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| Titel |
The isotopic composition of precipitation from a winter storm - a case study with the limited-area model COSMOiso |
| VerfasserIn |
S. Pfahl, H. Wernli, K. Yoshimura |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063212
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| Zusammenfassung |
| Stable water isotopes are valuable tracers of the atmospheric water cycle, and potentially
provide useful information also on weather-related processes. In order to further
explore this potential, the water isotopes H218O and HDO are incorporated into the
limited-area weather forecast and climate model COSMO. The new COSMOiso model
includes an advanced microphysical scheme, a convection parameterisation and
non-hydrostatic dynamics that facilitate simulations from sub-kilometre to synoptic spatial
scales.
In a first case study, the model is applied for simulating a winter storm event in January
1986 over the eastern United States associated with intense frontal precipitation. The
modelled isotope ratios in precipitation and water vapour are compared to spatially
distributed δ18O observations from a study by Gedzelman and Lawrence (1990). COSMOiso
very accurately reproduces the statistical distribution of δ18O in precipitation, and
also the synoptic-scale spatial pattern and temporal evolution agree well with the
measurements. Deviations at single stations can partly be attributed to errors in the
representation of mesoscale atmospheric structures in the model. Grounded on
this overall meteorological evaluation, the model is then used for investigating the
physical processes causing the synoptic-scale variability of δ18O during the selected
event.
Perpendicular to the front that triggers most of the rainfall, COSMOiso simulates a
gradient in the isotopic composition of the precipitation, with high δ18O values in the warm
air to the east and lower values in the cold sector behind the front. This spatial gradient is
connected to a temporal evolution with high δ18O values in the beginning and a decrease later
on at locations where the front passes by. Two major processes are identified that contribute
to creating the spatial pattern. First, the advection of cold, depleted water vapour to the west
of the front and warm, more enriched vapour further to the east, in concert with the
progressive removal of heavy isotopes by precipitation in the frontal band, cause a large scale
west-to-east gradient of δ18O of vapour and precipitation. Second, this large scale
pattern is modulated by microphysical effects, namely the isotope fractionation and
equilibration during the interaction of rain drops and water vapour beneath the cloud
base.
This investigation illustrates the usefulness of high resolution, event-based model
simulations for understanding the complex processes that cause synoptic-scale variability of
the isotopic composition of atmospheric waters. In future research, this will be particularly
beneficial in combination with laser spectrometric isotope observations with high temporal
resolution. |
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