Stacked slantwise convective circulations

It is well known that classical (ana) cold fronts tend to be characterized by mesoscale circulations in which upright line convection feeds a layer of concentrated rearward slantwise ascent. Occasionally, however, as in the case‐study presented in this paper, two and sometimes more of these mesoscale circulations coexist within the same cold‐frontal zone. The two slantwise circulations described were observed to be stacked one above the other with a vertical wavelength of less than 2 km. Although it is often suspected that the circulations at ana‐cold fronts are enhanced by mesoscale processes such as conditional symmetric instability (CSI) or ΔM‐adjustment, it is notoriously difficult to discriminate between these circulations and the larger‐scale transverse circulation within which they are embedded. The occurrence in this study of multiple circulations with small vertical scale helps to distinguish them from the large‐scale circulation, and this has motivated the detailed examination of this case. Mesoscale circulations of the kind described are difficult to detect: numerical weather prediction (NWP) models, even high‐resolution models, do not usually represent them, and conventional observations do not show them clearly. This study takes advantage of observations from a high‐resolution microwave Doppler radar plus a high‐resolution analysis of ultra‐high‐frequency wind‐profiler radar data, analysed in the context of output from an operational mesoscale NWP model. The study defines the mesoscale structure of the event sufficiently carefully to provide a basis for future idealized modelling studies to investigate the possible roles of CSI and ΔM‐adjustment, both of which appear to play a part in the maintenance of the circulations.