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| Titel |
Long-term trends of the extratropical jet streams through objective analysis |
| VerfasserIn |
Cristina Peña-Ortiz, Pedro Ribera, David Gallego, Paulina Ordoñez, María del Carmen Álvarez |
| 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 |
250051832
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| Zusammenfassung |
| Although the tropospheric jet streams are probably the more important single dynamical systems in the troposphere, their study at climatic scale has been usually troubled by the difficulty of characterising their structure. During the last years, a deal of effort has been made in order to construct long-term scale objective climatologies of the jet stream or at least to understand the variability of the westerly flux in the upper troposphere.
A main problem with studying the jets is the necessity of using highly derivated fields as the potential vorticity or even the analysis of chemical tracers. Despite their utility, these approaches are very problematic to construct an automatic searching algorithm because of the difficulty of defining criteria for these extremely noisy fields. Some attempts have been addressed trying to use only the wind field to find the jet. This direct approach avoid the use of derivate variables, but it must contain some stringent criteria to filter the large number of tropospheric wind maxima not related to the jet currents. These approaches have offered interesting results for the relatively simple structure of the Southern Hemisphere tropospheric jets or for the Northern winter jet streams.
In this work we present a new methodology able to characterise the position, strength and altitude of the jet stream at global scale on a daily basis. The method is based on the analysis of the 3-D wind field alone and it searches, at each longitude, relative wind maxima above 30 m/s in the upper troposphere, between 400 and 100 hPa. The frequency of occurrence of relative maxima during a certain period and at each grid point is interpreted as the probability of jet streaks to occur at each location. The probability of jet streaks at each longitude exhibits a distribution approximately normal around the latitudes with highest probabilities. Then, to obtain the average jet streaks calculate the mean position, velocity and altitude of the wind maxima around latitudes of maximum probability.
The algorithm has been applied to the NCEP/NCAR reanalysis from the period 1948-2008 to study the long-term trends in the position, velocity and altitude of the extratropical jet streaks in both hemispheres and during each month of the year. |
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