Climatology, characteristics and forcing mechanisms of warm-season cold-frontal convective cells in Germany

Abstract

Cold fronts provide an environment particularly favourable for convective initiation in the mid-latitudes and can also be associated with convective hazards such as flooding, wind, hail and lightning. In this thesis, the climatology, characteristics and forcing mechanisms of cold-frontal convection are analysed by combining a radar-based cell detection and tracking dataset and an automatic front detection method applied to reanalysis data. The climatology indicates that around twice as many cells develop on cold-frontal cell days on average compared to non-cold-frontal cell days. The maximum cell frequency is found to be marginally ahead of the typical surface front location, whereas the 700 hPa front location marks the minimum cell frequency and a clear shift in regime between cells with a weakened diurnal cycle on the warm side of the 700 hPa cold front and strongly diurnally driven cells on the cold side of the 700 hPa front. Lightning strike data are analysed in Western and Central Europe and show broadly consistent results. Pre-surface-frontal cells have the largest fraction associated with mesocyclones, intense convective cores and lightning in Germany. Namely, mesocyclones were detected in around 5.0% of pre-surface-frontal cells compared to only 1.5% of non-cold-frontal cells.

To unravel the results found in the climatology, the thermodynamic environments and lifting mechanisms of convective cells are analysed by considering different variables from reanalysis data. Pre-surface-frontal cells have the highest dewpoints and convective available potential energy, which is likely a contributing factor towards the larger fraction of cells with mesocyclones, intense convective cores and lightning. Higher mid-level relative humidity than climatology was found to be particularly important to allow convective cell development on the cold-side of the 700 hPa front. While large-scale lifting is typically thought to be most relevant in the warm-sector, here its relevance is also highlighted on the cold-side of the 700 hPa front. Most cells have relatively low sunshine duration prior to development, which indicates that solar heating may not be directly relevant for the majority of cell initiation.

A more complex statistical modeling approach, incorporating one moisture, one instability, and three lifting terms, reveals a lower level of proficiency near the surface front, consistent across four distinct statistical models. Feature (predictor) importance techniques show differences in the importance of each feature depending on the front relative region. Furthermore, certain features add more skill depending on the statistical model used. Finally, a 12 feature gradient boosted model is trained to predict the probability of convective cell occurrence. The model is shown to generally represent the spatial and temporal frequency of cells around the front on new, unseen testing data.

The results in this thesis are an important step towards a deeper understanding of the drivers of cold-frontal convection depending on the region relative to the front. Furthermore, light is shed on the applications of statistical models to predict the convective cell occurrence.