Satellite climatological aspects of the “polar low” and “instant occlusion”

Abstract

Middle resolution DMSP (Defense Meterological Satellite Program) visible and infrared imagery are analyzed in conjunction with surface and upper-air synoptic observations for two years' ( 1977. 1978/79) mid-season months (January, April. July, October) to derive synoptic climatological information on the "polar low" and "instant occlusion" phenomena. Polar-air vortices occur most frequently over the oceans in winter. Regional differences in the dominance of cloud signature sub-types confirm variations in the dynamics involved in polar-air cyclogenesis. as noted by previous workers. The "comma cloud" appears to occur more frequently in the North Pacific. while the spiraliform vortex predominates in the North Atlantic. The latter seems to be at least partly attributable to the effect of the eastern Arctic sea-ice boundary. as demonstrated by an analysis of imagery for two winters (D. J. F). each representative of an opposing mode of the North Atlantic Oscillation (1974/75. 1976/77).Composite synoptic "models" of polar lows reveal surface-pressure departures similar to those for cyclogenesis of the frontal wave type. However, a cold core and little vertical tilt distinguish the polar low in the mean. The "instant occlusion" exhibits surface-pressure departures that are deeper and comparable to the mature/dissipating stages of wave cyclone evolution. Spatial patterns of the upper-air anomalies associated with instant occlusions show two distinct vortices: one apparently derived from the polar low. the other from the incipient frontal wave with which it has merged. The seasonal and latitude-dependent cyclone intensity statistics (all types) are applied to the cloud vortex inventory of the North Atlantic-Europe sector for January 1979. The results support the r6le of the polar low as a synoptic indicator of anomalous winter circulation in the extratropics.

Mesocyclone activity over the North-East Atlantic. Part 1: vortex distribution and variability

“…The most active season was winter, confirming the perception that polar mesocyclones are a winter feature. This result is consistent with the limited climatologies of the Nordic Seas (Carleton, 1985;Polar Lows Project, 1986), the North Pacific (Businger, 1987), and Canadian waters (Parker, 1997). However, 600-800 km diameter cyclones were more common in the summer and so are unlikely to be triggered by the same processes as the majority of cyclones which occur in the cold seasons.…”

“…The mean diameter for multiple-sighted cyclones (410 km) was much greater than that for single-sighted cyclones (160 km) because the spread of the distribution towards the larger sizes was greater. These results are similar to those quoted by Carleton (1985).…”

“…The results presented here show that polar mesocyclones occur in all seasons, including the summer, a season not covered by the studies quoted above. However, Carleton (1985) noted a similar observation and Turner and Thomas (1994) also found this to be the case in the Antarctic.…”

“…However, in the other sizes (B200 and \ 800 km) cyclones either remained the same size, or changed from spiral to comma. This latter situation implies a residual frontal zone, after the circulation had decayed, and similar evolution of shape has been observed elsewhere (Carleton, 1985).…”

“…Some cyclones develop along the ice edge (Carleton, 1985;Businger and Reed, 1989), the (Turner and Row, 1989), such as Greenland, Norway and Iceland. Cyclones also occur in the lee of orographic barriers (Turner et al, 1993), and mesocyclones were observed near Greenland where large-scale flow was channelled around Cape Farewell.…”