Evolution and mesoscale structure of a polar low outbreak



An observational study of an unusual ‘polar low outbreak’ is described in which four lows formed in succession near south-east Iceland. Because all the lows tracked south-east, close to Scotland, an unusually dense network of surface observations was available for analysis, including radar. This complemented the geostationary and polar-orbiting satellite data that are more commonly used in polar low studies.

Each of the four polar lows displayed a different structure and evolution. The first was a circular convective low, the second a relatively weak surface signature of an upper-level feature, while the final two cyclones appeared as a wave train, with one cyclone decaying and the other developing into an intense disturbance.

Because of the high resolution of the surface data, it was possible to identify, unambiguously, mesoscale frontal features within the polar lows. These are described in detail. A close examination was made of the final intense low, which showed that its evolution and frontal structure closely matched those found in standard conceptual models of rapidly developing maritime cyclones. This polar low also showed evidence of a warm core, and symmetric instability within an extensive ‘cloud head’.