Evolution of a mesoscale upper tropospheric vorticity maximum and comma cloud from a cloud‐free two‐dimensional potential vorticity anomaly

A diagnostic case study is presented in which output from the limited‐area forecast version of the Meteorological Office operational Unified Model was used together with water vapour and cloud imagery from the Meteosat satellite. The case chosen for study was an archetypally simple example in which a long, narrow strip of air with high potential vorticity (PV) in the upper troposphere—strictly a tropopause depression—became unstable and rolled up into fairly ling‐lived mesoscale vortices. For much of the time the vortices were cloud‐free and they were dynamically pure examples of very small upper‐air PV anomalies moving relative to the strata beneath. As such they provided a good test of theoretical deductions from the PV invertability principle and of the performance of the dynamical components of the forecast model. The evolution of the mesoscale vortices as revealed in the satellite imagery was well handled by the Unified Model, indicating that the small scales were reproduced well by the model dynamics even though they were not fully, defined by the routine observations fed into the model. As the vortices began to form, the stretching deformation along the axis of the initial PV strip was very small, and the 900 km spacing of the vortices was broadly consistent with theoretical expectations for the wavelength of instabilities growing on such a PV strip. A detailed analysis of one of the mesoscale vortices throughout its 3‐day lifetime showed that its maximum potential vorticity and absolute vorticity decreased with time as it travelled within the circulation of large‐scale anticyclonic gyre. By the end of the second day, when cloud first formed, the vortex had become a weak upper tropospheric shear line, 500 km long, sandwiched between a pair of anticyclonic mesoscale vortices 900 km apart. The Meteosat imagery revealed a characteristic two‐component structure both in the water‐vapour channel and, when the cloud eventually formed, in the infrared channel too. This structure, which resembled that seen on other occasions, is explained in terms of a particular combination of vertical motion and advection in the region of sharp humidity gradients alongside dry stratospheric air that had intruded into the upper troposphere.