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| Titel |
The mesoscale structure of a mature polar low: Airborne measurements and numerical simulations |
| VerfasserIn |
Andreas Dörnbrack, Johannes Wagner, Alexander Gohm, Andreas Schäfler |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048469
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| Zusammenfassung |
| A unique data set of airborne light detection and ranging (LIDAR) and dropsonde observations
was obtained during the field campaign IPY-THORPEX (IPY-THORPEX was an
international field campaign funded by the Norwegian Research Council, DLR and EUFAR.
The DLR Falcon was employed in about 60 flight hours to observe Arctic fronts, polar lows
and terrain-induced flow disturbances, see e.g. http://www.pa.op.dlr.de/ipy_thorpex) over the
Norwegian and Barents Seas in February and March 2008. The presentation overviews
selected research flights focussing on the concomitant LIDAR observations of water vapour
and wind.
The mesoscale structure of a mature polar low was studied in detail on the basis of high
resolution airborne measurements and numerical modelling. Numerical simulations were
performed with the Integrated Forecast System (IFS) of the European Centre for
Medium-Range Weather Forecasts (ECMWF) and a high resolved, polar version of the
Weather Research & Forecasting Model (WRF) driven by the ECMWF data. The LIDAR
observations provided cross sections of water vapour mixing ratio, backscatter ratio and
horizontal wind speed around the polar low and through its centre. Mesoscale structures, such
as shallow convection in a cold air outbreak, deep convection in the eye wall and a dry
intrusion in the centre of the cyclone could be identified. WRF simulations reproduced these
structures and showed that the polar low had a warm, upper level core with descending
motions.
Several WRF sensitivity tests showed the influence of the initialisation time and sensible
and latent heat fluxes from the surface on the simulated polar low development. The polar low
was better simulated the more closely the simulation started at the mature stage.
Heat fluxes from the surface supported the polar low energetics especially at final
stages. When surface fluxes were switched off, the polar low weakened too much and
sea level pressure in the core was significantly too high. Quantitative comparisons
between simulations and LIDAR observations showed that cloud top heights and
vertical water vapour profiles were simulated properly both in shallow and deep
convection.
Special investigations focussed on the eye of the polar low. The centre was characterised
by rather stable stratification, horizontally constant potential temperatures and calm winds.
Wind speeds increased rapidly in the eye wall. The size of the eye was about 100-150 km
in diameter. Similar eye structures were already found in idealised simulations. |
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