Aleutian Island Bering Sea fisherman All coastal areas include: Kodiak Southeast Panhandle Polar low is a term used to describe a sub-synoptic, baroclinic polar vortice occurring in the winter season north of the Polar Front Jet and in the wake of a decaying low pressure (Businger and Reed 1989). The recent development (following the 1970’s) of very high-resolution satellite imagery and sensors in a wide variety of the spectrum bands associated with satellites has enabled scientist to study polar lows. During the cool season, October though March, with peak “polar-genesis” during November though February, two phases of polar-genesis occur. The first being the development of a polar vortex and the second being a polar low. Both phases occur to the north and/or the west of a modified Arctic or Polar Front Jet, or can usually be found on the west side of a dissipating synoptic low. Characteristic that distinguish the two phases include the vortex being an developing low, meaning the difference between the two, is a polar low will have a closed wind circulation, a pressure minimum, and surface wind speeds of 35 kts or greater (AF 11 OWS). Further identifying features of a polar low; they will most likely have an “eye,” and look something like an “Arctic Hurricane” (Dr. Steve Businger coined the term). There are several contributing factors for the development of a polar low. First, synoptic conditions are most favorable when on the west and cold side of a decaying baroclinic low pressure, positioned north of the Polar Front Jet. Next is to have a “vortex” of convectivity. This semi-organized convectivity or vortex, is essentially the same makeup as a severe thunderstorm in the lower 48, except these vortices do not play by the traditional severe storm rules. What I’m referring to is an occluded low is generally baroclinic in nature, stacking towards the west and into the colder air aloft, this upper level cold core closed low or upper level trough is the catalyst which will develop the polar vortex into a full fledged polar low or Arctic Hurricane. Next there are several ingredients which must be in place for the convection to be “self sustaining.” These include the following: warm, moist air at the lower levels, very cold arctic air flowing from the NW (either coming from cP air mass from mainland Alaska or mP air mass from northern Bering Sea ice shelf) in the mid-levels, this will ensure a steep lapse rate as parcels become thermodynamically unstable. If upper level temperature of -42 C or colder are moving out over open oceans, they are essentially acting as a giant sponge. (Rasmussen 1994). Once the cA air mass (form non water surface) advects over mP (ocean surface), this will promote deep convection as the air masses begin to equalize each other by means of condensation of latent heat thus transforming a cold core system into a low level warm cored system. Next (and the most pronounced differentiating characteristic) since polar low genesis occurs in the wake of a dissipating low, there will already be cyclonic turning of wind throughout the levels, in order to develop on it’s own, two ingredient must occur; one is reverse shear conditions, when thermal flow opposes surface flow, commonly occurs when a cold pocket of air moves south of a surface low (Duncan 1978). The other is convective instability of the second (CISK), which works something like a Carnot heat engine, which is what gives a tropical hurricane its fuel. This could possible explain why, on satellite imagery, there is a distinguishable “eye” and a high level outward flow of air aloft once reaching the tropopause level. Mature polar lows are extremely dangerous to ocean going ship. In the Bering Sea, the fisherman are the primary industry feeling the wintry wraith of the arctic hurricanes, often times these brave fisherman experience winds sustained winds from 40 to 60 knots, with unforeseeable higher winds, essentially like a rogue wave. Wave height can reach upwards of 50 feet, and intense episode of heavy snow and freezing spray give extreme icing conditions to mariners. One example of this; I was stationed aboard a 378 foot long Coast Guard cutter in 1998. During a winter patrol to the Bering Sea in January, the ship was in the wake of a polar low. Unable to outrun the storm, the captain steered the ship into the stormy and icy winds for three days. On the 4th day, when the winds had subsided and the sky’s cleared, every able bodied man and women onboard was tasked with chipping, hampering, and scraping an estimated 60 tons of rime icing off the entire horizontal and vertical surfaces of the ship. Some ice accumulations were more than a foot thick. One of the primary reasons for polar low formation is due the location being over open oceans. It is over these regions that models suffer from lack of surface and upper air observational data, thus misleading conventional models from “missing” a polar low formation. Therefore model analysis should not be relied upon by forecasters for the sole source of detection of these winter storms. There are three methods for detection of these storm formations. The first is experience at recognizing upper level conditions both on satellite and charts. The second is a forecaster’s ability to critically analysis satellite images, thereby issuing maritime warning to the regions being affected by these storms. Last is the forecaster’s ability to recognize both surface and upper air synoptic and sub-synoptic conditions favorable for formation of polar lows. I have been a forecaster in Kodiak for three years. From the first winter, I recognized that this ever-changing synoptic environment was different from the lower latitudes, such as the continuous United States. Unable to accurately prog systems moving through my forecast area (Western Aleutians eastward to Southeast Panhandle of Alaska) I began to wonder why. Then with further analysis I noted that some of the occluded systems would actually move in a northwestward direction! Through my personal observation, I’ve noted that this most often occurs in the winter months, when the PFJ is further southward between latitudes of 30° and 40°. Do to the meandering of the Rossby waves, as they propagate around the globe, one of the better set ups for a polar low formation in the Gulf of Alaska is for a Rossby wave to become very meridional just south of the Gulf of Alaska. Not only will the supply increase 1000- 500 mb thickness due to the “pineapple express,” but will also aid in a polar low moving in very erratic patters that are highly unpredictable. Therefore it has been interesting to note that during these conditions, maritime advisories and warning issued by the NWS will be very numerous and cover lots of ground around the forecasted region in anticipation of where the storm might move. In my limited research for information on polar low forecasting, there are few sites and training aids out there, most are case studies of past storms. As with tropical storms, polar low movement seems to be erratic and unpredictable, therefore I believe that there are not “hard fast” forecasting rules to forecast movement. As stated before, there are rules of thumb which have been identified by leading polar low researchers to help identify favorable conditions for such as storm formation. As a Coast Guardsman station in Kodiak, it is unfortunate for my position, and my replacement; since experience is the best method at identifying these storms, after I leave, then the “relearning” cycle will begin again thus hurting the life saving missions that rely on our weather forecast. References: Businger, S., 1987: The synoptic climatology of polar low outbreaks over the Gulf of Alaska and the Bering Sea. Businger, S., and R.J. Reed, 1989: Cyclogenesis in cold air masses. Weather forecasting. Rasmussen, Erik A., and Annette Cederskov, 1994: Polar Lows: A critical analysis. The life cycles of extra-tropical cyclones. Nicholas A. Bond and M. A. Shapiro, 1991: Polar Low over the Gulf of Alaska in Conditions of Reverse Shear. Forecasting Handbook for the Bering Sea, Aleutian Islands and Gulf of Alaska (Ch6.2) |