Statistics and dynamics of blockings with a point vortex model
We investigate a reduced point vortex model for statistical and dynamical analyses of atmospheric blocking phenomena. Thereby, we consider High-over-low and Omega blocking as relative equilibria of two and three point vortices. Under certain conditions, such point vortex systems move westward opposi...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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Stockholm University Press
2018-01-01
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| Series: | Tellus: Series A, Dynamic Meteorology and Oceanography |
| Subjects: | |
| Online Access: | http://dx.doi.org/10.1080/16000870.2018.1458565 |
| _version_ | 1818139288671354880 |
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| author | Mirjam Hirt Lisa Schielicke Annette Müller Peter Névir |
| author_facet | Mirjam Hirt Lisa Schielicke Annette Müller Peter Névir |
| author_sort | Mirjam Hirt |
| collection | DOAJ |
| description | We investigate a reduced point vortex model for statistical and dynamical analyses of atmospheric blocking phenomena. Thereby, we consider High-over-low and Omega blocking as relative equilibria of two and three point vortices. Under certain conditions, such point vortex systems move westward opposing the mean westerly flow and hence can become stationary. Based on the kinematic vorticity number, two novel, independent methods, the contour and the trapezoid method, are introduced in order to identify the vortices that form the blocking pattern, their local positions and circulation magnitudes. While the contour method takes into account the observed stationarity of blocking, the trapezoid method minimizes the total circulation of the vortex system following point vortex theory. Using an instantaneous blocking index, a total number of 347 blocking periods were identified in NCEP-NCAR Reanalysis data for the Euro-Atlantic region during the time period 1990–2012. This procedure provides the basis to corroborate the applicability of the point vortex model to atmospheric blocking in a statistical framework. The calculated translation speed of point vortex systems associated with the atmospheric blocking appears to match the zonal mean velocity reasonably well. This model explains the stationary behaviour of blocking patterns. A comparison between the theoretical and a statistical model further reveals that the circulation of the blocking high follows the principles of the point vortex model to a large extent. However, the low-pressure systems behave more variable. Moreover, the stability of point vortex equilibria is analysed regarding the relative distances by considering linear stability analysis and simulations. This reveals that the point vortex blocking model corresponds to an unstable saddle point. Furthermore, we take viscosity and a Brownian motion into account to simulate the influence of the smaller, subgrid-scale disturbances. As a result, a clustering near the equilibrium state emerges indicating the persistence of the atmospheric blocking pattern. |
| first_indexed | 2024-12-11T10:25:43Z |
| format | Article |
| id | doaj.art-b27ff3842a984f8cba703f1bff267354 |
| institution | Directory Open Access Journal |
| issn | 1600-0870 |
| language | English |
| last_indexed | 2024-12-11T10:25:43Z |
| publishDate | 2018-01-01 |
| publisher | Stockholm University Press |
| record_format | Article |
| series | Tellus: Series A, Dynamic Meteorology and Oceanography |
| spelling | doaj.art-b27ff3842a984f8cba703f1bff2673542022-12-22T01:11:10ZengStockholm University PressTellus: Series A, Dynamic Meteorology and Oceanography1600-08702018-01-0170112010.1080/16000870.2018.14585651458565Statistics and dynamics of blockings with a point vortex modelMirjam Hirt0Lisa Schielicke1Annette Müller2Peter Névir3Freie Universität BerlinFreie Universität BerlinFreie Universität BerlinFreie Universität BerlinWe investigate a reduced point vortex model for statistical and dynamical analyses of atmospheric blocking phenomena. Thereby, we consider High-over-low and Omega blocking as relative equilibria of two and three point vortices. Under certain conditions, such point vortex systems move westward opposing the mean westerly flow and hence can become stationary. Based on the kinematic vorticity number, two novel, independent methods, the contour and the trapezoid method, are introduced in order to identify the vortices that form the blocking pattern, their local positions and circulation magnitudes. While the contour method takes into account the observed stationarity of blocking, the trapezoid method minimizes the total circulation of the vortex system following point vortex theory. Using an instantaneous blocking index, a total number of 347 blocking periods were identified in NCEP-NCAR Reanalysis data for the Euro-Atlantic region during the time period 1990–2012. This procedure provides the basis to corroborate the applicability of the point vortex model to atmospheric blocking in a statistical framework. The calculated translation speed of point vortex systems associated with the atmospheric blocking appears to match the zonal mean velocity reasonably well. This model explains the stationary behaviour of blocking patterns. A comparison between the theoretical and a statistical model further reveals that the circulation of the blocking high follows the principles of the point vortex model to a large extent. However, the low-pressure systems behave more variable. Moreover, the stability of point vortex equilibria is analysed regarding the relative distances by considering linear stability analysis and simulations. This reveals that the point vortex blocking model corresponds to an unstable saddle point. Furthermore, we take viscosity and a Brownian motion into account to simulate the influence of the smaller, subgrid-scale disturbances. As a result, a clustering near the equilibrium state emerges indicating the persistence of the atmospheric blocking pattern.http://dx.doi.org/10.1080/16000870.2018.1458565Atmospheric blockingpoint vorticeskinematic vorticity numberstability analysisinstantaneous blocking indexcirculationvortex identificationvortex pattern recognition |
| spellingShingle | Mirjam Hirt Lisa Schielicke Annette Müller Peter Névir Statistics and dynamics of blockings with a point vortex model Tellus: Series A, Dynamic Meteorology and Oceanography Atmospheric blocking point vortices kinematic vorticity number stability analysis instantaneous blocking index circulation vortex identification vortex pattern recognition |
| title | Statistics and dynamics of blockings with a point vortex model |
| title_full | Statistics and dynamics of blockings with a point vortex model |
| title_fullStr | Statistics and dynamics of blockings with a point vortex model |
| title_full_unstemmed | Statistics and dynamics of blockings with a point vortex model |
| title_short | Statistics and dynamics of blockings with a point vortex model |
| title_sort | statistics and dynamics of blockings with a point vortex model |
| topic | Atmospheric blocking point vortices kinematic vorticity number stability analysis instantaneous blocking index circulation vortex identification vortex pattern recognition |
| url | http://dx.doi.org/10.1080/16000870.2018.1458565 |
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