The red-sky enigma over Svalbard in December 2002

On 6 December 2002, during winter darkness, an extraordinary event occurred in the sky, as viewed from Longyearbyen (78° N, 15° E), Svalbard, Norway. At 07:30 UT the southeast sky was surprisingly lit up in a deep red colour. The light increased in intensity and spread out across the sky, and at 10:00 UT the illumination was observed to reach the zenith. The event died out at about 12:30 UT. Spectral measurements from the Auroral Station in Adventdalen confirm that the light was scattered sunlight. Even though the Sun was between 11.8 and 14.6deg below the horizon during the event, the measured intensities of scattered light on the southern horizon from the scanning photometers coincided with the rise and setting of the Sun. Calculations of actual heights, including refraction and atmospheric screening, indicate that the event most likely was scattered solar light from a target below the horizon. This is also confirmed by the OSIRIS instrument on board the Odin satellite. The deduced height profile indicates that the scattering target is located 18–23km up in the stratosphere at a latitude close to 73–75° N, southeast of Longyearbyen. The temperatures in this region were found to be low enough for Polar Stratospheric Clouds (PSC) to be formed. The target was also identified as PSC by the LIDAR systems at the Koldewey Station in Ny-Ålesund (79° N, 12° E). The event was most likely caused by solar illuminated type II Polar Stratospheric Clouds that scattered light towards Svalbard. Two types of scenarios are presented to explain how light is scattered.publishedVersio

Polar low le Cygne: Satellite observations and numerical simulations

A polar low (PL) which occurred in October 1993 over the Norwegian Sea is investigated from an observational and a numerical point of view. This PL has several unique features: it developed early in the season, it lasted for about 3 days, and its trajectory was such that it passed over weather stations so that ‘conventional’ observations of the low are available. The conditions of the formation, development and decay of the PL are investigated using a double approach: satellite data from several instruments are used together to document the mesoscale structure of the low, and two versions of a limited-area model are run to investigate the dynamics of the low. Numerical model fields are compared to quantities derived from TIROS-N Operational Vertical Sounder, the Special Sensor Microwave/Imager, and satellite radar altimeter data. In spite of a better spatial resolution of the models, humidity and surface wind speeds are less organized in the simulations than in satellite retrievals. The number of vertical levels, especially for the lowest layers of the atmosphere, appears to be an essential component for a good simulation of the trajectory of the low. There is, however, good overall agreement between modelled and satellite-derived fields, and the good quality of the simulations allows inferences to be made regarding the essential physical and dynamical processes taking place during the formation and development of the PL. We find that the PL was the result of favourable flow conditions at the surface in the form of a shallow arctic front established south of the ice edge, together with an upper-level potential-vorticity anomaly setting the stage for a positive interaction. Later on, the strong surface sensible- and latent-heat fluxes contributed to the extensive vertical development. This study demonstrates the usefulness of the approach adopted here, which relies not only on simulations but also on observations to get a very complete description of such disturbances