Tropisch-extratropische Wechselwirkungen und ihre Bedeutung für intrasaisonale Variabilität und Extremereignisse in den Tropen und Extratropen
Zusammenfassung der Projektergebnisse
Tropical–extratropical interactions (TEIs) are an important part of the large-scale atmospheric circulation and are occasionally involved in the generation of extreme weather events in the Tropics and subtropics. This project investigated a wide range of TEI phenomena with respect to their dynamics, climatology, variability and predictability using both observational data and numerical models. One important TEI phenomenon is the penetration of extratropical upper-level disturbances to low latitudes. These systems are most frequent at the eastern edges of the major ocean basins in both hemispheres during boreal winter. Their frequency in the northern hemisphere is strongly modulated by El Niño on an interannual time-scale, while their occurrence over the North Atlantic is followed by phases of a positive North Atlantic Oscillation on a weekly time-scale. An important prerequisite for their generation is a long-wave trough-ridge configuration in the extratropics. In contrast to previous work latent heating appears to be only a secondary contributor to the amplification of waves into the Tropics. Upper-level troughs at low-latitudes can trigger elongated poleward stretching cloud bands called Tropical Plumes (TPs). A newly developed objective identification algorithm revealed that there are also many TPs over warm ocean surfaces, mainly related to tropical dynamics. In order to instigate significant poleward moisture transports and precipitation upper-level troughs need to be deep enough to reach far down into the troposphere. In terms of total moisture export, however, the warm regions in the western parts of the main ocean basins are most important as shown by a new trajectory climatology. Besides a direct relationship between low-latitude troughs and rainfall via dynamic lifting and vertical destabilization, a new indirect mechanisms has been discovered for the West African Tropics that involves a fall in surface pressure brought about by thermal advection, vertical motion and enhanced cloudiness, finally leading to an inflow of low-level moisture into the continent and subsequent deep convective rains. We found evidences that tropical rainfalls linked to extratropical dynamics are more predictable than other tropical phenomena. Low-latitude troughs can be involved in the lifting of fine surface dust in the Sahara. Associated cold air outbreaks can cause large-scale dust storms, while associated deep convection in summer can generate cold pools from evaporating precipitation that quickly spread under high winds. Other important mechanisms of dust mobilization are small-scale dust devils and dry-convective plumes in the boundary layer, as well as diurnal wind variations associated with low-level jets and the West African heat low. Soudano-Saharan Depressions were not found to be relevant for dust emissions.
Projektbezogene Publikationen (Auswahl)
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2007. Dust mobilization due to density currents in the Atlas region: Observations from the SAMUM 2006 field campaign. J. Geophys. Res., 112, D21109
Knippertz, P.; Deutscher, C.; Kandler, K.; Müller, T.; Schulz, O.; Schütz, L.
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2007. Tropical–extratropical interactions related to upper-level troughs at low latitudes. Dyn. Atmos. Oceans, 43, 36–62
Knippertz, P.
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2008. Dust mobilization in the West African heat trough – the role of the diurnal cycle and of extratropical synoptic disturbances. Meteorol. Z., 17(5), 553–563
Knippertz, P.
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2008. Identification and global climatology of upperlevel troughs at low latitudes. Meteorol. Z., Meteorol. Z., 17(5), 565–573
Fröhlich, L.; Knippertz, P.
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2009. Dust mobilization and transport in the northern Sahara during SAMUM 2006 – A meteorological overview. Tellus, 61B, 12–31
Knippertz, P.; Ansmann, A.; Althausen, D.; Müller, D.; Tesche, M.; Bierwirth, E.; Dinter, T.; Müller, T.; von Hoyningen-Huene, W.; Schepanski, K.; Wendisch, M.; Heinold, B.; Kandler, K.; Petzold, A.; Schütz, L.; Tegen, I.
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2009. Dynamics and predictability of a heavy dryseason precipitation event over western Africa - Sensitivity experiments with a global model. Mon. Wea. Rev., 137(1), 189–206
Meier, F.; Knippertz, P.
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2009. Prediction of dry-season precipitation in tropical West Africa and its relation to forcing from the extratropics. Wea. Forecasting, 24(4), 1064–1084
Knippertz, P.; Fink, A. H.
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2010. A Lagrangian climatology of tropical moisture exports to the Northern Hemispheric extratropics. J. Clim., 23(4), 987–1003
Knippertz, P.; Wernli, H.
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2010. Climatology of convective density currents in the southern foothills of the Atlas Mountains. J. Geophys. Res., 115, D11115
Emmel, C.; Knippertz, P.; Schulz, O.
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2011. Soudano-Saharan depressions and their importance for precipitation and dust: A new perspective on a classical synoptic concept. Quart. J. Roy. Meteorol. Soc., 137, 1431–1445
Schepanski, K.; Knippertz, P.
