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

Dry air descending within the circulation of a polar low was observed to gi ve 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 sys tem. The polar low had a compact vortex centred near 3 km, a cold core belo w this level, and a warm core above. Attribution of quasi-geostrophic forci ng showed that on the larger scale the polar low was dominated by upper-lev el forcing. The detailed mesoscale analysis showed that the polar low, duri ng 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 t ongues of this stratospheric, or near-stratospheric, air were penetrating s lantwise down a further several hundred metres, above the shallow cloud con stituting the southern end of an archetypal cloud head. It is argued that t his may be a locally important region of mixing between air originating nea r the tropopause and the boundary layer. In more general terms, the study d emonstrates the ability of the operational mesoscale model to represent var ious small-scale features. The study also helps in developing the interpret ation of the patterns of clear-air radar echo observed with VHF radar.