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 M-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 M-adjustment, both of which appear to play a part in the maintenance of the circulations.