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| Titel |
An online trajectory module (version 1.0) for the nonhydrostatic numerical weather prediction model COSMO |
| VerfasserIn |
A. K. Miltenberger, S. Pfahl, H. Wernli |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 6, no. 6 ; Nr. 6, no. 6 (2013-11-13), S.1989-2004 |
| Datensatznummer |
250085017
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| Publikation (Nr.) |
 copernicus.org/gmd-6-1989-2013.pdf |
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| Zusammenfassung |
| A module to calculate online trajectories has been implemented into
the nonhydrostatic limited-area weather prediction and climate model
COSMO. Whereas offline trajectories are calculated with wind fields
from model output, which is typically available every one to six
hours, online trajectories use the simulated resolved wind field at every model
time step (typically less than a minute) to solve the trajectory
equation. As a consequence, online trajectories much better capture
the short-term temporal fluctuations of the wind field, which is
particularly important for mesoscale flows near topography and
convective clouds, and they do not suffer from temporal interpolation
errors between model output times. The numerical implementation of
online trajectories in the COSMO-model is based upon an established
offline trajectory tool and takes full account of the horizontal
domain decomposition that is used for parallelization of the COSMO-model.
Although a perfect workload balance cannot be achieved for the
trajectory module (due to the fact that trajectory positions are not
necessarily equally distributed over the model domain), the additional
computational costs are found to be fairly small for the high-resolution
simulations described in this paper. The computational costs may, however,
vary strongly depending on the number of trajectories and trace variables.
Various options have been implemented to initialize
online trajectories at different locations and times during the model
simulation. As a first application of the new COSMO-model module, an Alpine
north foehn event in summer 1987 has been simulated with horizontal
resolutions of 2.2, 7 and 14 km. It is
shown that low-tropospheric trajectories calculated offline with one-
to six-hourly wind fields can significantly deviate from trajectories
calculated online. Deviations increase with decreasing model grid
spacing and are particularly large in regions of deep convection and
strong orographic flow distortion. On average, for this particular
case study, horizontal and vertical positions between online and
offline trajectories differed by 50–190 km and
150–750 m, respectively, after 24 h. This first
application illustrates the potential for Lagrangian studies of
mesoscale flows in high-resolution convection-resolving simulations
using online trajectories. |
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