Tue., Nov. 13 notes

Tuesday Nov. 13, 2007

The revised Experiment #2 reports have been graded and can be picked up in class.

Optional Assignment #6 was collected in class today.

The Experiment #4 reports are due on Thursday. If you haven't already returned the materials, you should try to do so by Wednesday. You can return the materials in PAS 588 and pick up the Supplementary Information sheet.

Thursday is also the first of the 1S1P Assignment #3 due dates. If you plan to do two reports this time, at least one report must be turned in on Thursday this week.

We started class with a new topic - thermal circulations.

Differences in temperature, such as might develop between a coast and the ocean or between a city and the surrounding country side, can create horizontal pressure differences. The horizontal pressure gradient can then produce a wind flow pattern known as a thermal circulation. These are generally relatively small scale circulations and the pressure gradient is so much stronger than the Coriolis force that the Coriolis force can be ignored. We will learn how thermal circulations develop and then apply to concept to the earth as a whole in order to understand large global scale pressure and wind patterns. You'll find the following discussion on p. 131 in the photocopied Class Notes.

A beach will often become much warmer than the nearby ocean during the day (the sand gets hot enough that it is painful to walk across in barefeet). Pressure will decrease more slowly with increasing altitude in the warm low density air than in the cold higher density air above the ocean.

Even when the sea level pressures are the same over the land and water (1000 mb above) an upper level pressure gradient can be created. The upper level pressure gradient force will cause upper level winds to blow from H (910 mb) toward L (890 mb).

The movement of air above the ground can affect the surface pressures. As air above the ground begins to move from left to right, the surface pressure at left will decrease (from 1000 mb to 990 mb in the picture below). Adding air at right will increase the surface pressure there (from 1000 to 1010 mb).

This creates a surface pressure gradient.

The surface winds blow from high to low. The surface winds and upper level winds are blowing in opposite directions.

You can complete the circulation loop by adding rising air above the surface low pressure at left and sinking air above the surface high at right. The surface winds which blow from the ocean onto land are called a sea breeze (the name tells you where the winds come from). Since this air is likely to be moist, cloud formation is likely when the air rises over the warm ground.

At night the ground cools more quickly than the ocean and becomes colder than the water. The thermal circulation pattern reverses direction. Surface winds blow from the land out over the ocean. This is referred to as a land breeze.

Here's another example of a thermal circulation

Cities are often warmer than the surrounding countryside, especially at night. This is referred to as the urban heat island effect. This difference in temperature can create a "country breeze."

The Asian monsoon (monsoon refers to a seasonal change in the direction of the prevailing winds) is a large scale circulation pattern and is much more complex than a simple thermal circulation. However you can use the thermal circulation concept to get a general understanding of what to expect at different times of the year.

In the summer India and SE Asia become warmer than the oceans nearby. Surface low pressure forms over the land, moist winds blow from the ocean onshore, and very large amounts of rain can follow.

The winds change directions in the winter when the land becomes colder than the ocean.

You can also use the thermal circulation to understand some of the basic features of the El Nino phenomenon (you find a discussion of the El Nino on pps 135-139 in the photocopied Classnotes).

First here is what conditions look like in the tropical Pacific Ocean in non-El Nino years.

Cold ocean currents along the west coasts of N. America and S. American normally converge at the equator and begin to flow westward (see top view above). As the water travels westward it warms. Some of the warmest sea surface waters are normally found the western Tropical Pacific. A temperature gradient becomes established between the W. and E. ends of the tropical Pacific. The crossectional view above shows the normal temperature and circulation pattern found in the equatorial Pacific Ocean. You would find surface high pressure in the east and low pressure in the west. Note that the wind circulation pattern is the same as the simple thermal circulation we studied above.

Colder than normal ocean waters in the E. Pacific is referred to as a La Nina event. La Nina conditions have developed at the present time. You can find up to date information on El Nino/La Nina conditions at a Climate Prediction Center website.
The Climate Prediction Center expects the La Nina conditions to have the following effects on winter weather in the western and southeastern United States: "Expected La Nina impacts during November - January include above average precipitation in the Northern Rockies, Northern California, and in southern and eastern regions of the Pacific Northwest. Below-average precipitation is expected across the southern tier, particularly in the southwestern and southeastern states."

In an El Nino year the cold currents don't make it to the Equator. Warm water is carried from the western Pacific to the eastern Pacific

Now surface high pressure is found in the west and surface low pressure and rising air is found in the E. Pacific (the reversal in the surface pressure pattern is referred to as the southern oscillation). Indonesia and Australia often experience drought conditions during El Nino events. In the desert SW we expect slightly wetter than normal conditions (perhaps 20% wetter than normal). Wetter conditions are also found in California and in the SE US.

At this point we took a little detour.
You might already have heard that water spins in a different direction when it drains from a sink or a toilet bowl in the southern hemisphere than it does in the northern hemisphere. You might also have heard that this is due to the Coriolis force or the Coriolis effect.

We've just finished learning about the Coriolis force. It does cause winds to spin in opposite directions around high and low pressure centers in the northern and southern hemisphere. The PGF starts the air moving (in toward low, out and away from high pressure) then the Coriolis force bends the wind to the right (N. hemisphere) or to the left (S. hemisphere).
Here's what you end up with in the case of low pressure (see p. 130 in the photocopied Classnotes):

The PGF is stronger than the CF. This results in a net inward force, something that is needed to keep winds blowing in a circular path.

Winds also spin around high pressure. The CF is critical in this case. The CF is stronger than the PGF and the CF points inward. The CF is what provides the needed inward force needed to keep the winds blowing in a circular path.

There are situations where the PGF is much stronger than the CF.
The CF is so weak it can be ignored. This is the case with tornadoes, winds spin around a core of very low pressure.

Winds can still spin around LOW. The PGF supplies the necessary net inward force.

The PGF points ouward with high pressure. Without the CF, there isn't any inward force, winds can't spin around high without the CF.

Water can spin in either direction in either hemisphere as it drains from a sink, a toilet bowl, or a bucket (with a hole in the bottom) such as was used in the video segment shown in class.

The CF doesn't play any role at all. The following figure shows that there is just an inwardly directed pressure gradient force.

If you look closely at water spinning in a bucket or sink you will notice that the surface of the water has a bowl shape. The water piles up and is deeper along the sides of the bucket than it is in the center. An inwardly directed pressure gradient force is created. The deeper water near the sides of the bucket produces a little higher pressure inside the water than the shallower water near the center of the bucket. This radial difference in pressure is what keeps the box of water spinning in a circular path.

Now back to another new topic covered in class today.
We'll apply the thermal circulation idea to a global scale and try to learn something about global scale pressure and wind patterns on the earth. Ordinarily you couldn't apply a small scale phenomena like a thermal circulation to the much larger global scale. However if we make some simplifying assumptions, particularly if we assume that the earth doesn't rotate or only rotates slowly, we can ignore the Coriolis force.

Some additional simplifications are also made and are listed below (p. 133 in the photocopied Classnotes)

Because the earth isn't tilted, the incoming sunlight shines on the earth most directly at the equator. The equator will become hotter than the poles. By allowing the earth to rotate slowly we spread this warmth out along the entire length of the equator rather than concentrating it in a spot on the side of the earth facing the sun. Because the earth is of uniform composition there aren't any temperature differences created between large bodies of water and land masses.

You can see the wind circulation pattern that would develop. The term one cell just means there is one complete loop in the northern hemisphere and another in the southern hemisphere.

Next we will remove the assumption concerning the rotation of the earth. We won't be able to ignore the Coriolis force now.

Here's what a computer would predict you would now see on the earth. Things are pretty much the same at the equator in the three cell and one cell models: low pressure and rising air. At upper levels the winds begin to blow from the equator toward the poles. Once headed toward the poles the upper level winds are deflected by the Coriolis force. There end up being three closed loops in the northern and in the southern hemispheres. There are belts of low pressure at the equator (the equatorial low) and at 60 degrees latitude (the subpolar low). There are belts of high pressure (the subtropical high) at 30 latitude and high pressure centers at the two poles (the polar highs).

We will look at the surface features in a little more detail because some of what is predicted, even with the unrealistic assumptions, is actually found on the earth.

We'll first look at surface pressures and winds on the earth from 30 S to 30 N, the tropics and subtropics. Then we'll look at the region from 30 N to 60 N, middle latitudes, where most of the US is located.

With a little study you should be able to start with a blank sheet of paper and reproduce this figure. I would suggest starting at the equator. You need to remember there is a belt of low pressure found there. Then remember that the pressure belts alternate: there are belts of high pressure at 30 N and 30 S.

Let's start at 30 S. Winds will begin to blow from High pressure at 30 S toward Low pressure at the equator. Once the winds start to blow they will turn to the left because of the Coriolis force. Winds blow from 30 N toward the equator and turn to the right in the northern hemisphere (you need to turn the page upside down and look in the direction the winds are blowing). These are the Trade Winds (northeasterly trade winds north of the equator and southeasterly trades south of the equator). They converge at the equator and the air there rises (refer back to the crossectional view of the 3-cell model). This is the cause of the band of clouds that you can often see at or near the equator on a satellite photograph.

The Intertropical Convergence Zone or ITCZ is another name for the equatorial low pressure belt. This region is also referred to as the doldrums because it is a region where surface winds are often weak. Sailing ships would sometimes get stranded there hundreds of miles from land. Fortunately it is a cloudy and rainy region so the sailors wouldn't run out of drinking water.

Hurricanes form over warm ocean water in the subtropics between the equator and 30 latitude. Winds at these latitudes have a strong easterly component and hurricanes, at least early in their development, move from east to west. Middle latitude storms found between 30 and 60 latitude, where the prevailing westerly wind belt is found, move from west to east.

You find sinking air, clear skies, and weak surface winds associated with the subtropical high pressure belt. This is also known as the horse latitudes. Sailing ships could become stranded there also. Horses were apparently either thrown overboard (to conserve drinking water) or eaten if food supplies were running low. Note that sinking air is associated with the subtropical high pressure belt so this is a region on the earth where skies are clear (Tucson is located at 32 N latitude, so we are affected by the subtropical high pressure belt).

The winds to the north of 30 N and to the south of 30 S are called the "prevailing westerlies." They blow from the SW in the northern hemisphere and from the NW in the southern hemisphere. The 30 S to 60 S latitude belt in the southern hemisphere is mostly ocean. Because there is less friction over the oceans, the prevailing westerlies there can get strong, especially in the winter. They are sometimes referred to as the "roaring 40s" or the "ferocious 50s" (the 40s and 50s refer to the latitude belt they are found in).

Here's the other map, it's a little simpler. Winds blowing north from H pressure at 30 N toward Low pressure at 60 N turn to the right and blow from the SW. These are the "prevailing westerlies." The polar easterlies are cold winds coming down from high pressure at the north pole. The subpolar low pressure belt is found at 60 latitude. This is also a convergence zone where the cold polar easterly winds and the warmer prevailing westerly winds meet. The boundary between these two different kinds of air is called the polar front and is often drawn as a stationary front on weather maps. A strong current of winds called the polar jet stream is found overhead. Strong middle latitude storms will often form along the polar front.

The following material wasn't covered in class on Tuesday. We may review some of it quickly at the start of class on Thursday.

We studied the 3-cell model because some of the features it predicts (despite the simpliying and unrealistic assumptions) are really found on the earth.

We assumed that the earth wasn't tilted. Let's look at how allowing the N. Pole to tilt toward the sun in June and away from the sun in December will affect the 3-cell model features.

The top figure shows the normal locations of the belts of Low and High pressure in the 3-cell model when the earth isn't tilted.

As the N. Pole tilts toward the sun in the middle figure, the sun's rays will strike the earth most directly at a location north of the equator (at the Tropic of Cancer). That is where the equatorial low will be found, north of its normal location. Similarly the other 3-cell model features will all be displaced to the north of their normal positions.

As the N. Pole tilts away from the sun in the bottom figure the 3-cell model features move south of their normal locations.

We'll see this north and south movement of the 3-cell model features in the next figure as well.

The 3-cell model predicts subtropical belts of high pressure near 30 latitude. What we really find are large circular centers of high pressure. In the northern hemisphere the Bermuda high is found off the east coast of the US (feature 3 in the figure), the Pacific high (feature 4) is positioned off the west coast. Circular low pressure centers, the Icelandic low (feature 2) and Aleutian low (feature 1), are found near 60 N. In the southern hemisphere you mostly just find ocean near 60 S latitude. In this part of the globe the assumption of the earth being of uniform composition is satisfied and a true subpolar low pressure belt as predicted by the 3-cell model is found near 60 S latitude.

The equatorial low or ITCZ is shown in green. Notice how it moves north (when the north pole is tilted toward the sun) and south of the equator (when the north pole is tilted away from the sun) at different times of the year.

The winds that blow around these large scale high and low pressure centers create the major ocean currents of the world. If you remember that high pressure is positioned off the east and west coast of the US, and that winds blow clockwise around high in the northern hemisphere, you can determine the directions of the ocean currents flowing off the east and west coasts of the US. The Gulf Stream is a warm current that flows from south to north along the east coast, the California current flows from north to south along the west coast and is a cold current. A cold current is also found along the west coast of South America (a disruption of this current often signals the beginning of an El Nino event); winds blow counterclockwise around high in the southern hemisphere. These currents are shown in the enlargement below.

The north and south movement of subtropical high has a big effect on weather in Arizona.

Tucson gets about 12 inches of rain in a normal year (we are well below normal at this time this year). About half of this comes during the "summer monsoon" season. The word monsoon refers to a seasonal change in wind direction. During the summer subtropical high pressure moves north of its normal position near 30 N latitude. Winds on the southhern side of the subtropical high have an easterly component. Moist air originating in Mexico and the Gulf of Mexico blows into Arizona. The sun heats the ground during the day, warm air in contact with the ground rises and produces convective thunderstorms.

The close proximity of the Pacific high, with its sinking air motions, is what gives California, Oregon, and Washington dry summers.

In the winter the subtropical high moves south of 30 N latitude. Winds to the north of the high blow from the west. Air originating over the Pacific Ocean loses much of its moisture as it crosses mountains in California (remember the rain shadow effect). The air is pretty dry by the time it reaches Arizona. Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest into Arizona.