Fri., Nov. 9 notes

Friday Nov. 9, 2007

The revised Expt. #2 reports have been graded and were returned in class.

The Experiment #4 reports are due next Wednesday, Nov. 14. If you haven't yet done the experiment you should try to do so now so that you can return the materials on Tuesday (in PAS 588) and pick up the Supplementary Information sheet.

Next Wednesday is also the first of the 1S1P Assignment #3 report due dates. If you plan to do two reports as part of this assignment, at least one report must be turned in on Wednesday.

The most recent Optional Assignment is due next Wednesday. Copies of the assignment are available in PAS 588.

We will finish the Chapter 6 material on winds today by covering surface winds. Upper level winds are determined by the pressure gradient force and the Coriolis force.

In the figure above, we drew in the pressure gradient force arrow (perpendicular to the contour lines and pointing toward low pressure). Then we can drew in an equal and oppositely directed CF so that the net force would be zero. We then saw that the CF was to the right of the wind and we could then say this was a N. hemisphere chart.

For surface winds, you must add the frictional force to the mix.

The frictional force will always point in a direction opposite the wind. Friction always try to slow moving objects (it doesn't cause you to speed up on your bicycle or to veer suddenly to the right or left). The strength of the frictional force depends on wind speed (stronger when the winds are fast and zero when the wind isn't blowing at all). Friction also depends on the type of surface the wind is blowing over (there is less friction when winds blow over the ocean than when blowing over land).

Note in the figure above that adding friction slows the wind. This in turn weakens the Coriolis force and the CF no longer balances the PGF. The PGF turns the wind slightly toward low pressure, the winds blow across the contours toward low pressure. You eventually end up with a new balance: CF together with F are able to balance the PGF and the net force becomes zero.

This figure summarizes most everything we have done. The upper level winds are shown in the figure at left. The upper level winds blow parallel to the contour lines.

At right surface winds around centers of high and low pressure are shown. You should remember from early in the semester that winds blow counterclockwise and inward around low pressure in the NH. They blow clockwise and outward around high pressure.

In the southern hemisphere the directions of spin change (clockwise around low and counterclockwise around high). Winds still blow converge into low pressure and diverge from centers of high pressure. This means that rising air (which expands and cools) and clouds will be found with centers of low pressure in both the northern and southern hemispheres.

Note the locations and directions of motion of the southern hemisphere warm and cold fronts. The winds spin clockwise around low pressure in the southern hemisphere. The coldest air is found in the south in the southern hemisphere.

Next we moved to 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."

This 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 Niña 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.