mixing of parcels whose isotopic content was set at the point of last saturation.