Background on Cyclones
Last week we looked at the surface strucutre of a cyclone. We also decided that somehing must be going on above the surface that is helping in the cyclone development. The flow at upper levels is in geostrophic balance, so there is no friction forcing convergence or divergence. Lets look at a typical flow pattern at 500mb:
There are ridges and troughs in the wavy geopotential height lines. The ridges and troughs are very important in analyzing weather systems. The wavy pattern is associated with convergence and divergence of flow.
So why is this important? If we have convergence of the flow at lower levels if forces vertial motion. Then we have the upward moving air piling up in the column above the low, once it reaches the tropopause it is forced out of the column and diverges.
Now we can locate the surface cyclones just by looking at an upper level map. If this were a 500mb map, we could say that the High and low are located in the locations below:
Storm Speed and Direction:
The storm speed and direction can also be identified on the 500 mb map. Cyclones move in the direction of the 500 mb flow, the 500 mb flow is also called the steering flow. The cyclone also moves at about half the speed of the 500 mb flow.
What else do these upper level maps tell us? Well if we know that there must be divergence directly above a surface cyclone, we know something else about the cyclone. The center of low pressure must tilt northwestward with height.
The next question is, how do we know if the cyclone will intensify or not?
Lets look at the structure of the 500 mb flow: Example
There is a wavy pattern embedded in the flow. There are two types of waves:
- If the divergence at upper levels is greater than the convergence at the surface, the cyclone will intensify.
- If the divergence at upper levels is less than the convergence at the surface, the cyclone will weaken.
They are a fundamental part of a fluid on a rotating sphere. There are typically 4-6 of these long waves all the way around the planet. They are also called Rossby Waves
These are found within the long wave pattern. They move quickly east within the longwaves. They weaken when they hit a longwave ridge and strengthen when they hit a longwave trough. These are easily seen on 500 mb charts.
Temperature Advection in Shortwaves
Consider a long wave trough situated over a stationary front, seen in (a). The height lines and the isotherms are parallel to eachother, we can say the atmosphere is barotropic.
At time (b) a shortwave trough move into the longwave trough and intensifies. The shortwave caused the isotherms to cross the height lines, we can say the atmosphere is baroclinic
West of the height trough, there is a region of Cold Air Advection (CAA). In this region the cold air is more dense and will cause sinking motions.
East of the height trough, there is a region of Warm Air Advection (WAA). In this region the warm air will produce rising motions.
Jet Streaks and Shortwaves
If a shortwave trough is intensified in a longwave trough, the height lines are forced together so here will be a large PGF in the base of a shortwave trough. We have seen before that large PGF corresponds to large wind speeds.
The figure at the right shows us that the largest wind speeds will be where the height lines are the closest together on an upper level map. The wind speed decreases outward from this point. Therefore we have a convergence of wind to the left/west of a trough and the divergence of wind to the east/right of a trough.
If we look at the full vertical structure we will see
that the divergence and convergence associated with a jet streak are directly above the low and high pressures at the surface.
Vorticity and Shortwaves
What is vorticity? Vorticity is the measure of a parcel's spin.
What does this tell us about the life of a cyclone? Well we can look for locations of maximum and minimum vorticity on upper level maps. Lets look at a 500 mb map:
- If a parcel has a counterclockwise spin, it has a positive vorticity. (left)
- If a parcel has a clockwise spin, it has a negative vorticity. (right)
We are interested in finding the regions of vorticity advection
Lets look at how this plays out in the total vertical structure: Vorticity advection and vertical motion
- Upward motion and divergence at upper levels are associated with the region of maximum vorticity advection.
- Downward motion and convergence at upper levels are associated with the region of minimum vorticity advection.
Now we can look at the full picture: