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 c ase-study presented in this paper. two and sometimes more of these mesoscal e circulations coexist within the same cold-frontal zone. The two slantwise circulations described were observed to be stacked one above the other wit h a vertical wavelength of less than 2 km. Although it is often suspected t hat the circulations at ana-cold fronts are enhanced by mesoscale processes such as conditional symmetric instability (CSI) or DeltaM-adjustment, it i s notoriously difficult to discriminate between these circulations and the larger-scale transverse circulation within which they are embedded. The occ urrence in this study of multiple circulations with small vertical scale he lps to distinguish them from the large-scale circulation, and this has moti vated the detailed examination of this case. Mesoscale circulations of the kind described are difficult to detect: numer ical weather prediction (NWP) models, even high-resolution models, do not u sually represent them, and conventional observations do not show them clear ly. This study takes advantage of observations from a high-resolution micro wave Doppler radar plus a high-resolution analysis of ultra-high-frequency wind-profiler radar data, analysed in the context of output from an operati onal mesoscale NWP model. The study defines the mesoscale structure of the event sufficiently carefully to provide a basis for future idealized modell ing studies to investigate the possible roles of CSI and DeltaM-adjustment, both of which appear to play a part in the maintenance of the circulations .