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Thickness and the Thermal Wind

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Thickness and the Thermal Wind

Nick Bassill

April 15th 2009

From: http://physics.uwstout.edu/WX/u6/U6_05.gif

When the PGF and Coriolis force are balanced, the atmosphere is said to be in “geostrophic balance”- The resultant wind is called the “geostrophic wind”

www.eoearth.org/upload/thumb/6/6f/Geostrophic_wind_flow.gif/250px-Geostrophic_wind_flow.gif

- The geostrophic wind always blows parallel to the isobars (lines of constant pressure)
- A stronger PGF (when the isobars are closer) results in a stronger geostrophic wind

www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

From: www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Spiral_Winds/Spiral_Winds.html

- So far we’ve looked at Sea Level Pressure maps (so pressure varies while the height is constant everywhere - 0 meters)
- However, meteorologists often look at constant pressure maps (so the height changes, rather than the pressure)
- As we’ll learn more about later, you can think of “high” heights as being analogous to high pressures, and “low” heights as being analogous to low pressures
- Similarly, the geostrophic wind will blow parallel to lines of constant height, with lower heights to the left of the direction of the wind

Heights and winds at 200 mb

Notice how much closer the winds are to geostrophic balance at this level, compared with the surface

www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

- Recall that warm air is less dense than cold air
- Therefore, a certain mass of warm air will take up more space than the same mass of cold air
- Atmospheric thickness is simply a measure of the vertical distance between two different pressure levels
- Based on the above, large thickness values correspond to a higher average air temperature than small thickness values

www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

The Horizontal surfaces are “heights” above sea level

The wavy surface is the 500 mb level

Cold Warm

The pressure surfaces are close together at the pole

… and further apart near the equator

This means that along a horizontal surface, a pressure gradient exists

So where would you expect lower thicknesses?

Or, to ask it another way, where would you expect to find lower pressures along a line of constant height?

COLD AIR

WARM AIR

1000 mb, 0 meters

Low Thicknesses High Thicknesses

500 mb

600 mb

Constant Height

500 mb

600 mb

COLD AIR

WARM AIR

1000 mb, 0 meters

This is a region of a strong horizontal pressure gradient

500 mb

600 mb

Constant Height

500 mb

600 mb

COLD AIR

WARM AIR

1000 mb, 0 meters

Therefore, we would expect a strong geostrophic wind here (the wind blows into the slide)

500 mb

600 mb

Constant Height

500 mb

600 mb

COLD AIR

WARM AIR

1000 mb, 0 meters

500 mb

600 mb

Jet Stream

Constant Height

500 mb

600 mb

This is a region of strong temperature contrast

COLD AIR

WARM AIR

1000 mb, 0 meters

Upper Jet Streams are frequently found above areas of strong temperature gradients in the lower atmosphere (aka, above fronts)

500 mb

600 mb

Jet Stream

Constant Height

500 mb

600 mb

A FRONT is present here!

COLD AIR

WARM AIR

1000 mb, 0 meters

Strong 850 mb temperature gradients

Strong 300 mb wind speeds

- Based on what we’ve learned, we can say that the change in strength of the geostrophic wind with height is directly proportional to the horizontal temperature gradient
- This relationship is known as the Thermal Wind
- The direction and strength of the thermal wind tells us about the temperature structure of the atmosphere
- A strong thermal wind means a stronger temperature gradient in the atmosphere (and therefore there is a strong geostrophic wind shear with height)

- It is easy to calculate, if you know the geostrophic wind at different levels
- Say we’re trying to calculate the thermal wind for the 1000-500 mb layer:
- Simply subtract the upper geostrophic wind (500 mb) vector from the lower geostrophic wind vector (1000 mb)

Thermal Wind

1000 mb geostrophic wind

500 mb geostrophic wind

It’s pretty easy!

- The Thermal Wind always blows with cold thickness to the left (and blows parallel to the constant lines of thickness)

- The thermal wind isn’t an actual, observable wind
- However, it does tell us useful things about the atmosphere, such as
- the strength of the temperature gradient in a layer

- and therefore the strngth of the geostrophic wind shear

- and more! (for later …)