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| Titel |
Numerical simulations of thermospheric dynamics: divergence as a proxy for vertical winds |
| VerfasserIn |
S. L. Cooper, M. Conde, P. Dyson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 6 ; Nr. 27, no. 6 (2009-06-22), S.2491-2502 |
| Datensatznummer |
250016562
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| Publikation (Nr.) |
 copernicus.org/angeo-27-2491-2009.pdf |
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| Zusammenfassung |
| A local scale, time dependent three-dimensional model of the neutral
thermosphere was used to test the applicability of two previously published
empirical relations between thermospheric vertical wind and velocity
divergence, i.e., those due to Burnside et al. (1981) and Brekke (1997).
The model self-consistently solves for vertical winds driven by heat and
momentum deposited into the neutral atmosphere by high latitude ion
convection. The Brekke condition accurately mimicked the overall "shape" of
the three-dimensional model vertical wind field although, as written, it
consistently overestimated the vertical wind magnitude by a factor of
approximately 5/3, for the heating scenarios that we considered. This same
general behavior was observed regardless of whether the forcing was static or
rapidly changing with time. We discuss the likely reason for the Brekke
condition overestimating the magnitude of our vertical winds, and suggest an
alternative condition that should better describe vertical winds that are
driven by local heating. The applicability of the Burnside condition was, by
contrast, quite variable. During static heating, both the magnitude and the
sign of the model vertical winds were predicted reliably at heights above
those of maximum energy and momentum deposition per unit mass. However, below
the thermal forcing, the Burnside condition predicted vertical winds of the
wrong sign. It also introduced significant artefacts into the predicted
vertical wind field when the forcing changed suddenly with time. If these
results are of general applicability (which seems likely, given the way these
relations are derived) then the Burnside condition could usually be used
safely at altitudes above hmF2. But it should be avoided below this
height at all times, and even at high altitudes during periods of dynamic
forcing. While the Brekke condition (or our modified version of it) could
likely be used in all circumstances, there are few experimental scenarios for
which this would be useful. This is because evaluation of the Brekke
condition would not usually be possible unless the vertical wind was already
known in advance. |
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