Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-7-6293-2007
http://www.atmos-chem-phys-discuss.net/7/6293/2007/
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http://www.atmos-chem-phys-discuss.net/7/6293/2007/acpd-7-6293-2007.pdf
6293
6327
2007-05-10
Formation of large (≃100 μm) ice crystals near the tropical tropopause
E. J. Jensen
eric.j.jensen@nasa.gov
L. Pfister
T. V. Bui
P. Lawson
B. Baker
Q. Mo
D. Baumgardner
E. M. Weinstock
J. B. Smith
E. J. Moyer
T. F. Hanisco
D. S. Sayres
J. M. St. Clair
M. J. Alexander
O. B. Toon
J. A. Smith
NASA Ames Research Center, Moffett Field, CA, USA
SPEC Inc., Boulder, CO, USA
Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Circuito Exterior, Mexico
Harvard University, Cambridge, MA, USA
Colorado Research Associates, Boulder, CO, USA
University of Colorado, Boulder, CO, USA
Recent high-altitude aircraft measurements with in situ imaging
instruments indicated the presence of relatively large
(≃100 μm length), thin (aspect ratios of ≃6:1 or larger)
hexagonal plate ice crystals
near the tropical tropopause in very low concentrations
(<0.01 L<sup>−1</sup>).
These crystals were not produced by deep convection or aggregation.
We use simple growth-sedimentation calculations as well as detailed
cloud simulations to evaluate the conditions required to grow the large
crystals.
Uncertainties in crystal aspect ratio leave a range of possibilities,
which could be constrained by knowledge of the water vapor concentration
in the air where the crystal growth occurred.
Unfortunately, water vapor measurements made in the cloud formation
region near the tropopause ranged from <2 ppmv to ≃3.5 ppmv.
The higher water vapor concentrations correspond to very large ice
supersaturations (relative humidities with respect to ice of about 200%).
If the aspect ratios of the hexagonal plate crystals are as small
as the image analysis suggests (6:1, see companion paper Lawson et al., 2007)
then growth of the large crystals before they sediment out of
the supersaturated layer would only be possible if the
water vapor concentration were on the high end of the range
indicated by the different measurements (>3 ppmv).
On the other hand, if the crystal aspect ratios are quite a bit larger
(≃14), then H<sub>2</sub>O concentrations toward the low of
the measurement range (≃2–2.3 ppmv) would suffice to grow
the large crystals.
Gravity-wave driven temperature and vertical wind perturbations
only slightly modify the H<sub>2</sub>O concentrations needed to grow
the crystals.
We find that it would not be possible to grow the large crystals
with water concentrations less than 2 ppmv, even with assumptions
of a very high aspect ratio of 15 and steady upward motion of
2 cm s<sup>−1</sup> to loft the crystals in the tropopause region.
These calculations would seem to imply that the measurements
indicating water vapor concentrations less than 2 ppmv are
implausible, but we cannot rule out the possibility that higher
humidity prevailed upstream of the aircraft
measurements and the air was dehydrated by the cloud formation.
Simulations of the cloud formation with a detailed model
indicate that the large crystals probably nucleated
on very effective ice nuclei.
Also, growth of the large crystals would not have been possible
if homogeneous freezing of aqueous aerosols and subsequent ice
crystal growth had rapidly depleted vapor in excess of saturation,
implying either very slow cooling during cloud formation or
that the aerosol physical state was different from those used
in homogeneous freezing laboratory experiments, such that the
vast majority of aerosols present
did not freeze even at very high ice supersaturations.
Improvements in our understanding of detailed cloud microphysical
processes require resolution of the water vapor measurement
discrepancies in these very cold, dry regions of the atmosphere.
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