Extratropical cyclones Cyclogenesis

1 extratropical cyclones

1.1 norwegian cyclone model
1.2 precursors development
1.3 vertical motion affecting development
1.4 modes of development
1.5 maturity

extratropical cyclones


the initial frontal wave (or low pressure area) forms @ location of red dot on image. perpendicular (at right angle) leaf-like cloud formation (baroclinic leaf) seen on satellite during stage of cyclogenesis. location of axis of upper level jet stream in light blue.


norwegian cyclone model

an upper level jet streak. div areas regions of divergence aloft, lead surface convergence , aid cyclogenesis.

the norwegian cyclone model idealized formation model of cold-core cyclonic storms developed norwegian meteorologists during first world war. main concept behind model, relating cyclogenesis, cyclones progress through predictable evolution move frontal boundary, mature cyclone near northeast end of front , least mature near tail end of front.

precursors development

a preexisting frontal boundary, defined in surface weather analysis, required development of mid-latitude cyclone. cyclonic flow begins around disturbed section of stationary front due upper level disturbance, such short wave or upper-level trough, near favorable quadrant of upper level jet. however, enhanced along-frontal stretching rates in lower troposphere can suppress growth of extratropical cyclones.

vertical motion affecting development

cyclogenesis can occur when temperature decreases polewards (to north, in northern hemisphere), , pressure perturbation lines tilt westward height. cyclogenesis occur in regions of cyclonic vorticity advection, downstream of strong westerly jet. combination of vorticity advection , thermal advection created temperature gradient , low pressure center cause upward motion around low. if temperature gradient strong enough, temperature advection increase, driving more vertical motion. increases overall strength of system. shearwise updrafts important factor in determining cyclonic growth , strength.

modes of development

the surface low have variety of causes forming. topography can force surface low when dense low-level high pressure system ridges in east of north-south mountain barrier. mesoscale convective systems can spawn surface lows warm core. disturbance can grow wave-like formation along front , low positioned @ crest. around low, flow become cyclonic, definition. rotational flow push polar air equatorward west of low via trailing cold front, , warmer air push poleward low via warm front. cold front move @ quicker pace warm front , “catch up” due slow erosion of higher density airmass located out ahead of cyclone , higher density airmass sweeping in behind cyclone, resulting in narrowing warm sector. @ point occluded front forms warm air mass pushed upwards trough of warm air aloft, known trowal (a trough of warm air aloft). developing low pressure areas share 1 important aspect, of upward vertical motion within troposphere. such upward motions decrease mass of local atmospheric columns of air, lower surface pressure.

maturity

maturity after time of occlusion when storm has completed strengthening , cyclonic flow @ intense. thereafter, strength of storm diminishes cyclone couples upper level trough or upper level low, becoming increasingly cold core. spin-down of cyclones, known cyclolysis, can understood energetics perspective. occlusion occurs , warm air mass pushed upwards on cold air airmass, atmosphere becomes increasingly stable , centre of gravity of system lowers. occlusion process extends further down warm front , away central low, more , more of available potential energy of system exhausted. potential energy sink creates kinetic energy source injects final burst of energy storm s motions. after process occurs, growth period of cyclone, or cyclogenesis, ends, , low begins spin down (fill) more air converging bottom of cyclone being removed out top since upper-level divergence has decreased.

occasionally, cyclogenesis re-occur occluded cyclones. when happens new low center form on triple-point (the point cold front, warm front, , occluded front meet). during triple-point cyclogenesis, occluded parent low fill secondary low deepens main weathermaker.

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