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| Titel |
Slow wave dynamics stalls tropical tropopause ice clouds |
| VerfasserIn |
Peter Spichtinger, Martina Krämer, Stephan Borrmann |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038598
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| Zusammenfassung |
| Water vapour is the most important natural green house gas. However, in the stratosphere an
increase in water vapour would possibly result in a cooling. The major entrance of trace
substances into the stratosphere is the tropical tropopause layer (TTL), localized between the
main level of convective outflow, 150Â hPa, and 70Â hPa. The TTL water vapour budget, and
thus the exchange with the stratosphere, depends crucially on the occurrence and properties
of ice clouds in this cold region (T < 200Â K).
It is believed that homogeneous freezing of liquid solution particles, which predominate the
particle population, is the preferred pathway of ice formation. High water vapour
supersaturation with respect to ice is required to initiate homogeneous ice nucleation. The
number of emerging ice crystals depends on temperature and the ambient relative humidity
over ice (RHi). Strong increase in RHi due to rising vertical velocity will produce large
amounts of ice crystals. In the TTL, very slow large-scale updraughts prevail (-¤ 0.01 m/s),
which would lead to low ice crystal concentrations (-¤ 0.1cm-3). However, tropical deep
convection initiates intrinsic gravity waves and consequently, we would expect much higher
vertical velocities and therefore higher ice crystal number concentrations. Since
the many ice crystals rapidly grow by water vapour diffusion it is also expected
that the initially high ice supersaturation quickly reduces to saturation after ice
formation.
Contrarily, during the last years high and persistent ice supersaturations were observed in
the cold TTL in several airborne field campaigns inside and outside of ice clouds (Peter et al.,
2006), creating a discussion called the ‘supersaturation puzzle’. A step forward in that
discussion was made recently: Krämer et al. (2009) observed ice crystal concentrations much
lower than expected (most often < 0.1cm-3), but consistent with the measured high
supersaturations. These observations turned the ‘supersaturation’ into a ‘nucleation puzzle’.
The ‘nucleation puzzle’ is currently intensely discussed and other nucleation pathways
suppressing, modifying or replacing homogeneous freezing are proposed. All these
approaches to explain the TTL ice nucleation are of chemical or microphysical
nature.
Here, we present intense model studies of ice cloud formation under dynamical
conditions typical for the TTL. From direct comparison of model simulations and
observations we claim that the special TTL dynamics – namely a superposition of very slow
large-scale updraughts with high-frequency short waves – can produce the observed low
numbers of ice crystals solely by ‘classical’ homogeneous freezing.
References:
Krämer, M., Schiller, C., Afchine, A., Bauer, R., Gensch, I., Mangold, A., Schlicht,
S., Spelten, N., Ebert, V., Möhler, O., Saathoff, H., Sitnikov, N., Borrman, S.,
de Reus, M. and P. Spichtinger, 2009: On Cirrus Cloud Supersaturations and Ice
Crystal Numbers. Atmos. Chem. Phys., 9, 3505-3522.
Peter T., Marcolli C., Spichtinger, P., Corti, T., Baker M.B., Koop, T., 2006: When dry
air is too humid. Science 314 (5804), 1399-1402. |
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