Mesoscale structure of a polar low with strong upper‐level forcing

Dry air descending within the circulation of a polar low was observed to give a striking signature in the echo return from an upward‐looking VHF radar. Data from this radar, along with weather‐radar data, satellite imagery and output from limited‐area and mesoscale versions of the Met Office Unified Model, are synthesized to depict the mesoscale structure of the weather system. The polar low had a compact vortex centred near 3 km, a cold core below this level, and a warm core above. Attribution of quasi‐geostrophic forcing showed that on the larger scale the polar low was dominated by upper‐level forcing. The detailed mesoscale analysis showed that the polar low, during its mature phase, was affected by a stratospheric intrusion that brought air from near tropopause level down to 3 km to give the striking signature in the VHF radar echo pattern. The echo pattern also suggested that thin tongues of this stratospheric, or near‐stratospheric, air were penetrating slantwise down a further several hundred metres, above the shallow cloud constituting the southern end of an archetypal cloud head. It is argued that this may be a locally important region of mixing between air originating near the tropopause and the boundary layer. In more general terms, the study demonstrates the ability of the operational mesoscale model to represent various small‐scale features. The study also helps in developing the interpretation of the patterns of clear‐air radar echo observed with VHF radar.