The Origin of Western European Warm-Season Prefrontal Convergence Lines

Johannes M. L. Dahl Atmospheric Science Group, Department of Geosciences, Texas Tech University, Lubbock, Texas

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Jannick Fischer Atmospheric Science Group, Department of Geosciences, Texas Tech University, Lubbock, Texas

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Abstract

The authors investigate the origin of prefrontal, warm-season convergence lines over western Europe using the Weather Research and Forecasting Model. These lines form east of the cold front in the warm sector of an extratropical cyclone, and they are frequently the focus for convective development. It is shown that these lines are related to a low-level thermal ridge that accompanies the base of an elevated mixed layer (EML) plume generated over the Iberian Peninsula and northern Africa. Using Q-vector diagnostics, including the components that describe scalar and rotational quasigeostrophic frontogenesis, it is shown that the convergence line is associated with the rearrangement of the isentropes especially at the western periphery of the EML plume. The ascending branch of the resulting ageostrophic circulation coincides with the surface velocity convergence. The modeling results are supported by a 3-yr composite analysis of cold fronts with and without preceding convergence lines using NCEP–NCAR Reanalysis-1 data.

Current affiliation: Institute of Meteorology, Free University of Berlin, Berlin, Germany.

Corresponding author address: Johannes Dahl, Atmospheric Science Group, Dept. of Geosciences, Texas Tech University, Box 41053, Lubbock, TX 79409. E-mail: johannes.dahl@ttu.edu

Abstract

The authors investigate the origin of prefrontal, warm-season convergence lines over western Europe using the Weather Research and Forecasting Model. These lines form east of the cold front in the warm sector of an extratropical cyclone, and they are frequently the focus for convective development. It is shown that these lines are related to a low-level thermal ridge that accompanies the base of an elevated mixed layer (EML) plume generated over the Iberian Peninsula and northern Africa. Using Q-vector diagnostics, including the components that describe scalar and rotational quasigeostrophic frontogenesis, it is shown that the convergence line is associated with the rearrangement of the isentropes especially at the western periphery of the EML plume. The ascending branch of the resulting ageostrophic circulation coincides with the surface velocity convergence. The modeling results are supported by a 3-yr composite analysis of cold fronts with and without preceding convergence lines using NCEP–NCAR Reanalysis-1 data.

Current affiliation: Institute of Meteorology, Free University of Berlin, Berlin, Germany.

Corresponding author address: Johannes Dahl, Atmospheric Science Group, Dept. of Geosciences, Texas Tech University, Box 41053, Lubbock, TX 79409. E-mail: johannes.dahl@ttu.edu
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