Laboratory 8 Activities *

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.

Ans 1a. The three cells are:

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.

Ans 1b: The "Ideal 3-Cell Global Circulation Model" is based on an all-ocean, non-tilted, rotating earth. The main differences found between the "actual" and "ideal" atmospheric circulations are from land influences (uneven heating between ocean & land) and from the earth's tilt, which results in the seasons. In general terms, 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 simple three-cell model.

In the full disk GOES imagery, you can see:

• The intertropical convergence zone (ITCZ) just north of the equator. This is usually the easiest global feature to pick out.
• Basically clear areas over the central Atlantic and Pacific at about 25-40 degree North Latitude that correspond to the subtropical ridge predicted by the three-cell circulation.
• More cloudiness is evident over the land at the 25-40 north latitude because the heating of the land by the sun causes greater instability and more upward vertical motion than the simple 3-cell global circulation would predict.
• Notice how the circulation of a tropical storm (Isidore - located in the Western Caribbean) is interacting and distorting both the ITCZ to the south and the mid-latitude systems to the north.
• The ITCZ is north of the equator, which is a slight variation in the simple three-cell circulation.
• Notice the evidence of strong tropical convection over Equatorial South America with many circular clouds. The circular clouds are indicative of very light winds. This is consistent with the simple 3-cell circulation, where the equatorial zone should have predominantly upward motion and light (horizontal) winds (the doldrums).
• It is hard to pick out, but there is evidence of a low-pressure system at about 60 north where one would expect to find a band of low pressure based upon the simple 3-cell circulation.
• By using the upper level clouds (and general observations), you may also notice that the upper-level winds in the midlatitudes do not match the winds predicted by the Ferrel Cell of the simple 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. 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 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.)

Usually the most distinct, consistent feature seen on a global satellite image is the band of clouds forming over the ITCZ (doldrums). You should readily see a line of clouds within the equatorial region. The polar front may be seen as a series of comma-shaped clouds with the center of the comma usually around 40-60 degrees latitude. You may also notice much cloudiness over some summer land areas due to the increased heating there (remember land heats & cools faster than ocean areas). Generally the subtropical highs from around 30 ° S and N are regions of clear skies and light winds called the "horse" latitudes.

The cloud patterns in the image at the links above match up very well to that of the Fig. 9.7b, July pattern. The ITCZ is just north of the equator from the central Pacific to the western Atlantic. The ITCZ bulges northward then dips southward as it moves into Africa then back northward again into the Indian Ocean. This pattern matches well with the average pressure patterns depicted in the text. The subtropical ridge system is evident with the mostly clear band that circles the global at 25-40 South. It is harder to see that corresponding band in the northern hemisphere because of the greater land coverage which produces more convection and cumuliform (puffy) clouds. The cloud bands in the polar regions match well with the convergent flow you would expect between the polar easterlies and mid-latitude westerlies in each hemisphere.

Ans 2b. The two most prevalent upper level jet streams are the polar front jet (PFJ) and subtropical jet (STJ).

Which one is the most global in its circulation pattern? The polar front jet (PFJ) is more or less continuous around the globe and is evident by the cloud band that is more or less continuous.

Can you see evidence of this on the satellite picture? The nearly continuous PFJ is quite evident in the image.

Do you see any surges of clouds from the tropical regions to the midlatitudes that would likely be associated with the subtropical jet? Yes, there are at least three surges that can be see on the image. They are marked STJ on the analyzed image.

Ans 2c. Explain the connection between the polar front and polar jet stream.

The cause of the polar front is the thermal contrast between the polar and tropical regions. This thermal contrast also is the fundamental cause of the polar jet stream near the top of the tropopause. The thermodynamics controlling the behavior of the atmosphere keeps the position of the polar jet stream about 50 -200 miles poleward of the surface position of the polar front.

Ans 2d. What is the connection between the polar front and the fronts we see on surface weather maps associated with midlatitude pressure systems?

When an area of very strong winds, called a jet max or jet streak, forms it results in areas of upper tropospheric convergence and divergence. When the area of divergence moves over the thermal transition zone of the polar front, low pressure forms near the surface and a wave forms on the front. Then the portion of the polar front behind the center of the low pressure becomes a synoptic-scale cold front and the front ahead of the low becomes a synoptic-scale warm front.

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.

Ans 3a. November 1982 (fig a): El Nino Year - El Nino was in progress.

November 1988 (fig b): La Nina Year - the eastern Pacific was colder than normal.

El Nino was occurring in 1982. Note that the 82F-degree isotherm indicates much higher sea-surface temperature across much of the equatorial Pacific during 1982. In contrast, in 1988, surface waters of <77 deg. F dominated much of the eastern Pacific near the equator. Another view would be to look at the ocean surface temperatures off the coast of Ecuador. Warmer temperatures would indicate when an El Nino was in progress.

Ans 3b. According to the Climate Prediction Center (CPC) ENSO-neutral conditions are expected during the next 3-6 months.

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?

Ans 4a. Diagram B represents the pressure patterns of a Sea Breeze.

Ans 4b. Circulation will reverse and look something like Diagram A, a land breeze. The temperature relationship will be warm water and cold land.

Ans 4c. Hadley and Polar cells.

Ans 4d. The indirect thermal circulation is where the cold air is forced to rise and the warm air is forced to sink. The forcing mechanism is strong divergence in the upper troposphere over the cold air and strong convergence in the upper troposphere over the warm air. On the synoptic scale the complete indirect circulation will cover several hundred miles.

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?

Ans 5a. What is a monsoon? A monsoon means there is a pronounced (at lease a 120 degree shift) in the mean wind direction between the winter and summer season. All monsoons are the result of direct thermal circulations reversing in direction from winter to summer. They form because of the reversal of the thermal differences between land and water reversing from winter to summer.

Ans 5b: As shown in the table of the monthly average precipitation for Seoul, Korea, there is a distinct rainy season that approximately correlates with the rainy season in India. If you compare the precipitation patterns of India (text pg. 242) and Seoul, you can readily see the similarities in their seasonal precipitation. The Asian monsoon is influencing the seasonal precipitation in Seoul. Many meteorologists argue that Seoul is one of the northernmost large cities to experience a seasonal monsoon. However, the seasonal wind changes don't meet the strict criteria of the monsoon. Also, if you look at the northern location of Seoul and the wind patterns in Figure 9.6 & Figure 9.7 of your textbook, you could argue that Korea isn't really a monsoon regime. However, even if you follow the strict definition of a monsoonal regime, you can't reasonably argue against the fact that Korea is greatly influenced by the Asian Monsoon and this influence is at least in part the reason for the heavy precipitation in July and August at Seoul. For reference, here is a graph of Seoul's precipitation climatology.

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.

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