General Circulation
Patterns of wind in the atmosphere
We start in the 1700’s. People sailed across the oceans in tall
sailing ships.
I show this for my
METR 110 class when I
do general circulation.
What can you say about
this image?
Hadley’s simple idea:
Hot air rises, cold air sinks
<----These are now called
Hadley cells.
With this model,
what would winds be
like in the U.S.?
Do we observe prevailing
northeasterly winds in the U.S.?
Here are the observed prevailing wind directions. See any northeasterlies?
http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl
Our theory of the general circulation must explain the observed pattern.
How about this?
This is a more sophisticated view,
called the Three Cell model.
It still has flaws (to be
discussed next)
On a cross section, the low latitude Hadley cell and the Ferrel cell
can explain some of the observed features in the atmosphere.
But the Polar Hadley cell is weak and doesn’t fit the powerful northern
latitude weather systems. We will need a better theory.
Where is the rising arm of the low-latitude Hadley cell?
The rising part is called the Intertropical Convergence Zone or ITCZ (ITC)
The ITCZ migrates toward the summer hemisphere.
What effects will the ITCZ migration have on various parts of the world?
Notice the Lows in
the south with the
cloud cover.
That’s the ITCZ on
that day.
From 2008. You can see these feature on any day.
Subtropical Highs
ITCZ
This is a recent composite image. It looks similar to the
one from 8 years ago.
Here’s the same map
as you saw before.
This time, focus on
the Highs
Another view of the 3-cell model
The ITCZ occurs at the rising end of the Hadley cell and the
Subtropical Highs occur at the sinking end. What happens at the
poleward end of the the Ferrel cell?
Subpolar Low ------->
Subtropical
High --------->
ITCZ ----->
On the left is the ideal 3-cell
model. It’s better than
Hadley’s 1-cell idea, but not
particularly realistic.
On the right is a model based
on 3 cells, but including
continents. The subpolar Low
actually is two Lows in the
Northern Hemisphere.
Here’s a loop from February
2006. Find the Subtropical Highs
and the Subpolar Lows.
<-------------------------Siberian High
(Not a Subtropical High)
Subpolar Low (Aleutian Low)
Subpolar Low (Icelandic Low)
<---------Subtropical High
Here’s a blank map of the Southern Hemisphere. Where are the
appropriate Subpolar Lows and Subtropical Highs?
Subtropical Highs
are always over
warm oceans.
The Southern
Hemisphere is
much more like
Hadley’s ideal
world – mostly
ocean! So the
Subpolar Low is
more continuous
than in the North.
Lots of storms
surround
Antarctica during
Southern
Hemisphere
winter. The Highs
are farther north.
Here is a Polar view of the average sea-level pressure in the Northern
Hemisphere.
In January (left), the Subpolar Lows are large and strong. In July
the Subtropical Highs are large and strong. The Siberian High in
January turns into a Monsoon Low in July
What should we do about the weak part of the 3-cell model – that the
strong systems in the Polar half of each hemisphere are poorly explained?
Answer: Modify the theory.
In low latitudes, the Hadley cells dominate. This is the Hadley
regime. Our 3-cell theory works well here.
Poleward of the Subtropical Highs, transitory weather systems
dominate. This is called the Rossby regime, after C.G. Rossby.
In the Rossby regime, surface weather systems extend into the upper
troposphere. That flow is smoother than the surface.
Siberian High
Aleutian Low
Icelandic Low
Parallel to the lines, the air is moving very fast at 500 mb. This fast
flow travels all the way around the hemisphere. It is called the
Circumpolar Vortex but the media have renamed it the Polar Vortex
The Average winter
500 mb pattern
shows 3 troughs and
3 ridges. It’s a 3
wave pattern.
The positions of the
average waves are
determined by the
continental shapes
and the Himalaya
mountains.
A three-wave pattern refers to the wavelength. But waves can be high
amplitude or low amplitude.
Low amplitude is zonal, mostly west to east flow along the
latitudes (zones)
High amplitude is meridional, with strong north-south flow along the
longitudes (meridions)
Meridional flow
Zonal flow
The Circumpolar Vortex itself fluctuates. The colors indicate wind
speed. Jet Streams are embedded in the vortex. At the surface, this
is where you get storms.
Watch this
trough
Here’s a loop from early January 2014.
On this day in
Oneonta the high
temperature was 5ºF.
The low was -3ºF.
In NYC the high was
19ºF, the low was a
record 4ºF.
The 300 mb height in
the trough is less
than 8200 meters, the
lowest in the
hemisphere
Here’s the upper air Circumpolar Vortex (left) in 2006 with the
accompanying surface (right). Where are the strong surface storms?
The cold air pool over the pole spreads southward. Low-pressure waves
in the flow create surface storms which intensify around Iceland and the
Aleutian Islands.
This one is from last night. It’s a little different from the last example. Yet
some common features are also present.
With the upper air
trough and ridge
positions tied to the
topography, storms
tend to move along very
specific tracks.
The hatched areas
are cyclogenesis
regions. This is
January.
In July the tracks are
farther north and
more numerous.
Good exam question:
Explain why the July
storm tracks are
farther north and
more numerous than
the January tracks
using the concept of
the Circumpolar
vortex.
Storms are known by types which are named for their cyclogenesis
regions and track areas.
It’s called a Pandle Hooker
(sometimes called a Colorado
Hooker)
This one (Texas
Panhandler) is a sanitized
name. Some of you already
know the more common
and risque name for this
type of storm.
Clippers
Fig. 7.19 here
Hookers
(Great Basin)
Nor’easters
Gulf
Here’s a surface map loop showing a Colorado Hooker.
Watch the state of Colorado.
This one is an Alberta Clipper from January of 2014
This 2011 Nor’easter produced 12-18” of snow in NYC and Long Island
Who gets affected by the three most common types
of storms? This is class participation. You tell me.
1. Nor’ easters (do the easy one first. Name cities)
2. Colorado Hookers
3. Alberta Clippers
Here are some cities for you to pick:
They don’t get these three types of storms on the west coast. So
where do the west coast storms originate? How about in Arizona?
Do they ever get rain?
Next: Monsoons