Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer
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 understand feedbacks in the c...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/5139/2016/acp-16-5139-2016.pdf |
| Summary: | 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 understand feedbacks in the climate system. We
measured the oxygen and hydrogen isotope composition of water vapor at a very
tall tower (185 m) in the upper Midwest, United States, to 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 this
region. Several simple isotope models and cross-wavelet analyses were used to
assess the importance of the Rayleigh distillation process, evaporation, and
PBL entrainment processes on the isotope composition of water vapor. The
vapor isotope composition at this tall tower site showed a large seasonal
amplitude (mean monthly <i>δ</i><sup>18</sup>O<sub>v</sub> ranged from −40.2 to
−15.9 ‰ and <i>δ</i><sup>2</sup>H<sub>v</sub> ranged from −278.7 to
−113.0 ‰) and followed the familiar Rayleigh distillation relation
with water vapor mixing ratio when considering the entire hourly data set.
However, this relation was strongly modulated by evaporation and PBL
entrainment processes at timescales ranging from hours to several days. The
wavelet coherence spectra indicate that the oxygen isotope ratio and the
deuterium excess (<i>d</i><sub>v</sub>) of water vapor are sensitive to synoptic
and PBL processes. According to the phase of the coherence analyses, we show
that evaporation often leads changes in <i>d</i><sub>v</sub>, confirming that it is a potential tracer of regional evaporation. Isotope mixing models indicate
that on average about 31 % of the growing season PBL water vapor is
derived from regional evaporation. However, isoforcing calculations and
mixing model analyses for high PBL water vapor mixing ratio events
( > 25 mmol mol<sup>−1</sup>) indicate that regional evaporation 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 evaporation-derived water vapor implies
that evaporation 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. |
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| ISSN: | 1680-7316 1680-7324 |