We explain how summertime convergence lines can be understood and forecasted.

What is it about?

Several case studies are analyzed in detail, looking at the vertical forcing ahead of the cold front (with quasi-geostrophic theory using Q-vector analysis). A climatological perspective is also considered. It is found that the deformation of the isotherms with the developing frontal system causes quasi-geostrophic ascent in the warm sector. This leads to wind convergence near the ground, which can serve as a trigger for deep moist convection in an unstable airmass typically characterized by an elevated mixed layer (EML).

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Photo by Cody Schroeder on Unsplash

Photo by Cody Schroeder on Unsplash

Why is it important?

Pre-frontal summertime convergence lines are commonly analyzed by the European weather service agencies and frequently lead to organized thunderstorms with damaging hail and wind. We show that quasi-geostrophic theory, especially Q-vector analysis, can be a useful tool to understand and forecast these events.