Transcript Chapter 9

Chapter 6
Pressure, and the forces that explain the wind
pressure 
weight
area
thus
gravity* m ass
area
m ass
m ass
density 

volum e area * depth
or
pressure 
z
area
m ass
 density* depth
area
thus
pressure  gravity* density* depth
 p   g  z
or
1 p

g
 z
Hydrostatic balance:
The upward pressure gradient force is
equal and opposite to the gravity
p
kg
m
mb
  g  1.2 3 10 2  120
z
m
s
km
aneroid
barometer
aneroid barograph
mercury
barometer
What is the typical SLP?
How much does it vary ?
780
Average air pressure in Laramie
We need to reduce
station pressure
to a standard
height, for
instance sea level
Why?
Because winds are
driven by horizontal
pressure
differences
Isobars and pressure patterns
Where are you more likely to find a pressure value of
994 mb? At A or B ?
Becoming acquainted with contouring and frontal analysis
•
•
http://cimss.ssec.wisc.edu/wxwise/contour/index.html
http://cimss.ssec.wisc.edu/wxwise/fronts/fronts.html
Defining patterns on a surface weather chart
•
•
•
lows and highs
trofs and ridges
saddle
trof
examine the current weather analysis
What drives the wind?
Pressure gradient force (PGF) and wind
1 p
PGF  
 x
here, x=100 km and p=4 mb
The PGF is directed from high to low pressure, and is
stronger when the isobars are more tightly packed
in reality, winds do not blow from high to low, at least not along the shortest path
… so there must be other force(s)
Coriolis force
CF  fv
f : Coriolis _ param eter
v : wind _ speed
Geostrophic wind balance: a balance between the PGF and the Coriolis force
link
1 p
fv 
 x
or
1 p
v
f x
L
Buys-Ballot law
•
•
When you face downwind, the low will be on your left
Vice versa in the southern hemisphere
you (seen from above)
The geostrophic wind blows along the isobars (height contours), counterclockwise around lows
(in the NH), and at a speed inversely proportional to the spacing between the isobars (height
contours)
1 p
v
f  x
in the southern
hemisphere, the low is
on your right when you
look downwind
L
There is a third force, important only near the ground
Friction slows the wind
1008
1004
1000
Interplay between 3 forces
• Pressure gradient force
• Coriolis force
• Friction (near the ground)
Guldberg-Mohn balance
Check out how they affect the wind!
Trajectories
spiral out of a
high,
and into a low
 ~ 10° over oceans
 ~ 30° over land
 > 30° near mountainous terrain
finally, a fourth force: centrifugal force
CFF
PGF
Coriolis
slower-than-geostrophic wind
(subgeostrophic)
PGF
Coriolis
faster-than-geostrophic wind
(supergeostrophic)
CFF
The jet stream wind is subgeostrophic in trofs, and supergeostrophic in ridges
slow
fast
fast
slow
Where does the air, spiraling into a low, end up?
height
rising motion leads to
cloudiness and precipitation
subsidence leads
to clear skies
Fig. 10.11
300 mb height, 9 Nov 1975, 7 pm
Find the trofs
Fig. 10.13
fast
slow
upper-level divergence,
low-level convergence
surface low
300 mb height, 9 Nov 1975, 7 pm
Fig. 10.13
Today’s surface weather analysis
http://www.rap.ucar.edu/weather/surface/sfc_den.gif
http://weather.uwyo.edu/surface/front.html
Today’s upper-air maps
http://weather.uwyo.edu/upperair/uamap.html
Upper-level winds,
and upper-level charts
Upper level charts are NOT plotted at constant height, eg 18,000 ft. Rather, they
display the topography of a pressure surface, eg 500 mb
Approximate conversion of
pressure level to altitude
Pressure
Approximate Height
Approximate
Temperature*
1013 mb
0 m (sea
level)
0 ft
15 °C
59 °F
1000 mb
100 m
300 ft
15 °C
59 °F
850 mb
1500 m
5000 ft
5C
41 F
700 mb
3000 m
10000 ft
-5 C
23 F
500 mb
5000 m
18000 ft
-20 C
-4 F
300 mb
9000 m
30000 ft
-45 C
-49 F
200 mb
12000 m
40000 ft
-55 C
-67 F
100 mb
16000 m
53000 ft
-56 C
-69 F
1000 mb – near sea level
850 mb - ~5,000 ft
700 mb - ~10,000 ft
500 mb - ~18,000 ft
300 mb - ~30,000 ft
200 mb ~ 40,000 ft
pressure at a fixed height (sea level)
elevation of the 1000 mb surface
contours: sea-level pressure
color fill: 1000 mb height
Why do isobar and height contour charts look (almost) the same?
1560 m
high
low 1500 m
New York
Boston
height
sea level
Pressure decreases with height at about 10 mb every 100 m
Locate the trofs
Thickness and temperature
thickness
between 2 material
surfaces
(1000- 500 mb)
temperature
L
Pop quiz: why is their a ‘pit’ in the 500 mb
surface over Antarctica?
- because it is much colder there than over Australia and other
surrounding places
- because of the ozone hole
- because there is less sunshine
- I give up
calm
calm
L
Jet stream
is due to the
cold pool
below
(circumpolar
vortex)
calm
calm
Jet stream
•
•
why does it exist?
why does it vary in strength?
The jet stream is the result of a horizontal temperature gradient
… and thus a thickness gradient
thickness = 20.3 * Tmean
thickness is in meters
between 1000 and 500 mb
Tmean is the layer-mean
temperature in Kelvin
5000
100
5200
5400
5600
150
1000 mb height (m)
near the ground: weak PGF, weak wind
5800
500 mb height (m)
near 18,000 ft: strong PGF, strong wind
• Where is the 1000-500 mb thickness lower? Where is it higher?
• Where is the colder airmass – where is the warmer one?
5000
B
100
B
5100
5200
5400
5600
150
5800
A
A
1000 mb height (m)
500 mb height (m)
Calculate thickness at A and B
• at A: Z500-Z1000 = 5850-150 = 5700 m
• at B: Z500-Z1000 = 5100-100 = 5000 m
… answer: the lower atmosphere
is less thick at B up north
indeed, it is colder where
the air is less thick
B
A
700 mb mean temperature (C)
Relation between wind and temperature ...
Key : colder air is less thick, therefore upper
level winds will blow cyclonically around cold pools
For instance, look at the pole-to-pole variation of temperature with height (in Jan)
Around 30-45 N, temperature drops northward, therefore westerly winds increase in strength
with height
The N-S temperature gradient is
large between 30-50N and 1000300mb
Therefore the westerly wind increases
rapidly from 1000 mb up to 300 mb
J
cold
warm
cold
J
‘thermal wind’
The increase of wind with height parallel to
the isotherms, cyclonically around cold pools
Illustration : compare the 300 mb height over the northern
hemisphere ...
… to the temperature
700 mb
Now explain why a jet stream is found above a frontal zone
wind speed
(kts)
The jet stream is there because of low-level temperature differences
polar
front jet
(PFJ)
Pop quiz: why is the jet stream stronger in winter?
•
because the north-south temperature gradient is larger
•
because cold air is lighter and can be blown around easier
•
because there is less sunshine
•
because there are fewer thunderstorms that act as obstacles to
the upper-level flow.
Pop quiz: why is the jet stream stronger in winter?
because the north-south temperature gradient is larger
because cold air is lighter and can be blown around easier
because there is less sunshine
because there are fewer thunderstorms that act as obstacles to
the upper-level flow.
Change the equator-to-pole temperature gradient,
and see what happens to the jet stream!
Pop quiz: according to climate change models and observations, the
arctic is warming up faster than low latitude regions. What does this
imply about the strength of the jet stream and the intensity of
storms spawned by the jet stream?
• they weaken
• they strengthen
• it can go either way
• I give up
Summary
•
There are four key forces driving the wind:
–
–
–
–
•
pressure gradient force (to start the motion)
Coriolis force
friction (only near the ground)
centrifugal force
As a result the wind blows counterclockwise around lows (in the NH)
– friction makes the low-level wind spiral into lows
– the centrifugal force slows the wind in trofs, and speeds it up in ridges
•
Weather changes (as we know it) is the result of passing jet streams,
with
– rising motion & clouds ahead of a trof, with a low at the surface
– sinking motion & clear skies upstream of a trof, with a high at the surface
– the deep vertical motion is due to changes in wind speed in the jet, as the wind
in trofs (ridges) is slower (faster) than expected from geostrophic balance
•
The jet stream tends to occur above regions with a large temperature
difference
– The jet blows counterclockwise around cold pools (in the NH)
Let’s cover chapter 7 (global winds) and
skip chapter 8 (air-sea interaction)
then we ‘ll do chapter 9 (air masses and fronts) and
chapter 10 (mid-latitude weather)