Background on Wave Cyclones and Dynamic Lows

1. Background Information and Review from Metr 201/400

a. What is a wave cyclone?

A "wave cyclone" (aka "frontal cyclone") is a cyclone that forms and moves along a front. The circulation about the cyclone center tends to produce a wavelike deformation of the front which is observable in the evolution of the cloud pattern. The wave cyclone is the most frequent form of extratropical cyclone (or low).

b. How does nature create surface cyclones?

Please use structured thinking to organize or reorganize your thoughts on 'wave cyclones'. From Metr 201/301, or even Metr 302/356 remember that reason for the formation of any surface "cyclone" (a region of low pressure or heights totally encircled by at least one isobar) is related to net divergence out of the air column above the cyclone.

In concept, the troposphere can be divided into thirds: (a) an upper third in which significant horizontal divergence and convergence occurs (and in which flow is basically horizontal and wind speeds tend to be the greatest); (b) a middle troposphere in which the flow is nearly non-divergent (and in which synoptic scale vertical motions are maximum and the geostrophic approximation works best); and, (c) a lower third in which there is significant horizontal divergence and convergence, but of a lesser magnitude, generally, than in the upper troposphere (because of surface friction, the shape of the isobars, and the speed of the wind...to be explained in Metr 403) and in which the wind speeds tend to be the weakest.

c. Dynamical explanation for net divergence and the formation of surface cyclones

The discussion in the previous sectioni merely relates to the kinematics of the flow, one property of which is divergence. In Metr 201/301/302/356 we have also summarized the dynamics that explain why divergence occurs. There are two ways in which divergence aloft can be produced at the synoptic and macroscales.

One way is related to differential heating and cooling effects (resulting in net divergence above the earth's surface) producing a class of cyclones and anticyclones termed "thermal lows and highs". The other is related to aspects of the flow related to the presence of jet streams. The net divergence associated with the latter results in a class of surface lows and highs often referred to as "dynamic lows and highs" by opertional meteorologists.

 
For those students who have taken Metr 201/400 and 698/798, you were taught that the dynamical underpinning for both classes of pressure systems is found in the quasi-geostrophic height tendency and omega equations. See this link to see the conceptual equivalent of these two equations.

 

d. The polar front and polar jet stream

Wave cyclones are a type of dynamic low pressure area. To understand these cyclones, recall that the polar front surrounds the globe. The surface expression of this boundary is the wave like frontal boundary which encircles the globe in discontinuous fashion.

The expression of the polar front aloft is the polar jet stream (please remember that there is one polar jet stream when one averages over a long period of time....but that on any one day, there may be multiple branches of the jet). Thus, there is a relationship between the location of the polar jet and the surface polar front and one can always estimate (or "guesstimate") the position of the surface polar front by placing it on the warm air side of the jet, as we did in Metr 201/301.

Of course, you're learning in Metr 402 that a true measure of the mean temperature of the atmosphere (between two isobaric levels) is not the, say, 500 mb height, but the 1000-500 mb thickness (related to the mean virtual temperature of the layer between 1000 mb and 500 mb). Thus, one should really make a first guess approximation of the location of the surface by drawing it on the south side of the packing of the 1000-500 mb thickness contours (which are really tantamount to isotherms).

The relation of surface winds to thickness contours allows one to assess temperature advection, and to make a better estimate of both the location and type of surface fronts (Note: In reality, finding surface fronts requires a careful analysis of actual temperature fields, instead of the layer mean temperatures inferred from thickness or height maps. Here is the NCEP analysis; note that the general position of and type of fronts for the eastern two thirds of the US was well "guessed", but the complications associated with the actual wind and temperatures in southwest TX made our "first guess" poor there). You can check this link to review the procedure we learned in Metr 301 for finding and analyzing surface fronts.

e. Formation of wave cyclones

We also learned that the polar jet stream is not, generally, in a perfectly zonal configuration on any given day (though it may average out to be zonal in mean seasonal patterns). Observations show that the flow patterns in the jet stream are sinusoidal. This very geometry produces divergence and convergence patterns, as depicted in this schematic chart and will be discussed at length in Metr 403/503,

The interesting thing about the divergence patterns associated with the polar jet is that since the polar jet is intimately connected with the surface polar front, the divergence patterns associated with the jet, should stimulate the formation of surface lows on or nearby the polar front. The cyclonic circulation around the low will then circulate cold air masses south on the west side of the low and warm air masses north on the east side of the low. This is the birth of the wave cyclone.

Partially because of kinematics, and also dynamics, the wave cyclone typically evolves in shape, intensity and size from birth to death over a 4 to 7 day period, and, again, the details of this will have to wait until Metr 403/503.

f. Formation of clouds in association with wave cyclones

Since the basic reason for cloud formation is cooling, substantial cloud formation occurs in association with the development of wave cyclones, if air masses drawn into their circulation have substantial amounts of water vapor. The reason for this, of course, is that the vertical motion associated with the general synoptic divergence will be associated with mid-tropospheric lift or lofting, and, also, warm air will be lofted along the fronts.

g. Satellite analysis techniques in relation to previous discussion

Obviously, then, the cloud patterns that result from the factors considered in (a), (b), (c), (d), (e) and (f) will allow us to "go backwards" from a satellite view of a wave