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Unstable Waves
CONDITIONS NECESSARY FOR FRONTOLYSIS

Aerographers Mate, Module 05-Basic Meteorology
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cold  front  overtakes  the  warm  front  and  closes  the warm sector, an occlusion is formed (view E). This is the time of maximum intensity of the wave cyclone. As the occlusion continues to extend outward, the cyclonic    circulation    diminishes    in    intensity    (the low-pressure area weakens), and the frontal movement slows  down  (view  F).  Sometimes  a  new  frontal  wave may begin to form on the westward trailing portion of the cold front. In the final stage, the two fronts become a single stationary front again. The low center with its remnant  of  the  occlusion  has  disappeared  (view  G). Table  4-2  shows  the  numerical  average  life  cycle  of  a typical unstable wave cyclone from initial development to cyclolysis. It is only intended to be used as a guide in areas where reports are sparse. FRONTOGENESIS AND FRONTOLYSIS LEARNING    OBJECTIVE:    Describe    the conditions   necessary   for   frontogenesis   and frontolysis, and identify the world fronto-genetical zones. CONDITIONS NECESSARY FOR FRONTOGENESIS Frontogenesis is the formation of a new front or the regeneration  of  an  old  one.  Frontogenesis  takes  place only when two conditions are met. First, two air masses of different densities must exist adjacent to one another; and second, a prevailing wind field must exist to bring them together.   There are three basic situations, which are conducive to frontogenesis and satisfy the two basic requirements. The windflow is cross isothermal and flowing from cold   air   to   warmer   air.   The   flow   must   be   cross isothermal,  resulting  in  a  concentration  of  isotherms (increased  temperature  gradient).  The  flow  does  not have to be perpendicular; however, the more perpendicular the cross isothermal flow, the greater the intensity of frontogenesis. The winds of opposite air masses move toward the same  point  or  line  in  that  cross-isothermal  flow.  A classic example of this situation is the polar front where cold    polar    air    moves    southward    toward    warmer 4-24 Time   (hours) Central   Pressure   (mb) Direction   of   Movement (toward) Speed   of   Movement (knots) NE   to   SE   or (quad) 30-35 1,012-1,000 Wave   Cyclone 1000-988 NNE   to   N (arc) 20-25 984-968 N   to   NNW (arc) 10-15 998-1,004 0-5 Occlusion Mature   Occlusion Cyclolysis 0 12-24 24-36 36-72 The symbol   indicates   that   the   filling   center   drifts   slowly   in   a   counterclockwise   direction   along an   approximately   circular   path   about   a   fixed   point. AG5t0402 Table 4-2.—Numerical Characteristics of the Life Cycle of an Unstable Wave Cyclone T1 T2 T3 T4 T5 H L X X L H Y T6 Y T1 T2 T3 T4 T5 T6 H L Y X X L H X T1 T2 T3 T4 T5 T6 H L Y X L H A. IDEALLY FRONTOGENETIC B. CRITICALLY FRONTOGENETIC C. IDEALLY   FRONTOLYTIC AG5f0423 Y Figure 4-23.—Perpendicular deformation field.







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