Classification of mesoscale vortices in polar airstreams and the influence of the large-scale environment on their evolutions

Abstract

Satellite imagery was used to detect and study 133 mesoscale cloud vortices which occurred in the northeastern Atlantic Ocean and adjacent seas between 1 December 1981 and 5 January 1982. Only vortices occurring in cold air, separate from synoptic‐scale frontal cloud bands, were considered. The complete history of the evolution of each cloud vortex was recorded in terms of its location, size, cloud pattern, and cloud type (shallow or deep; stratiform or convective). Once all vortices were documented, conventional meteorological data and analyses were consulted. One objective of the study was to determine whether satellite imagery could be used to estimate the central pressure of the cyclones accompanying the mesoscale vortices. Other objectives were to determine if there were significant differences in (1) the mesoscale cloud structure or (2) the large‐scale environment of vortices which remained insignificant versus those that developed into polar lows. Results indicate that the deficit pressure of the polar vortex could be estimated from satellite imagery and that there were recognizable differences between developing and non‐developing vortices. Only a few cloud configurations accompanied polar lows, whereas the occurrence of certain other configurations at early stages in the vortex existence indicated that these vortices would remain insignificant. The synoptic‐scale environment appeared to play a considerable role in determining which incipient vortices continued to develop. Moderate baroclinicity (horizontal temperature gradients and vertical wind shear), relatively weak winds, large lapse rates, and favorable vorticity advections accompanied developing polar lows. Non‐developing vortices had unfavorable vorticity advections. Of these, some vortices were able to grow initially to moderate disturbances which remained steady thereafter. The apparent difference between these and insignificant vortices was that the steady vortices were more baroclinic and had steeper lapse rates.

Polar low tracks over the Nordic Seas: a 14-winter climatic analysis

“…Of the 190 PL events in our study, 56 are dual, of which nine are classified as 'wave systems', and 24 are multiple, of which 8 events are classified as 'merry-go-round' (Forbes and Lottes, 1985). With the exception of the latter systems, dual and multiple systems exhibit roughly the same spatial and temporal distributions as other PLs in the list ( Fig.…”

Associations between large‐scale atmospheric circulation and polar low developments over the North Atlantic during winter

“…The association of polar lows with changes in the locations and amplitude of planetary waves [e.g., Forbes and Lottes , 1985], SST anomalies, sea‐air temperature difference and sea ice extent provides a context for a low‐frequency signature associated with major climatic teleconnection patterns. Rather surprisingly, these low‐frequency associations of polar lows have been better studied for the Southern Hemisphere than for the Northern Hemisphere [ Carleton , 1996].…”

Observation of polar lows by the Advanced Microwave Sounding Unit: potential and limitations

“…Such climatologies remain seldom and are, at the time being, of two types. The first type relies on extensive subjective IR‐visible satellite imagery (Carleton, 1985; Forbes and Lottes, 1985; Harold et al, 1999). Caveats of such studies are their shortness (typically a couple of years), essentially because of the non‐automatic character of the method and the uncertainty when attempting to strictly repeat such a technique.…”