Air Masses and FrontsOr “bombs”Accumulation at Cotton’s at 7300’ (my house) was 54” snow (137 cm) and145 Part 5. Extratropical Cyclones Extratropical Synoptic Scale Disturbances Cyclone – rotates in the same sense as the earth’s rotation (counterclockwise in Northern Hemisphere, clockwise in Southern Hemisphere). Extratropical Cyclones are poleward of 25° (Tropics). Air Masses and Fronts In the classical Norwegian concept, cyclones are born along a front between warm and cold air masses. Normally, the temperature and humidity of the air near the surface change only slowly over great distances of 1,000 miles (1,600 km) or more, leading to the concept of broad, relatively uniform air masses. Temperature and moisture are their main distinguishing features. Basically, we differentiate between warm (topical) and cold (polar) air masses and between humid (maritime) and dry (continental) air masses. Combining these, the four principal air masses according to geographic origin are: Continental polar Maritime polar Continental tropical Maritime tropical Classification of air masses has been carried much farther than presented in this simple scheme, but with little advantage in most circumstances. Frequently two air masses, especially tropical and polar, develop a sharp boundary or interface, where the temperature difference between them becomes concentrated. Such boundaries were named fronts by the Norwegians.146 Extratropical cyclones form by baroclinic instability – where north/south temperature gradient becomes so strong that an infintitely small perturbation amplifies into a synoptic-scale wave disturbance. • Assume that this perturbation exists in a uniform zonal flow u. • Assume wavelike perturbation moves at speed c=u. • Assume N/S velocity associated with perturbations will distort T patterns.147 Northerly flow advects colder air southward. Southerly flow advects warmer air northward.148 Structure of baroclinic waves • If uc>, the wave in the isotherm pattern will be advected toward the east. • If uc<, the wave in the isotherm pattern will lie to the west of its position for .uc! In lower troposphere uc< (usually). • In the upper troposphere uc> (usually). • In the middle troposphere uc! (usually).149 Vertical structure of developing baroclinic wave is:150 Jet Streams and Jet Streaks Where do extratropical cyclones form? • Near jet streams. • Particularly near jet streaks. Jet Streak – a region within jet stream where the wind speeds are a local maximum.151 Jet streaks are a focusing mechanism for cyclogenesis. ~200 mb152153 Lifecycle – Norwegian Cyclone Model154155156 Precip. Clouds Temp. (1) Continuous Low stratus overcast, fog Cool highs (2) Very light showers Broken shallow cu, intermittent blue skies Cool highs (3) None Fair wx. Cu. > 75% sunshine Warm highs, cold lows (4) Spotty conv. showers, pre-frontal squall lines Hazy sunshine, thin overcast Hot (5) Cbs. in summer and winter Cb’s dark sky Hot turning cool after Fropa157158 Occluded Fronts159 • The common notion is that such fronts form when the cold front catches up with part of the warm front during cyclogenesis. • However there are few examples of cold fronts overtaking warm fronts to form occlusions. • It appears most occluded fronts are new fronts which form as surface lows separate from the junctions of their respective warm and cold fronts and deepen further back into cold air.160161162 Figure X.20 provides another illustration of extratopical cyclone development including regions of thickness advection and vorticity advection.163 The conditions associated with the development of extratropical storms is as follows: • Favorable conditions: o The existence of a thickness gradient in the lower troposphere (i.e, a front); particulary when it is anticyclonically curved; o The presence of an upper-level trough with cold advection to its rear and warm advection ahead; and o Release of latent heat near the center of the surface low by deep cumulonimbus and stratiform precipitation. • Unfavorable conditions: o A weakening thickness gradient as a result of low-level divergent flow; and o The absence of an upper-level trough or a trough with cold advection ahead ot it, and warm advection behind resulting in a trough which will decrease in intensity with time. Extratropical cyclones are different from hurricanes and tropical storms because their energy is primarily from the juxtaposition of cold and warm air masses (i.e., a horizontal thickness gradient). Tropical cyclones, in contrast, derive their energy though heating around the central core as a result of deep164 cumulonimbus. In addition, the wind field of extratropical cyclones, although spread over a large area, has weaker maximum speeds since the pressure gradient is not as strong as found in mature, well-developed hurricanes. Oceanic extratropical cyclones are less of a danger to shipping than hurricanes because the seas are not as chaotic since the wind direction does not vary through 360° around a small center as it does for the tropical storm.165 Symmetric and Conditional Symmetric Instability (CSI) Consider the more complicated problem of a parcel of air in the region of a cold front or what we called a baroclinic zone. We noted previously that wherever there is a strong, horizontal, temperature gradient, the winds increase in speed with height. Thus, a parcel in geostrophic equilibrium will experience an equilibrium angular momentum that also increases with height. Figure 5.1 illustrates a west-to-east cross section through a Figure 5.1. Schematic of a west-east, vertical cross-section illustrating symmetric instability. Solid lines represent absolute angular momentum, M, of the undisturbed flow. Dashed lines represent the air density of the undisturbed flow. Movement of parcels from positions A, B, and C are indicated by the small arrows, while the accelerations of the parcels are indicated by the double arrows. The acceleration is decomposed into a vertical component due to gravity (G) and a horizontal component due to imbalances of absolute angular momentum (M′). (Adapted from Sanders, F. and L.F. Bosart, 1985: Mesoscale structure in the megalopolitan snowstorm of 11-12 February 1983. Part I: Frontogenetical forcing and symmetric instability. J. Atmos. Sci., 42, 1050-1061.) baroclinic zone. The solid lines M1, M2, M3 represent surfaces