EAS 486: Alternatives to Norweigian Polar Front Theory

1. Alternatives and Adaptations of Norweigian Polar Front Theory
   1. Original Theory: Bjerknes (1919, Geophy. Soc., 1, 1-8.) ; Bjerknes and Solberg (1922, Geophys. Pub., 3, 1-18.)
   2. Fine Scale Structure
      1. All gradients observed on a scale on a few hundred meters for a front passing an instrumented tower in Boulder, CO (Figure 2.6 from Shapiro et al. 1985)
      2. References
         1. Extratropical Cyclogenesis: The Erik Palmèn Memorial Volume, 1990, Fig. 10.9, p. 175
         2. Original Reference: Shapiro (1984, MWR, 1634-1639); Shapiro et al (1985, MWR, 1166-1183.)
   3. Adaptation to Marine Cyclogenesis - Shapiro-Keyser Model)
      1. History
         1. Series of experiments from Virginia-North Carolina coast to England
         2. GALE (Genesis of Atlantic Lows Experiment) - middle 1980's along VA/Carolina coast
         3. Looking for secondary cyclone development along coastal front--didn't get much
         4. ERICA (Extratropical Redevelopment and Intensification of Cyclones in the Atlantic) - late 1980's along Gulf Stream to east of mid-Atlantic coast
            1. Accompanying Canadian experiment offshore of New England, Canadian Maritimes
         5. UK front/cyclone experiment - late 1980's
      2. Key structural adaptations (Shapiro and Keyser 1990)
         1. Figures:
            1. Extratropical Cyclogenesis: The Erik Palmèn Memorial Volume, 1990: Figs. 10.20 through 10.27 (Chapter 10 by Shapiro and Keyser is the Original Reference)
            2. Bluestein, Synoptic-Dynamic Meteorology in Middle Latitudes, Vol. II, 1993: Figs. 1.103 (p. 125) and 2.33 (p. 278)
      3. Warm air seclusion
         1. Air originally to the southwest of low can never quite be replaced by cold air since the circulation of the storm pulls the cold air around a larger radius of curvature.
         2. Even in mature stage, end up with warm air wrapped into storm center
      4. Bent-back front
      1. Warm front remains definable to west of surface low on northern flank of warm-air seclusion
      2. Rather than occluding, the original warm front gets pulled southward along northwest flank of low since the warm air to the south is not replaced
      5. Frontal fracture
         1. At relatively early stage of development, northern portion of cold front becomes disconnected from warm front
         2. Warm secluded air about the same temperature as northern edge of warm sector
         3. Cold front doesn't reconnect until occlusion stage along the east flank of low late in development
      6. Some evidence of similar structure in a few continental cyclones
   4. Polar lows and Polar Troughs
      1. References
      2. Reed, Extratropical Cyclogenesis: The Erik Palmèn Memorial Volume, 1990: Section 3.3.3, 36-39
      3. Bluestein, Synoptic-Dynamic Meteorology in Middle Latitudes, Vol. II, 1993: Figs. 1.104 and 1.105 (126-127)
      4. Original Research by Reed (1979, MWR, 38-52) and Mullen (1979 MWR 1636-1647; 1982 MWR 1664-1676; 1983 MWR 1537-1553)
      5. Cyclones or troughs in polar air streams
   1. Identification
      1. First identified from Pacific satellite imagery (late 1970's)
      2. First seen as area of enhanced convection (open cellular)
      3. Eventually takes form as either "comma cloud" or "baroclinic leaf"
      4. In comma cloud stage, can interact with a frontal wave on original cold front and suddenly take on occluded cyclone structure without intermediate stages "instant occlusion"
   2. Location
      1. Develops poleward of surface cold front within the temperature/thickness gradient within front
      2. Usually in cyclonic vorticity advection area associated with a secondary short wave in mean trough position
      3. Also can be in left exit region of straight jet (on cold side of jet)
      4. Most often seen over oceans
      5. Mullen (1982) documents a case moving from the Canadian Prairie Provinces into the Dakotas and Minnesota.
   3. Believed to be the same as the "polar low" well known by British and Norwegian meteorologists in the North Atlantic
   4. Debate about forcing
   5. Baroclinic instability from synoptic scale forcing noted above
   6. Convection present at start, so some question if it was a "cold season hurricane" driven by latent heat release
   7. Diagnostic studies (Reed and Blier 1986 MWR 1681-1708 2 articles) showed
      1. that moderate baroclinic instability does exist
      2. Cold air over warmer water produces large amount of surface heat and moisture flux to lower troposphere
      3. Cold air over warmer water also destabilizes lapse rate through a large depth in the atmosphere.
   8. Mullen (1982) documents one in the Canadian Prairies over land
   5. STORM Model (Hobbs et al 1996 BAMS)
   1. Adaptation of Polar Front model to central US
   2. Three Main Components
      1. Drytrough (Martin et al, Feb 1995, MWR)
         1. Unified name for feature that begins as a lee trough, then becomes a dry line as the induced southeasterly flow to the east of the drytrough pulls in enough Gulf air to provide a dew point contrast
         2. Pre-drytrough rain band
            1. Development of convection along dry line eventually propagates with mid-level southwesterly winds into cold sector
            2. Often appears as a comma shaped system with active convection along tail
      2. Arctic Front--Wong et al. (May 1995, MWR)
         1. Cold front not typically found on southern flank of surface system
         2. West to southwest winds in lee of Rockies force downslope flow
         3. Can be slightly cooler than mT air in warm sector, but not the coldest air in the system
         4. Arctic front remains or develops on western flank of low
         5. Can only advance as system moves away from Rockies so that cold air can be pulled southward to the west of the system
      3. Cold Frontogenesis Aloft (CFA)-- Locatelli et al. (Sept. 1995, MWR) (growl)
         1. Claimed production of squall line to east of drytrough created by "tipped forward" cold front
         2. Cold advection occurs at 700 mb or 850 mb before the surface front can advance
         3. Would destabilize atmosphere in warm sector (cooler air moves in over warm air)
         4. More likely to happen with "Pacific Front" than traditional arctic front
            1. Original short wave in zonal flow, so no cold air behind cold front, just air pulled over mountains and down the lee side
            2. Prefer "downsloped" or "Chinooked" air
         5. My opinion: all papers have questionable frontal analysis
            1. Locatelli et al (June 2002, WAF) uses a numerical model to show that the Palm Sunday tornado outbreak was produced by a CFA-triggered squall line
            2. Also admits that CFA is rarely seen in actual observations (I don't recall it at all!)
   6. Cold-air damming
      1. Seen to lee of Appalachians (documented elsewhere)
      2. When surface high/polar air mass retreats northeastward into New England and Canadian Maritimes, cold air flows parallel to mountains on lee side
      3. When next cyclone begins warm-air advection, cold air remains trapped in the lee of the mountains
      4. Warm air from Gulf of Mexico or Gulf Stream rides over the top of the cold air, producing cold (and often frozen) precipitation in Piedmont from Maryland to northeast Georgia, depending on set-up
      5. Cold air flow highly ageostrophic (see Bluestein, Figure 2.74, p. 360).
      6. Sometimes can produce equatorward wind that qualifies as a low-level jet streak (but cold)
      7. New frontal zone often develops parallel to Atlantic coast or onshore between dammed cold air and warmed air over Gulf Stream => coastal front
      8. Equivalent in lee of Rockies produces cold air surges, but no warm air source for coastal front
         1. Often produces Texas Norther or "blue norther" conditions
         2. Little precipitation; strong north winds
         3. Winds blow southward at right angles to isobars to the south

Last updated: February 24, 2009

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