Climatology, characteristics and forcing mechanisms of warm-season cold-frontal convection in Europe

Convection frequently initiates in proximity to cold fronts during the European warm-season and can also be associated with hazards such as flooding, rain, and hail. Despite this, the frequency and underlying processes that drive such events are not well-understood. To understand the typical nature, frequency and forcing mechanisms of convection depending on the region relative to the front, automatic front detection methods, a convective cell detection and tracking dataset (KONRAD), and lightning data are combined between 2007–2016.

The climatology shows that convective cells are most frequent in Germany marginally ahead of the surface front. Furthermore, the 700 hPa frontal line marks the minimum frequency of convection and a shift in regime between cells with a strong diurnal cycle on the cold-side of the 700 hPa front and a weakened diurnal cycle on the warm-side of the 700 hPa front. The results are consistent for lightning data on a sub-European domain. Given cell detection ahead of the surface front, cells are up to 3 times more likely to be associated with a mesocyclone compared to non-cold-frontal cells in Germany. Cells with 55 dBZ cores are over 1.5 times more likely.

To unravel the complex relationships between different predictor variables and the probability of convection a logistic regression model is developed. Feature importance techniques are utilised to understand which variables carry the most importance depending on the region relative to the front. We find solar heating carries more importance towards the model’s predictive power behind the 700 hPa front than ahead of the 700 hPa front. The opposite is true for the elevation term, which acts as a proxy for the influence of orography on convective initiation. By giving the model information on the number of surrounding grid points associated with convection, a proxy for cell interactions, the most skill is added near the surface front.

These results are an important step towards a deeper understanding of the underlying processes that drive cold-frontal convection and improved forecasting.