Polar lows, its structure and mechanism of cyclogenesis.

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A polar low over the Labrador Sea was observed on 14-15 February 1992. Satellite images obtained from the AVHRR showed the cyclogenesis and development of the polar low. The cloud streaks over the Labrador sea suggested that a convergence formed between the westerly from Eastern Canada and the northeasterly from Greenland before when the polar low developed. A vortex formed along the convergence zone and developed into the polar low of which the horizontal diameter was about 500 km.On the basis of the JMA global objective analysis (GANAL), an intense upper cold vortex was located over the Labrador sea when the polar low occurred. The GANAL data showed a dry and cold westerly was prevailing over the Labrador sea and that the convergence occurred between the westerly from Eastern Canada and the northeasterly from Greenland. The polar low formed along the convergence zone. The polar low was shallow disturbance which was confined to below 700 hPa.In order to clarify the detailed structure and the processes of cyclogenesis and development of the polar low, we made numerical simulation experiments using the Japan Spectral Model. JSM simulated well the development of the Labrador polar low. First, a convergence occurred between the westerly from Eastern Canada and the northeasterly from Greenland. Then, the polar low developed along the convergence zone. There was a strong stable layer above 700 hPa and the disturbance of the polar low was confined to below 700 hPa.As far as this case of the Labrador polar low, we could infer that the convergence was important for the cyclogenesis and that the latent and sensible heat fluxes were important for the development. It is not clear presently the role of the upper cold vortex in the cyclogenesis of the polar low.