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ABSTRACT

Four cases satisfying the necessary conditions for development of Arctic outbreak polar lows have been investigated from numerical simulations. Characteristic synoptic conditions are a mature extratropical cyclone situated over Scandinavia and a northerly, baroclinic flow in the Norwegian Sea. This flow has a convective planetary boundary layer (PBL) caused by heat fluxes from the warm ocean in cold Arctic air masses coming from the sea ice areas to the north and west. The height of the PBL may reach 700 hPa and even more. The synoptic situation is also characterized by a dry intrusion of stratospheric air west of the cyclones. When the tropopause is defined by the surface of potential vorticity (PV) equal 2 PVU (1 PVU = 10−6 m2 s−1 K kg−1), the intrusion lowers the tropopause typically down to levels between 450 and 750 hPa, bringing large positive anomalies of PV to levels normally in mid-troposphere. These synoptic conditions are ideal for cyclogenesis from mutual interaction of positive upper air and boundary layer PV anomalies. The Rossby height is high, which means that small-scale polar lows may form. It is found that simply the height between the tropopause and the top of the convective PBL is a good indicator of the risk of polar low formation. This height was found to be 2500 m or more in two of the cases when no polar lows were observed. When polar lows developed in the other two cases, heights of 1000 m or less were measured (in numerical simulations). The simulated development of these two polar lows has been investigated. The polar low cyclogenesis was found to be caused by the baroclinic instability in agreement with the conceptual model of Montgomery and Farrel. Diabatic intensification is important and seems to be necessary to seclude the warm core disturbance which is characteristic of Arctic outbreak polar lows. The seclusion process implies that warm air is being surrounded by cold air.