Upper-level extratropical disturbances over the low-latitude North Atlantic: Generation mechanisms and predictability
Zusammenfassung der Projektergebnisse
Weather disturbances in the upper troposphere (around 7 km above sea level) frequently penetrate from the region of main storm activity over the extratropical North Atlantic into low latitudes, where they can be involved in significant weather events such as heavy precipitation over the western and northern parts of Africa, or Saharan dust outbreaks. This project is the first that systematically explored these disturbances using the huge dynamical information content of long-term data from the European Centre for Medium-Range Weather Forecasts operational ensemble prediction system (EPS) and from the recently established multi-model THORPEX Interactive Grand Global Ensemble (TIGGE). Ensemble prediction is a relatively new development in weather forecasting and aims at estimating uncertainties through multiple model runs from slightly different initial conditions or even by using different atmospheric models (as in the case of TIGGE). This study investigated the dynamics, forecast quality and predictability of low-latitude upper disturbances exploring a wide range of verification and analysis tools based on potential vorticity (PV), which is a widely used dynamical tracer for airmasses penetrating equator- and downward into the troposphere. Both statistical approaches and case studies of high-impact weather events were pursued. The main conclusions from this work are: (i) The ECMWF ensemble prediction system shows a generally underdispersive behaviour in the region of the upper disturbances. This means that the forecast errors are on average larger than the spread of the ensemble for these particular situations. (ii) Forecast errors are reduced by about 50% if each TIGGE centre’s own analysis is used instead of the ECMWF analysis, particularly in regions of large PV gradients. This result calls for more thorough reflections in future work on what to use as the ‘truth’ in forecast evaluation. (iii) The breaking of large-scale upper-level waves in the atmosphere that is often responsible for the generation of the disturbances studied here is often too weak in the forecasts, leading to a northeastward shift. (iv) Ensemble-mean multi-model forecasts of a fourday accumulated precipitation event appear accurate enough for a successful severeweather warning in contrast to some single-model ensemble predictions. (v) Condensational heating in clouds upstream early in the forecasts appear to be more important than high-pressure systems downstream for strengthening the upper disturbances in the medium range (3–7 days forecasts). The results and the analysis tools developed in this project have improved the understanding of low-latitude upper-level disturbances and their impacts. The project has also advanced the verification of ensemble predictions. Both can contribute to an improvement of numerical weather forecasting in the long term.
Projektbezogene Publikationen (Auswahl)
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2009: An unusual Saharan dust outbreak into central Europe and heavy precipitation at the southern side of the Alps in May 2008: A TIGGE case study. 3rd THORPEX International Science Symposium, 14–18 Sept. 2009, Monterey, CA, USA
Wiegand, L., Twitchett, A., Schwierz, C. and Knippertz, P.
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2009: Upper-level disturbances over the lowlatitude North Atlantic: Forecast quality, predictability and dynamics. 9th EMS Annual Meeting, 28. Sept.–02. Oct. 2009, Toulouse, France, EMS2009-98
Wiegand, L. and Knippertz, P.
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2010: Upper-level disturbances over the lowlatitude North Atlantic: Forecast quality, predictability and dynamics. EGU 6th General Assembly, 02–07 May 2010, Vienna, Austria, Geophysical Research Abstracts, Vol. 12, EGU2010-5573
Wiegand, L.; Knippertz, P.
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2011: Heavy precipitation at the Alpine south side and Saharan dust over central Europe: A predictability study using TIGGE. Wea. Forecasting, 26, 957–974
Wiegand, L., A. Twitchett, A., C. Schwierz, C. and Knippertz, P.
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2011: The key role of diabatic processes in modifying the upper-tropospheric wave guide: a North Atlantic case-study. Quart. J. Roy. Meteor. Soc., 137, 2174–2193
Grams, C. M., Wernli, H., Böttcher, M., Campa, J., Corsmeier, U., Jones, S. C., Keller, J. H., Lenz, C.-J. and Wiegand, L.
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2012: Equatorwards-breaking rossby waves in the North Atlantic storm track: Forecast quality, predictability and dynamics. PhD thesis, University of Leeds
Wiegand, L.
