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Middle-Latitude Cyclones - II. Review and Outline. The polar front model (Norwegian model) of a developing mid-latitude cyclonic storm represents a simplified but useful model of how an ideal storm progresses through the stages of birth, maturity and dissipation.
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Review and Outline • The polar front model (Norwegian model) of a developing mid-latitude cyclonic storm represents a simplified but useful model of how an ideal storm progresses through the stages of birth, maturity and dissipation. • Cyclogenesis, lee-side lows, northeasters, bombs. • For a surface mid-latitude cyclonic storm to form, there must be an area of upper-level divergence above the surface low. For the surface storm to intensify, this region of upper level divergence must be greater than surface convergence. • When the polar-front jet stream develops into a looping wave, it provides an area of upper-level divergence for the development of surface mid-latitude cyclonic storms. • The curving nature of the polar-front jet stream tends to direct surface mid-latitude cyclonic storms northeastward and surface anticyclones southeastward. • Skip the sections on “Vorticity, Divergence, and Developing Mid-Latitude Cyclones” and “Polar Lows”
What happens with the high and low pressure centers over time? • The development of a cyclonic wave is determined by the motion of the air aloft: • the air flow aloft is parallel to the isobars • the air flow aloft is neither into L nor away from H. • Air flowat the surfaceis across the isobars. • air convergence near low pressure centers • air divergence near high pressure centers Where does it go? Where does it come from?
Vertical structure of deep dynamic lows • The air cannot disappear or come out of nowhere: the surface flow is related to the vertical flow and consequently to the air flow aloft. • The surface winds are coupled to the winds aloft • What type of upper-level air flow would favor the formation and development of a surface wave cyclone?
Favorable conditions for a strong midlatitude storm aloft Divergence Convergence L H Divergence Convergence surface • Air convergence at the surface is aligned with air divergence aloft. • Air divergence at the surface is aligned with area of convergence aloft • Surface L intensifies (weakens) if the divergence aloft is stronger (weaker) than the surface convergence. • Surface H intensifies (weakens) if the convergence aloft is stronger (weaker) than the surface divergence.
But … • We know that at upper levels the winds are blowing along the isobars. • The air aloft is NOT converging into nor it is diverging away from pressure centers (L or H). • What is the right pattern of the upper level winds that will result into a strong storm?
Is it this one? • Low over Low, High over High. • No convergence or divergence aloft. • The surface Low will fill up and disappear • The surface High will be depleted and disappear • Such a configuration does NOT support the storm. • Similarly, Low over High, High over Low won’t work. • Surface and upper-level pressure centers cannot be aligned above each other.
Is it this one? • Perfect geostrophic flow – parallel isobars, constant wind speed • Again, no divergence or convergence (i.e. piling up or removal of air). • This configuration will NOT support a strong storm at the surface • Conclusion: the isobars must curve, creating regions where they spread apart and/or get closer together.
500 mbar pressure map • The air converges as it flows toward the L trough. • The air diverges as it flows away from the L trough. Convergence Divergence
Upper Level Waves • Long waves (Planetary, Rossby waves) • Typically the longwaves have 3-6 wavelengths around the Earth. • The result from the uneven heating of the Earth and the rotation of the planet. • Long waves move very slowly or are stationary. • Often created by mountain ranges • Shorter waves • Imbedded in the long waves • Short waves travel faster • They can intensify the troughs of the long waves.
The Vertical Structure of a Middle-latitude Storm • The surface L is aligned with the diverging part of the jet stream flow. • The surface H is aligned with the converging part of the jet stream flow. • H and L centers are not aligned! Upper-level centers are generally shifted to the west
Another way of looking at it • B) storm intensifies: convergence over surface H, divergence over surface L. • C) storm dies out: Low over Low.