The climatology and nature of warm-season convective cells in cold-frontal environments over Germany

Cold fronts provide an environment particularly favourable for convection in the mid-latitudes with advection of low-level moisture, synoptic-scale ascent and modification of atmospheric instability among some of the most important mechanisms. However, the importance of different mechanisms at different locations relative to the front (i.e., pre-frontal, frontal and post-frontal), is not currently well-understood. Cold-frontal convection has attracted the attention of several case studies but spatial and temporal climatologies over longer timeseries of data are very limited, especially during the warm-season. A further question of interest is how cold-frontal environments may alter the nature of convection in terms of convective organisation, cell intensity and the associated convective hazards.

By combining automatic front detection methods and the KONRAD cell detection and tracking algorithm a climatology of cold-frontal convective cells is produced in Germany between 2007–2016 for April–September. We find that on average around twice as many cells develop on cold-frontal cell days compared to non-cold-frontal cell days. Using the 700hPa level as a reference point we show the maximum cell frequency is 350–400km ahead of the 700hPa frontal line which is marginally ahead of the mean surface front location and maximum in surface convergence. The 700hPa front location marks the minimum in the cell frequency and a clear shift in regime between cells with a weakened diurnal cycle on the warm-side of the 700hPa cold front and strongly diurnally driven cells on the cold-side. High cell frequencies are found several hundreds of kilometres ahead of the surface front and cells in this region are most likely to be associated with mesocyclones, intense convective cores and lightning where CAPE is climatologically higher.