Lab 8 Objectives *

Lab 8 Key Concepts *

Lab 8 Reading Resources *

Lab 8 Activities Guidance *

Laboratory 8 Activities *

• Discuss the basic assumptions used in establishing the ideal 3-cell global circulation model.
• Discuss the importance and be able to analyze both small and large scale atmospheric/oceanographic events, including:

El Niño / Southern Oscillation (ENSO)

Sea breeze / land breeze

Winter/summer monsoons

Trade winds

Jet streams

• Compare the features of the ideal 3-cell global circulation model with actual global weather and climate data.
• Be able to list and identify the location of the upper tropospheric jet streams that are part of the general circulation.
• Be able to identify the polar front and explain its connection to the polar (front) jet stream and the general circulation.

• A watery (all ocean, no continents), non-tilted, non-rotating Earth would result in a simple thermally driven one-cell (per hemisphere) global circulation model, as shown in animation in Figure 7.1 (in Course Content, Course Modules, Module 2, Lesson 7). The simple one-cell model shows warm air rising at the equator and cold air sinking over the poles. This simplified model is the first conceptual model developed of the General Circulation. It is called Hadley cell circulation model of general circulation. The Hadley cell is also described on page 240 in WoW.

• The global circulation actually has a three-cell structure. The cells are:

• The ideal 3-Cell Global Circulation Model results from differential solar heating on a rotating, but still non-tilted, and all-ocean Earth, as shown in Figure 7.2 below, from Course Content, Course Modules, Module 2, Lesson 7.

• The Hadley and Polar Cells are direct thermal circulations with warmer air rising and colder air sinking. The Ferrel Cell is a hydrodynamically driven or "forced" cell where the colder air is forced to rise and the warmer air is forced to sink.
• The reason that the simple Hadley cell breaks down into three cells is because of:

• The result is a net sinking motion at 30 degrees latitude and a net rising motion at 60 degrees latitude. So the mid-latitude circulation (Ferrel cell) is an indirect thermal circulation with the relatively warmer air at 30 degrees sinking and colder air at 60 degrees rising. Note I did say that in this case the warmer air sinks and colder air rises; this is a special point that many students find surprising. The complete general circulation picture has rising motion over the equator, sinking motion at 30 degrees, rising motion at 60 degrees and sinking motion at the poles.
• The three-cell circulation model is associated with the following pressure belts.

• The three-cell circulation model and pressure belts form the following wind belts:

• When examining the upper level winds, you may notice that the upper-level winds in the midlatitudes do not match the winds predicted by the Ferrel Cell of the ideal three-cell model. The three-cell model predicts winds should blow from the east. But in the real atmosphere the prevailing upper-level winds have a predominately westerly component. This would appear contrary to the idealized 3-cell model of circulation where the Ferrel Cell shows flow toward the equator in the upper levels, which would give easterly winds because of Coriolis deflection. But, there are westerly winds aloft even in midlatitudes because the temperature contrast between the poles and tropics creates an upper-level pressure gradient strong enough to overwhelm ("hide") the upper level return flow of the midlatitudes to the equator.

• In summary, the simple idealized circulation gives us a rough model of the circulation of the atmosphere, but the land /water distribution and migratory (moving) pressure systems alter the real circulation somewhat from the idealized three-cell model.

• Look at the animated satellite image in Figure 7.3 (in Course Content, Course Modules, Module 2, Lesson 7). The figure shows actual global-scale motions from satellite imagery. Also look at the Figure 9.2 page 240 in WoW. Note the following:

• For additional guidance look at the POES global stitched satellite images at the links below.

The following are some additional sites where you can find real-time full disk satellite images:

• The El-Nino-La Nino - Southern Oscillation refers to two aspects of an ocean cycle that directly impacts the tropical Pacific ocean and indirectly effects global weather patterns. El Nino and La Nina are used to describe the warm and cold phases of the eastern tropical Pacific sea surface temperature cycle. See Figure 7.4 (from Course Content, Course Modules, Module 2, Lesson 7) that shows an excellent interactive animated diagram of the ocean temperature and wind related differences between the "normal" and El Nino conditions. More information on ENSO can be found in WoW pages 245-249.
• Southern Oscillation is used to describe the atmospheric cycle of high and low atmospheric pressure that oscillates in an east-to-west fashion and reverses approximately every 3 - 10 years.
• Initially El-Nino-La Nino and the Southern Oscillation were looked at as independent phenomena, but it is now realized they are connected as part of a large-scale air-sea interaction cycle.
• "Normal" Conditions: The coast of South America usually experiences a cold current called the Peruvian current. There is also upwelling of cold, deep water during their summer. This cold, upwelling is important for fisheries, since the cold water is nutrient rich. In December, there is usually a short period of warming caused by a warm, countercurrent. This event occurs near Christmas, so the natives call it El Nino in honor of the Christ child.
• El Nino Conditions: At intervals of 3 to 10 years the warm December current stays and becomes unusually strong. It is these strong events, where the water along the west coast of South America are warmer than normal, that we now refer to as El Nino.

• Here is a map showing the changes in global weather patterns during El Nino years.
• La Nina Conditions. La Nina is the name for the opposite of El Nino, when the surface waters off the west coast of South America become abnormally cold. Essentially the La Nina is an extreme form of the "Normal" condition. La Nina can also effect the intensity of weather patterns far from the tropics.
• For current information on the El Nino cycle do to he link below at Climate Predication Center (CPC) for the latest El Nino - La Nina Advisory: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/
• Myths and Misconceptions about El Nino

• The presentation at the two alternate links below gives more illustrations of the causes and effects of ENSO:

All wind motions in the atmosphere can be classified into three basic forms:

• The direct thermal circulation is a thermodynamically driven circulation or convective circulation. In the direct thermal circulation the colder air sinks displacing warmer air and causing the warmer to rise. You've probably heard this concept stated as: "Hot air rises". The direct circulation is the most fundamental circulation of the atmosphere and occurs at all scales of the atmosphere. The direct circulation that results from the difference in heating between the equator to poles drives the general circulation of the atmosphere.
• The hydrodynamically forced circulations are caused by pressure gradients induced by the motion of the air. The most common cause of hydrodynamically forced circulation is convergence in one location and the upper troposphere, which forces downward motion and divergence in another location in the upper troposphere, which forces upward motion. For example a "jet steak" (jet maximum wind area near the top of the troposphere) will cause upper-level divergence and associated convergence that result in hydrodynamically forced circulations. Many times a hydrodynamically forced circulation will cause an "indirect (thermal) circulation", where the cold air is forced to rise and warm air forced to sink. Indirect circulations are important to the development and intensification of synoptic scale fronts and mid-latitude cyclones.
• Turbulent motion is just what it sounds like. It is rapidly changing small-scale wind gusts. If we could see the air, turbulent motion would look something like a breaking wave on a shoreline. Turbulence dissipates kinetic energy in the atmosphere. If there were no turbulence, the wind would increase without bounds. So when a gust of wind hits you and dissipates, you are helping maintain kinetic energy balance in the atmosphere.

This presentation gives an illustration of the direct and indirect circulations:

• Both direct thermal (convection) cells and indirect thermal circulations occur on all spatial scales in the atmosphere from the very large global scales to localized circulations, such as sea and land breezes.
• Sea breezes are direct thermal circulations that form during the day when the ground temperatures are warmer than the nearby ocean temperature. Warm air tends to rise over the land while cooler air tends to sink over the ocean. This situation typically causes a breeze from the sea, or a "sea breeze".
• During the night, the opposite occurs, since the land will tend to cool down quicker compared to the ocean. This situation will sometimes cause a breeze from the land, or a "land breeze". So land breezes are direct thermal circulations that form more often at night. The sea breeze is typically more pronounced than the land breeze.
• Land/Sea breeze situations can also occur with large lakes, e.g. any of the U.S. Great Lakes or Lake Victoria in Africa.
• The basic thermodynamics of the sea and land breeze are shown below:

• Monsoons are a seasonal reversal of winds. These occur because of the heating or cooling and associated pressure changes caused by the large-scale land - water distribution. The resulting pressure changes alter the global circulation pattern, resulting in the local winds changing from season to season. The reversal in winds typically causes a "dry" and "wet" season for those regions that have monsoons. Sometime the term "monsoon" is incorrectly thought to mean "heavy precipitation". Some additional information on monsoons can be found in WoW on page 242.
• This presentation gives an illustration on two of the more well known monsoons in Asia and North America:

Please continue to try to make the connection between the concepts we are learning in the course and how they can help you understand your local weather. Think about how weather forecasts at your location are related to the general circulation of the atmosphere.

Go to this link "help for weather observations and forecasts" for more help on topics relevant to taking observations and making forecasts.

1a. What are the three cells of the general circulation?

For each of the three cells, list whether it is a direct or indirect thermal circulation.

1b. Background: Look at the full-disk infrared (IR) geostationary satellite image at the link below. [If you are interested, view the water vapor image, also.] Compare the IR image to the idealized 3 cell global circulation theory (model).

Alternate Site if links above are inactive:

Keep in mind that regions of high pressure tend to be clear, and that regions of low pressure tend to be cloudy. Don't be concerned if the global circulation features don't "jump out" at you; the real world is much more complicated than our simple 3-cell global circulation model. That's part of the point of this exercise. But you should see some features that clearly fit with the three-cell theory.

Question: What are the similarities and differences of the surface features found between the "ideal" 3-Cell Global Circulation Model pressure / wind patterns and the clouds on the image above?

• In your answer, discuss which features seem to match well with the 3-cell theory and which are hard to see. As a hint, in general the farther away you get from the equator, the more variable the weather patterns. So you would expect the tropics to be more consistent with the three-cell theory than the polar regions.
• Explain what you believe to be responsible for any differences. Look for features such as, strong heating over land areas causing upward motion and clouds, where the 3-cell model predicts sinking motion and clear conditions.
• One thing that is NOT going to match the idealized 3-cell circulation is the 3-cell model's prediction of upper-level easterly winds in the midlatitudes.

In this exercise we will view a Polar Orbiter (POES) Composite picture (called the "stitched Global IR" satellite image) and see how positions of the continental landmasses modify the "idealized pressure patterns".

The satellite image is "stitched" together from a series of polar orbiting (POES) images. So they are of higher resolution than the full disk GOES images. You can see the images are not all taken at the same time, but they do highlight the global features well. Click on the links below for a view of the unanalyzed and analyzed images and look for the global features. The "smaller" file will load more quickly but will have less detail.

Unanalyzed images:

Now view my analysis of the satellite images on one of the sites below:

2a. Compare the above satellite image cloud patterns with the surface pressure and surface wind flow information in Figures 9.7a and 9.7b [p. 244] of your text. Discuss how the satellite image cloud patterns compare with what you would expect to see after studying Figures 9.7a and 9.7b. For example, are there clear conditions in the image where there are high pressure systems in the figures? Is the ITCZ visible? Is there evidence for low-pressure systems at around 60 north?

2b. Look for evidence of the two most important types of upper tropospheric jet streams, the polar front jet stream (PFJ) and subtropical jet stream (STJ). Which jet stream should be the most global in its circulation pattern? Can you see evidence of this on the satellite picture (at the link above)? Do you see any surges of clouds from the tropical regions to the midlatitudes that would likely be associated with the subtropical jet? Give a brief discussion on your findings.

2c. Explain the connection between the polar front and polar jet stream. I indicated the polar front (PF) and polar front jet (PFJ) with the same line in the analyzed satellite image. But in reality the polar front jet will be displaced slightly from the surface position of the polar front. See Lab 8 Guidance, section 2 for the actual relationship of the polar front to the polar front jet.

2d. What is the connection between the polar front and the fronts we see on surface weather maps associated with midlatitude pressure systems? (Note. Answer to 2d is given below. Read it over; it will be essential to understanding how midlatitude cyclones form.)

3a. Figures 9.15a and 9.15b [p. 257] show isopleths of sea surface temperature for November 1982 and November 1988. Given that El Niño was in progress during one of these months, which month was it? Explain your answer based on the isotherms of sea surface temperature. Hint: Make sure you compare the sea surface temperatures in the two diagrams right along the equator. One should clearly be warmer than the other.

3b. Go to the CPC El Nino advisory at http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/

Just read the first couple of paragraphs of the web page to see what the current phase or "episode" of El Niño - La Nina is occurring. Do they indicate we are in Warmer (El - Nino conditions), Colder (La Nina conditions), or Normal conditions? Note: This is an active site that is regularly updated, so sometimes it is hard to find the information on what phase of El Nino we are in, but usually the "phase" or "episode" is described in the first couple of sentences. I don't expect you to study the entire website.

4a. Assume that it's a hot and sunny afternoon, with temperatures over land soaring past 90º F (32º C ). Which diagram, A or B, best represents the thermal circulation that will develop during the afternoon?

Sea breeze A: http://www.borg.com/~glenn/umuc/171/seabreezal.jpg

Sea breeze B: http://www.borg.com/~glenn/umuc/171/seabreeznl2.jpg

Sea breeze A: http://web1.meso.com/wind-personal/glenn/171/seabreezal.jpg

Sea breeze B: http://web1.meso.com/wind-personal/glenn/171/seabreeznl2.jpg

4b. Now assume that it's the middle of the night, and temperatures over land have dropped to around 70º F (21º C). Water temperatures, however, continue to hover around 80º F. Describe briefly what the direct thermal (convective) circulation will look like at that time.

4c. The sea breeze cell is a direct thermal circulation. It will resemble which of the general circulation cell(s) that are also direct thermal circulations: Hadley, Ferrel and/or Polar cells?

In the table below, you are given the mean monthly liquid precipitation (in inches) at Seoul, South Korea (latitude 37.6ºN-to locate Seoul, see Color Plate 1.B).

5b. The precipitation climatology was one of the primary factors considered when planning the 1988 Summer Olympics that were to be held in Seoul. The Summer Olympics are traditionally held during the months of July and August, but for Seoul were delayed until September. Why do you think there is so much precipitation in July and August in Seoul and it is so relatively dry in the other months?

Using the instruments found in your "weather kit," or any other source of local weather information, take a weather observation of the weather elements indicated below. Think about how your observation relates to the larger synoptic scale patterns and the general circulation of the atmosphere.

Go to this link "help for weather observations and forecasts" for more help on topics relevant to taking observations and making forecasts.

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