Transcript Atmospheric Measurements

Thickness and the
Thermal Wind
Nick Bassill
April 15th 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”
www.eoearth.org/upload/thumb/6/6f/Geostrophic_wind_flow.gif/250px-Geostrophic_wind_flow.gif
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
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
The New Force Balance
From:
www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Ene
rgetics/Spiral_Winds/Spiral_Winds.html
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
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
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
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
A Conceptualization
The Horizontal
surfaces are “heights”
above sea level
The wavy surface is
the 500 mb level
Cold
Warm
The Big Picture
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
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
1000 mb,
0 meters
WARM AIR
Low Thicknesses
High Thicknesses
500 mb
600 mb
Constant
Height
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
WARM AIR
This is a region of a
strong horizontal
pressure gradient
500 mb
600 mb
Constant
Height
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
WARM AIR
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
1000 mb,
0 meters
WARM AIR
500 mb
Jet
Stream
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
This is a region of
strong temperature
contrast
600 mb
Constant
Height
WARM AIR
Upper Jet Streams are frequently found above areas of strong
temperature gradients in the lower atmosphere (aka, above
fronts)
500 mb
Jet
Stream
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
A FRONT
is present
here!
600 mb
Constant
Height
WARM AIR
Strong 850 mb
temperature
gradients
Strong 300 mb
wind speeds
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)
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)
Thermal Wind
1000 mb
geostrophic
wind
500 mb geostrophic
wind
It’s pretty easy!
A Useful Feature
• The Thermal Wind always blows with cold
thickness to the left (and blows parallel to
the constant lines of thickness)
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 …)