Paired Vapor-Precipitation Isotope Data from Manus, Papua New Guinea
Abstract:We measured the stable isotopic composition of water vapor, precipitation and seawater over a period of 10 days at the highly instrumented Atmospheric Radiation Measurement (ARM) site located on Manus Island, Papua New Guinea. The island site, at 4 meters above sea level, sits close to the open ocean and is minimally impacted by terrestrial processes. Over the period of measurement, the mean δD value of near-surface water vapor (δDv) was -109.7±39.7‰, overlapping average tropospheric emissions satellite (TES) δDv values for the lower troposphere. We observed a 155‰ range in δDv values, coinciding with a transition through a Madden-Julian Oscillation event into a quiescent period. Low δDv values appear to reflect post-condensation exchange and the highest δDv values measured were in equilibrium with seawater δD. Of all surface meteorological variables, δDv was most strongly correlated with specific humidity, reflecting the importance of rainout and distillation in driving δDv values. During precipitation events, δDv dropped abruptly following maximum precipitation rates, likely due to mesoscale subsidence or rain evaporation.
Intra-event δD values of precipitation (δDp) were most strongly correlated with δDv, and only weakly correlated with surface meteorological variables, highlighting the important role of δDv in controlling δDp values. We did not observe an amount effect through all precipitation events, as the lowest δDp values often lagged maximum precipitation rates. Precipitation deuterium excess values were significantly correlated with surface relative humidity and temperature, as well as cloud base height measurements, with higher deuterium excess values corresponding to higher cloud altitudes.