General Circulation Patterns of wind in the atmosphere

We start in the 1700’s. People sailed across the oceans in tall sailing ships. They had a very good idea of the prevailing winds. It was their business. A fellow named Hadley had a theory of why the winds blew the way they did.

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? Our theory of the general circulation must explain the observed pattern.

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.

ITCZ From You can see these feature on any day. Subtropical Highs

This is a recent composite image. It looks the same as 8 years ago Replace with current cmoll.

Here’s the same map as you saw before. This time, focus on the Highs

Another view of the 3-cell model ITCZ -----> Subtropical High > 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 >

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 Find the Subtropical Highs and the Subpolar Lows. < Subtropical High Subpolar Low (Icelandic Low) < Siberian High (Not a Subtropical High) Subpolar Low (Aleutian Low)

Here’s a blank map of the Southern Hemisphere. Where are the appropriate Subpolar Lows and Subtropical Highs? The Southern Hemisphere is much more like Hadley’s ideal world – mostly ocean! So the Subpolar Low is more continuous than in the North. Subtropical Highs are always over warm oceans.

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. Icelandic Low Aleutian Low Siberian High

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 flowZonal 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.

Here’s a loop from early January Watch this trough

On this day in Oneonta the high temperature was 5 º F. The low was -3 º F. In NYC the high was 19 º F, the low was a record 4 º F. 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 (replace). 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. This one (Texas Panhandler) is a sanitized name. Some of you already know the more common and risque name for this type of storm. It’s called a Pandle Hooker (sometimes called a Colorado Hooker)

Fig here Clippers Hookers Nor’easters (Great Basin) 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