Ch. 13 Ahrens: Mid-Latitude Cyclone

• POLAR FRONT: a semicontinuous global boundary separating cold polar air from warm subtropical air

• POLAR FRONT THEORY: published shortly after World War I, in Bergen, Norway by Vilhelm Bjerknes, his son Jacob, Halvor Solberg and Tor Bergeron.

• Figure 13.1.a: begins with stationary polar front; high pressure north and south of the pressure trough (i.e., stationary front) sets up cyclonic wind shear.

• Figure 13.1.b: a wavelike "kink" forms on the front, in response to an approaching short wave; the low that forms on the front is known as a "frontal wave" or "incipient cyclone

• Figure 13.1.c: in about 12-24 hours, a fully developed "open wave" forms; low now has several closed isobars around its center; precip develops in advance of the warm front and in a narrow band along the cold front; a "warm sector" develops between the two fronts

• Figure 13.1.d: the low "deepens" (i.e., central pressure drops); the wind circulation increases, as more and more closed isobars encircle the low center; the faster moving cold front begins to overtake (i.e., "occlude") the slower moving warm front; clouds and precip cover a large area

• Figure 13.1.e: the system reaches its most intense point and it begins to "occlude" as the cold front overtakes the warm front; the low begins to "fill" (i.e., central pressure rises) and the dissipation stage begins; clouds and precip in "comma" shape; a secondary low may develop on the "triple point" (i.e., where all three fronts meet) and go through a "life history" of its own

• Figure 13.1.f: the system dies out, as it is far removed from the "warm sector" and the supply of energy provided by the rising warm, moist air; this entire "life cycle" can last from a few days to over a week

• CYCLOGENESIS: the development or strengthening of a mid-latitude cyclone

• EXPLOSIVE or BOMB CYCLOGENESIS: defined to be a pressure drop of 24 mb in 24 hours, normalized at 60 deg N Latitude (Bergen, Norway); translates to about 19 mb in 24 hours at 40 N.

• SUPER BOMB CYCLOGENESIS: defined to be a pressure drop of 24 mb in 12 hours, normalized at 60 deg N Latitude (Bergen, Norway); translates to about 19 mb in 12 hours at 40 N; on Sep. 10, 1978, low pressure off the NJ coast (over the Gulf Stream) dropped 60 mb in 24 hours, battering the ocean liner "QE-II" and sank the fishing vessel "Captain Cosmo"

• STORM TRACKS: Gulf Low and Hatteras Low (Nor'easters); Texas Low; Colorado Low; Gulf of Alaska Low; "Alberta Clipper" (the most common storm track); Figure 13.3

• Any process that warms the air can contribute to a decrease in surface air pressure, whereas any process that cools the air can contribute to an increase in surface air pressure.

• CONVERGENCE and DIVERGENCE: refer to your SHW Ch. 7 notes

• CONFLUENCE and DIFLUENCE: refer to your SHW Ch. 7 notes

• JET STREAKS: refer to your SHW Ch. 7 notes

• Rule of Thumb: surface cyclones tend to move in direction of 500 mb flow at about half the speed of the 500 mb winds. (avg summer speed: 16 knots; avg winter speed: 27 knots)

  (Ch. 13 Ahrens: Mid-Latitude Cyclone Continued)

• PLANETARY WAVES ("Rossby Waves"): longwave troughs and ridges that encircle the hemisphere; anywhere from three (3) to six (6) longwaves at any time around the hemisphere; wavelength (trough-to-trough or ridge-to-ridge) in the thousands of kilometers; tend to be stationary or move eastward at less than 4 degrees Longitude per day (i.e., about 8 knots) or move westward (i.e., retrograde); Figure 13.6

• SHORT WAVES: smaller disturbances or ripples imbedded in the longwaves; tend to move eastward at a speed proportional to the average wind flow at 700 mb; when short wave drops into longwave trough, constructive interference causes trough to deepen; Figure 13.7a & b

• BAROTROPIC: isotherms are parallel to height contours; air density does not vary

• EQUIVALENT BAROTROPIC: when a barotropic pattern is moving (e.g., cut-off low)

• BAROCLINIC: isotherms cross height contours, creating areas of warm air advection (WAA) and cold air advection (CAA); air density does vary; Figure 13.7b

• BAROCLINIC INSTABILITY: occurs in a flow pattern when warmer air rises and colder air sinks; this sets up areas of convergence and divergence, which in turn intensifies the corresponding surface high and low pressure areas; Figure 13.8b

• The sinking of cold air and the rising of warm air provides ENERGY to the developing cyclone, as potential energy is converted to kinetic energy; furthermore, if clouds form, condensation in the ascending air releases latent heat, which warms the air; the warmer air lowers the surface pressure; the cyclone intensifies; Figure 13.11

• The overall effect of differential temperature advection is to intensify the wave.

• CAA into a trough, "deepens" the trough; WAA into a ridge, "builds" the ridge

• CONVEYOR BELT MODEL: a 3-D model of a developing wave cyclone

• Warm Conveyor Belt: the warm conveyor belt originates at the surface in the warm sector (mT air mass) and flows northward, slowly rising along the sloping warm front, gradually turning toward the northeast or east, parallel to the upper wind flow; the water vapor in the rising air condenses, forming clouds and precip; Figure 13.12

• Cold Conveyor Belt: the cold conveyor belt originates at the surface to the northeast of the surface low and north of the warm front (mP); it moves slowly westward from off the ocean; as it moves into the vicinity of the surface low, rising air gradually forces the cold conveyor belt upward, turning as it ascends, to form the comma-shaped cloud pattern; part of the airstream may rise high enough to get caught in the southwesterly flow aloft and swings northeastward, thus splitting the cold conveyor belt; Figure 13.12 and 13.13

• Dry Conveyor Belt: this upper-level airstream slowly descends from the northwest behind the surface cold front, where it brings general clearing weather; if a branch of the dry air sweeps into the storm, it produces a clear air that "sharpens" the comma-head, creating a "dry slot"; Figure 13.12

• Blizzard of 1993: known as "The Storm of the Century"; the central pressure dropped to a minimum of 960 mb (28.35"), a pressure comparable to a Category 3 hurricane, but the surface winds were more like a Category 1 hurricane in places; the storm produced 27 tornadoes (mostly in FL, where a "storm surge" was noted as the low moved ashore from off the NE-Gulf of Mexico); the storm produced deep snow from AL & GA, north to eastern Canada; every major airport along east coast was shut down; over 3 million people lost power; an estimated $800 million in damage and 270 storm-related deaths; Figures 13.13, 13.14, 13.15 & 13.16