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Title: Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer

item GRIFFIS, TIMOTHY - University Of Minnesota
item WOOD, JEFFREY - University Of Minnesota
item Baker, John
item LEE, X - Yale University
item XIAO, K - University Of Minnesota
item CHEN, Z - University Of Minnesota
item WELP, L - Purdue University
item SCHULTZ, N - Yale University

Submitted to: Atmospheric Chemistry and Physics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2016
Publication Date: 4/25/2016
Citation: Griffis, T.J., Wood, J.D., Baker, J.M., Lee, X., Xiao, K., Chen, Z., Welp, L.R., Schultz, N. 2016. Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer. Atmospheric Chemistry and Physics. 16:5139-5157.

Interpretive Summary: As the earth's surface warms climate models predict that there will be changes in the hydrologic cycle that could have important consequences. A major unanswered question is the extent to which atmospheric changes will be tempered by terrestrial ecosystems, i.e. - feedback effects. We collected several years of measurements of the regional water vapor isotope composition of the atmosphere from a 185 m radio tower in Minnesotaand used the results in conjunction with atmospheric models and ancillary ground-based measurements to determine the importance of local and regional scale processes versus larger-scale motion (transport of ocean-derived water vapor) in determining regional humidity in the planetary boundary layer. Results show that oceanic sources predominate during the non-growing season, but during the growing season regional evapotranspiration can provide as much as 40-60% of the water vapor in the boundary layer, with a seasonal growing season average of 30%.These results suggest that land use changes such as increased irrigation or adoption of management practices that extend the growing season, such as cover crops, may exert an impact on atmospheric processes by increasing boundary layer water vapor content.

Technical Abstract: Increasing atmospheric humidity and convective precipitation over land provide evidence of intensification of the hydrologic cycle – an expected response to surface warming. The extent to which terrestrial ecosystems modulate these hydrologic factors is important to understanding feedbacks in the climate system.We measured the oxygen and hydrogen isotope composition of water vapor froma very tall tower (185 m) in the Upper Midwest, United States to help diagnose the sources, transport,and fractionation of water vapor in the planetary boundary layer (PBL) over a 3-year period (2010 to 2012). These measurements represent the first set of annual water vapor isotope observations for the region. Models and cross wavelet analyses were used to assess the importance of Rayleigh, evapotranspiration (ET), and PBL entrainment processes on the isotope composition of water vapor. The vapor isotope composition at this tall tower site showed a very large seasonal amplitude (mean monthly _18 Ov ranged from -40.1 to -15.5‰ and _2 Hv ranged from -278.7 to -109.1‰)and followed the familiar Rayleigh distillation relation with water vapor mixing ratio at the annual time-scale. However, this relation was strongly modulated by ET and PBL entrainment processes at time-scales ranging from hours to several days. The wavelet coherence spectra indicate that the oxygen isotope ratio and the deuterium excess (dx ) of water vapor are sensitive to synoptic and PBL processes. According to the phase of the coherence analyses, we show that ET often leads changes in dx, confirming that it is a potential tracer of regional ET. Isotope mixing models indicate that on average about 31% of the growing season PBL water vapor is derived from regional ET. However, isoforcing calculations and mixing model analyses for high PBL water vapor mixing ratios events (> 25 mmol mol''1 indicate that regional ET can account for 40% to 60% of the PBL water vapor. These estimates are in relatively good agreement with that derived from numerical weather model simulations. This relatively large fraction of ET-derived water vapor implies that ET has an important impact on the precipitation recycling ratio within the region. Based on multiple constraints, we estimate that the summer season recycling fraction is about 30%, indicating a potentially important link with convective precipitation.