The Origin of Western European Warm-Season Prefrontal Convergence Lines

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.

Increase in the number of extremely strong fronts over Europe? A study based on ERA‐Interim reanalysis (1979–2014)

“…This is in agreement with the findings of Catto and Pfahl [] who note that extreme‐precipitation‐producing fronts have up to 35% stronger frontal strengths. This implies that an increase in the number of extremely strong fronts (Figures and ) potentially is one driving agent behind an increase in the number of extreme precipitation events, in particular, because summertime convective thunderstorms frequently form in prefrontal zones [ Browning and Monk , ; van Delden , , ; Dahl and Fischer , ].…”

What are the Best Thermodynamic Quantity and Function to Define a Front in Gridded Model Output?

“…A similar concern regarding the lack of similarity in θ and θ e for frontal analysis has been noted by Heimann (1992) in the case of a prefrontal foehn. The western European prefrontal convergence lines associated with north African air masses (Dahl and Fischer 2016) may also have this behavior, although maps of surface θ e were not presented. Thus, in some situations where the prefrontal air is particularly dry (illustrated here with situations from the Great Plains and prefrontal foehn, but not limited to those), using θ e in frontal analyses might misrepresent the situation.…”

Do changing circulation types raise the frequency of summertime thunderstorms and large hail in Europe?

“…As an example, thunderstorms in Western Europe are often characterized by a circulation type that features an upstream synoptic-scale trough that brings low-level moisture northward (van Delden 2001). This layer of air is initially capped by an elevated mixed layer with high instability until a front or a prefrontal convergence line produces the required lift to initiate storms (van Delden 1998, Dahl and Fischer 2016, Schemm et al 2016. In studies of severe weather events across Europe, the synoptic scale was found to be important in providing the required atmospheric moisture, e.g.…”