Thickness and the thermal wind
Download
1 / 28

Thickness and the Thermal Wind - PowerPoint PPT Presentation


  • 107 Views
  • Uploaded on

Thickness and the Thermal Wind. Nick Bassill April 15 th 2009. A Quick Review …. 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”.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Thickness and the Thermal Wind' - lucas-callahan


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Thickness and the thermal wind

Thickness and the Thermal Wind

Nick Bassill

April 15th 2009




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
The Geostrophic Wind atmosphere is said to be in “geostrophic balance”

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


The new force balance
The New Force Balancewww.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


Constant pressure vs constant height maps
Constant Pressure vs. Constant Height Mapswww.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

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

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


Thickness
Thicknesswww.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.shtmlwww.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml


A conceptualization
A Conceptualizationwww.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 big picture
The Big Picturewww.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml


The pressure surfaces are close together at the polewww.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

… and further apart near the equator

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


Consider an example
Consider an Examplewww.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

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

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 (the wind blows into the slide)

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 temperature gradients in the lower atmosphere (aka, above fronts)


Strong 300 mb wind speeds temperature gradients in the lower atmosphere (aka, above fronts)


The thermal wind
The Thermal Wind temperature gradients in the lower atmosphere (aka, above fronts)

  • 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)


Thermal wind
Thermal Wind temperature gradients in the lower atmosphere (aka, above fronts)

  • 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 temperature gradients in the lower atmosphere (aka, above fronts)

1000 mb geostrophic wind

500 mb geostrophic wind

It’s pretty easy!


A useful feature
A Useful Feature temperature gradients in the lower atmosphere (aka, above fronts)

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


Thermal wind continued
Thermal Wind Continued temperature gradients in the lower atmosphere (aka, above fronts)

  • 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 …)


ad