Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes
<p>Understanding what controls the water vapor isotopic composition of the sub-cloud layer (SCL) over tropical oceans (<span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span>) is a first step towards understanding the water vap...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/12235/2019/acp-19-12235-2019.pdf |
| Summary: | <p>Understanding what controls the water vapor isotopic composition of
the sub-cloud layer (SCL) over tropical oceans (<span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span>) is
a first step towards understanding the water vapor isotopic composition
everywhere in the troposphere. We propose an analytical model to predict
<span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span> motivated by the hypothesis that the altitude from
which the free tropospheric air originates (<span class="inline-formula"><i>z</i><sub>orig</sub></span>) is an important
factor: when the air mixing into the SCL is lower in altitude, it
is generally moister, and thus it depletes the SCL more efficiently.
We extend previous simple box models of the SCL by prescribing the
shape of <span class="inline-formula"><i>δ</i><i>D</i></span> vertical profiles as a function of humidity profiles
and by accounting for rain evaporation and horizontal advection effects.
The model relies on the assumption that <span class="inline-formula"><i>δ</i><i>D</i></span> profiles are steeper
than mixing lines, and that the SCL is at steady state, restricting
its applications to timescales longer than daily. In the model, <span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span>
is expressed as a function of <span class="inline-formula"><i>z</i><sub>orig</sub></span>, humidity and temperature
profiles, surface conditions, a parameter describing the steepness
of the <span class="inline-formula"><i>δ</i><i>D</i></span> vertical gradient, and a few parameters describing
rain evaporation and horizontal advection effects. We show that <span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span>
does not depend on the intensity of entrainment, in contrast to several
previous studies that had hoped that <span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span> measurements
could help estimate this quantity.</p>
<p>Based on an isotope-enabled general circulation model simulation,
we show that <span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span> variations are mainly controlled by mid-tropospheric
depletion and rain evaporation in ascending regions and by sea surface
temperature and <span class="inline-formula"><i>z</i><sub>orig</sub></span> in subsiding regions. In turn, could <span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span>
measurements help estimate <span class="inline-formula"><i>z</i><sub>orig</sub></span> and thus discriminate between
different mixing processes? For such isotope-based estimates of <span class="inline-formula"><i>z</i><sub>orig</sub></span>
to be useful, we would need a precision of a few hundred meters in
deep convective regions and smaller than 20 m in stratocumulus regions.
To reach this target, we would need daily measurements of <span class="inline-formula"><i>δ</i><i>D</i></span>
in the mid-troposphere and accurate measurements of <span class="inline-formula"><i>δ</i><i>D</i><sub>0</sub></span>
(accuracy down to 0.1 ‰ in the case of stratocumulus clouds,
which is currently difficult to obtain). We would also need information
on the horizontal distribution of <span class="inline-formula"><i>δ</i><i>D</i></span> to account for horizontal
advection effects, and full <span class="inline-formula"><i>δ</i><i>D</i></span> profiles to quantify the uncertainty
associated with the assumed shape for <span class="inline-formula"><i>δ</i><i>D</i></span> profiles. Finally,
rain evaporation is an issue in all regimes, even in stratocumulus
clouds. Innovative techniques would need to be developed to quantify
this effect from observations.</p> |
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| ISSN: | 1680-7316 1680-7324 |