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. 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”

The life of a mid-latitude (wave) cyclone

Satellite image from Chapter 1: Fig. 1.13.

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 flow at the surface is 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 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. Convergence Divergence L surface aloft H

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.

Another way of looking at it B) storm intensifies: convergence over surface H, divergence over surface L. C) storm dies out: Low over Low.

Summary of the Processes in a Cyclonic Wave