Thickness and the Thermal Wind

1. Thickness and the Thermal Wind Nick Bassill April 15th 2009

2. A Quick Review …

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

4. 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

5. The Geostrophic Wind • 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

6. 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

7. 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

8. The New Force Balance From: www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Spiral_Winds/Spiral_Winds.html

9. Constant Pressure vs. Constant Height Maps • 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

10. 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

11. Thickness • 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

12. 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

13. A Conceptualization The Horizontal surfaces are “heights” above sea level The wavy surface is the 500 mb level Cold Warm

14. The Big Picture

15. 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

16. Consider an Example 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

17. Low Thicknesses High Thicknesses 500 mb 600 mb Constant Height 500 mb 600 mb COLD AIR WARM AIR 1000 mb, 0 meters

18. 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

19. 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

20. 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

21. 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

22. Strong 850 mb temperature gradients

23. Strong 300 mb wind speeds

24. The Thermal Wind • 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)

25. Thermal Wind • 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)

26. Thermal Wind 1000 mb geostrophic wind 500 mb geostrophic wind It’s pretty easy!

27. A Useful Feature • The Thermal Wind always blows with cold thickness to the left (and blows parallel to the constant lines of thickness)

28. Thermal Wind Continued • 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 …)