(Inge Johannessen, GIUB)

In June 1994, during the cyclone symposium in Bergen, Professor Erik Rasmussen put our attention to an interesting polar low (PL) incident. This case, eventually to be known as the "Le Cygne Polar Low", had been briefly studied by N. W. Nielsen and Anna Hilden at the Danish Meteorological Institute. Their work appeared in a report published in the May 1994 issue of Vejret. The case seemed very promising and we immediately set forth performing numerical experiments with the Norwegian Limited Area Model (NORLAM50). The initial results turned out to be very successful. Both the 50 km and nested 25 km grid simulations managed to reproduce the PL's characteristics in great detail. A summary of the results was presented at the 1996 EPLWG workshop in St. Petersburg, and published in the accompanying proceedings. We will here present a short resume of our main conclusions, including some recent work. According to our results, the Le Cygne PL, exhibiting a total lifespan of some 84 hours (13-16 Oct. 1993), was a result of favourable flow conditions at the surface, together with an upper-level anomaly setting the stage for a positive self-exciting feedback (phase locking). A shallow arctic frontal boundary had prior to the PL development established itself just south of the ice-edge, northeast of Svalbard. A low-level jet in connection with this front resulted in strong north-northeasterly winds (18 m/s) parallel to the ice-edge. The temperature gradient along this secondary front was strengthened considerably by a relatively warm, east-southeasterly wind flow generated by the movement of an intense extratropical cyclone into northern Scandinavia. Thus, this shallow, arctic baroclinic zone intensified as a result of the ongoing confluence. In the meantime, an extensive cold upper-level vortex north of Svalbard moved slowly, but steadily southwards. See Figure A for an overview of the situation prior to initial development. The vertical stability recorded by a radiosonde ascent from Bear Island at 14/10/00UTC, indicate an approximately neutral atmosphere up to 500-450 hPa, interupted by a weak inversion layer between 900-750 hPa.

Our view is that the upper-level cold core vortex assists in perturbating the shallow arctic front. Their subsequent interaction explains why this developing low-level wave propagates southwards, from near the ice-edge zone and into the Norwegian Sea. See Figure B, which shows the in- tensifying pressure wave having moved southwards. The entire life cycle of the PL is most fruitfully understood with the aid of the potential vorticity concept. As the PV anomaly moves south into the Norwegian Sea, a phase-locking between the growing surface disturbance and upper PV ano- maly occurs. Strong surface sensible and latent heat fluxes (about 1000 W/m2) feed the growing PL, contributing to its extensive vertical development. Observational evidence for this is found in satellite imagery, showing deep cumulus cells protruding through the high level cloud field during its early stage. The initial synoptic flow conditions are that of reversed shear, with a forward tilt in the vertical. The upper-level vortex then undergoes an interesting transition, evolving from a synoptic scale and into a mesoscale anomaly while maturing. Both stretching and heat release processes contribute to this scale transition. Additional satellite observations so gratefully supplied to us by Chantal Claud reveals an impressive correlation when compared to our numerical results. Since the workshop in St. Petersburg, we have performed additional numerical experiments in order to isolate other probable causes of development. This included the possible role of orographically induced instabilities (Novaja Zemlya, Kola Peninsula) and coastal trough influence (which was present just off the Northern Norwegian coast. A mid-tropospheric wave moving east above the Norwegian Sea towards the mainland, generated vertical motion and the surface trough signature). None of these experiments uncovered convincing proof supporting any of these alternative explanations. Though, we should mention that the coastal trough eventually merges with the southwardmoving, intensifying surface wave just off Bear Island. This most likely has a favourable influence during this early stage. In a recently published paper by N. W. Nielsen in the 27. June issue of J. Geophys. Res.(1997), the author proposes an explanation of the PL development based on the generalised forms of the equations for omega and geopotential tendency. We agree with Nielsen's findings regarding the physical aspects of the PL's mature phase and the role of CISK/ASII. But our experiments show that in order to capture it's complete life-cycle, and especially the early stages of development, one must investigate the atmospheric conditions at a much earlier time. We propose an alternative explanation, based on potential vorticity thinking, which we believe gives an elegant and simplistic but neverthless powerful insight into the origins and structures of the Le Cygne polar low. An article about this case is presently being prepared.