Wed., Apr. 12 notes

Wed., Apr. 12, 2006

    Today was the first due date for 1S1P Assignment #3 reports. You could turn in one or two reports today. If you didn't turn in two reports today, you can turn in one report next Wednesday (Apr. 19).

    The optional assignment from Monday's class was also due today.

    The reading assignments link has been updated.

    Look through these notes carefully, there is a hidden 1S1P assignment in them somewhere. There are also many short segments of text that have been turned blue to make it slightly more difficult to find the link to the assignment.

We are mostly finished with the material on Coriolis force and other forces that cause horizontal winds; we may come back to the question of water draining out of sinks in the northern and southern hemisphere on Friday.

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 can also refer to p. 131 in the photocopied 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 above). Adding air at right will increase the surface pressure there (from 1000 to 1010 mb). This creates a surface pressure gradient and surface winds begin to blow from right to left (the opposite of what is going on above the ground).

You can complete the picture by adding rising air above the surface low and sinking air above the surface high. Because the surface winds come from the ocean they are referred to as a sea breeze. These winds would probably be pretty moist so clouds would be likely over land above the surface low.

At some point during the night, the ocean often ends up warmer than the land. The thermal circulation reverses direction. The surface winds are then called a land breeze and clouds and rain form out over the ocean.

Here are a couple of sample thermal circulation questions

Based on the upper level wind direction in the figure answer the following questions. This problem has been changed slightly (made a little more difficult) from the one given in class).
Is the pressure at Point A GREATER LESS EQUAL than(to) the pressure at Point B?
Is the pressure at Point B GREATER LESS EQUAL than(to) the pressure at Point C?

Answering the first half of the question should be easy if you remember the upper level wind is created by a horizontal pressure gradient force. The pressure gradient force always points from high toward low. Thus the pressure at Point A is GREATER than the pressure at Point B.

Answering the second half should also be easy if you remember that pressure always decreases with increasing altitude in the atmosphere, so the pressure at Point B is LESS than the pressure at Point C. You might be tempted to say high pressure at Point B is pushing the wind toward lower pressure at Point C, but that wouldn't be correct. What does cause the wind to sink from Point B toward Point C? The answer is gravity.

--------

Your job here is to draw the thermal circulation that would develop in the vicinity of an island surrounded by cooler ocean water. To answer a question like this you don't want to go through the 4 or 5 steps in the development of a thermal circulation like we did at the beginning of class. You want something simple that can get you started, something that you can remember; something like "hot air rises."

Draw in the rising air above the hot part of the picture. Then you can complete the thermal circulation loops as shown above.

Here are some additional examples of wind patterns that resemble thermal circulations. These examples weren't shown or discussed in class.

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.

In the winter, high pressure forms over the land, and dry winds blow from land out over the ocean.

This is an example of a monsoon wind system, a situation where the prevailing winds change directions with the seasons.

Cities will sometimes become warmer than the surrounding countryside, especially at night. This difference in temperature can create a "country breeze."

Now we'll apply the thermal circulation concept to the earth as a whole and learn about the "one-cell model" of the earth's global pressure and wind circulation pattern. You'll learn what the "one-cell" refers to shortly. A model is just a simplified depiction or representation of the earth's global scale circulation.

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.

You can see the wind circulation pattern that would develop (really just the same situation as the second sample problem studied earlier). 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 moving the air will be deflected to the right or left by the Coriolis force and won't make it to the poles. There end up being three closed loops in the northern and in the southern hemispheres. There are belts of low pressure at the equator (equatorial low) and at 60 degrees latitude (subpolar low). There are belts of high pressure (subtropical high) at 30 latitude and high pressure centers at the two poles (polar highs).

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

We'll first look at pressure and winds on the earth from 30 S to 30 N, the region enclosed by the dotted line and labelled Map #1 above. On Map #2 we'll look at the region from 30 N to 60 N, where most of the US is located.

This is Map #1. 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. 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. Here is the hidden 1S1P assignment. 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).

Here's Map #2, 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 called the "prevailing westerlies." In the southern hemisphere the prevailing westerlies blow from the northwest. The 30 S to 60 S latitude belt in the southern hemisphere is mostly ocean. 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 subpolar low pressure belt is found at 60 latitude. Note 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. Many middle latitude storms will form along the polar front.

Despite the simplifying assumptions in the 3-cell model, some of the features that it predicts (particularly at the surface) are found in the real world. This is illustrated in the next figure. A handout with this figure will be distributed in class on Friday. It wasn't discussed in class on Wednesday. It probably won't be discussed much in class on Friday either.

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 (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 IRCZ is shown in green. Notice how it moves north and south of the equator 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.

Something else not discussed in class is the effect that movement of the Pacific high (north of 30 N in the summer and south of 30 N in the winter) can have on weather in Tucson and much of the rest of the desert SW. This is illustrated below (on the back side of the handout to be distributed in class on Friday).

In the winter Arizona is north of the Pacific high and the winds that blow through our region have a westerly component. They originate over cool Pacific Ocean water and then must cross mountains in California. These winds are often pretty dry by the time they reach Arizona, dew points are usually in the 20s, 30s and 40s.

In the summer winds become easterly because the Pacific High moves well to our north. These winds transport moisture from the Gulf of Mexico into Mexico. Moisture then makes its way into Arizona from Mexico. Dew points rise into the 50s and 60s.

This seasonal change in wind direction is another example of a monsoon wind system.

Most of our yearly rainfall comes from summer thunderstorms in July, August, and September. We have a secondary "wet" season during the winter months of December, January, and February.