Chapter 2 INTERPRETING SATELLITE IMAGERY Goals: To get hands-on experience interpreting visible, infrared, and water vapor satellite imagery. A brief summary of guidelines for interpreting clouds on satellite images is given below.

SUMMARY OF GUIDELINES FOR INTERPRETING CLOUDS ON SATELLITE IMAGERY

Low Clouds

The base of a deck of low clouds typically looks rather dark, which means that a limited amount of visible light is getting through. Thus, a significant amount of sunlight must be reflected (scattered) back to space by low clouds. Thus, low clouds look bright on visible images.

On infrared images, low clouds appear gray because their tops are relatively warm.

Low clouds are typically not represented on water vapor images because they are too low in the atmosphere to be detected.

Cirrus Clouds

From the ground, a layer of thin cirrus clouds usually looks white, meaning that sufficient sunlight is getting through to make their bottoms rather bright to an observer on the ground. So, because ample sunlight is getting through, the amount of sunlight that is reflected (scattered) back to space by cirrus must be somewhat less than what is reflected back by more reflective low clouds. Thus, cirrus look just a bit off-white (compared to low clouds) on visible imagery.

On infrared images, cirrus clouds appear white because their tops are very cold (they are high in the atmosphere).

Cirrus usually appear white on water vapor images because their high-altitude ice crystals emit "cold" 6.7 micron radiation.

Tall cumulus clouds or Cumulonimbus clouds

From the ground, the base of a cumulonimbus cloud usually appears very dark, which means that little visible light is getting through. Thus, such thick clouds must be reflecting (scattering) a large amount of visible light back to space, so tall cumulonimbus clouds appear bright white on visible images.

The high tops of cumulonimbus clouds are cold and thus appear bright white on infrared imagery.

The high tops of cumulonimbus clouds appear white on water vapor images because their high-altitude ice crystals emit "cold" 6.7 micron radiation.

Data:

Visible, infrared, color-enhanced infrared, and water vapor imagery for several cases, plus high-resolution visible imagery.

Case 1: Comparing visible, infrared, water vapor, and enhanced infrared: A typical case
Case 2: Comparing visible, infrared, water vapor, and enhanced infrared: Tropical Storm Ignacio
Case 3: High-resolution visible imagery and albedo: Salt Flats, mountains, and snow
Case 4: Comparing visible and infrared imagery: Ship trails and Erica
Case 5: The usefulness of water vapor imagery: Low-level muggies

1. Looking at the water vapor image, is the air at high altitudes over northwestern Ohio dry or moist? Explain your reasoning.

2. Looking at the visible image, are there clouds over northwestern Ohio? Argue, using only information from the water vapor image and the visible image, why these clouds are probably low clouds.

3. Substantiate your answer to the previous question using the infrared image.

4. Look again at the water vapor image. Note the narrow swath of moist air aloft that extends from central Texas to the lower Mississippi Valley and into the central Appalachians. Note the bright blob (which represents clouds) in Mississippi. Do you think that this blob represents cumulonimbus clouds (thunderstorms) or a patch of thick, high cirrus (feathery ice-crystal) clouds? Look at the enhanced infrared image and frame your answer based on this image. Also make your case using the visible image. Hint: For comparison sake, there are streamers of cirrus clouds moving into western Montana from southwestern Canada.

5. Use one of the images to decide what surface, land or water, has the higher albedo (on average) with respect to radiation in the range of wavelengths from 0.4 micrometers to 0.7 micrometers. Which image did you use?

1. Look first at the water vapor image, on which the fuzzy countenance of the remnants of Ignacio can be seen along the coast of central California. Ignacio subsequently moved into the Pacific Northwest, bringing rain to parts of Oregon and Washington. From this direction of movement, we can assume that the high-altitude winds that were steering weather systems at the time were blowing rather swiftly from the south-southwest. Based on this assumption, what would you forecast for San Francisco for later in the day and at night - breaking clouds or just plain cloudy? What feature on the water vapor image would have likely arrived and influenced San Francisco later that day and at night. Explain.

2. Now look at the infrared image. It is difficult to say, at first glance, whether there are any clouds along and just off the coasts of Baja California and southern California (south of the remnants of Ignacio). By looking at another type of satellite image, can you be more specific about whether there are clouds are not? Which image did you use to decide?

3. Now look again on the infrared image at the clouds over the Pacific Ocean west of the remnants of Ignacio. Do these Pacific clouds have lower or higher cloud tops than the clouds associated with the remnants of Ignacio? Explain your answer.

4. Now peruse the enhanced infrared image. Considering the entire span of this image, where are the coldest tops located? Based on its shape, this feature likely represents a cluster of thunderstorms (cumulonimbus clouds). At the same time, there were also thunderstorms associated with the remnants of Ignacio. Of these two thundery areas, which probably contained the more potent thunderstorms (assume that the taller the thunderstorms, the more powerful they are). Explain your answer in terms of cloud-top temperatures.

1. Consider this high resolution visible satellite image which shows the Bonneville Salt Flats in Utah, taken on July 4, 1997. At one time in earth's long and illustrious history, the Great Salt Lake covered a much larger area of Utah than it does today. As the waters of the lake receded, large deposits of salt were left behind, helping to create the famous Bonneville Salt Flats, seen in the image as the very bright blob west of the Great Salt Lake. What characteristic of salt is responsible for the Flats appearing so bright on this image?

2. Now look near the Utah-Wyoming border. There is a dark splotch in Utah than spans from east to west. This splotch is the Uinta Mountains. If you look closely, there are tiny veins of white that ripple north-south through the Uintas. What do you think these veins of white represent? Explain.

3. Now consider another high resolution visible satellite image of Utah, this one taken in the aftermath of the blizzard that struck the central Rockies on October 24-25, 1997. You will learn in a future chapter that snow is usually heaviest atop tall mountains, given that moist air is forced to ascend to great altitudes, a perfect route for making heavy snow. Based on what you see, pick out the Wasatch Range by giving the direction of the mountain chain's orientation (east-west, northwest-southeast, etc). Note that part of the Wasatch chain is visible from Salt Lake City.
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   In visible images, it is sometimes difficult to tell the difference between clouds and snow on the ground (under mostly clear skies). Of course, if you have access to a sequence of visible images, clouds will typically move in time while snow cover won't. On the other hand, while looking at a single visible image, meteorologists can determine whether a blotch of white is snow cover by identifying rivers or lakes, which early in the cold season, are ice- and snow-free and therefore appear as dark fingers amidst white snow cover.

4. Consider the linear swath of white stretching from Colorado to Iowa on this visible image taken on October 28, 1997. Within this white swath, there are telltale signs that this is mostly snow cover and that the sky is actually mostly clear over the Great Plains. Look in eastern Nebraska. Is there any feature that looks like a river? If so, consult an atlas and give the name of the river.

5. On the same visible image, now look at Iowa. There is a finger of dark that stretches toward the southeastern corner of the state. Is this a river within a snowpack? If so, name the river.

6. This swath of snow was produced by a storm that dumped 21.9 inches of snow at Denver's Airport on Friday and Saturday, October 24-25, 1997. Most of eastern Colorado was also hard hit. Can you find a river in Eastern Colorado? If so, describe where it is and give its name.

Now look at the infrared image taken at the same time. The ship trails are not evident. Putting your knowledge of infrared cloud imagery to work, can you explain why the streaks do not stand out from other low clouds in the vicinity?

Now inspect the surface weather map at 17Z, showing station models. Consider closely the locations in Florida that are within the dark swath shown in the previous water vapor image. The number to the lower left of the cloud circle in the station model is called the dew point. The dew point is an absolute measure of the amount of water vapor in the air (that is, the higher the dew point, the more water vapor in the air). We will study the dew point in detail in Chapter 5, but for now, be advised that dew points in the 65-70^oF range indicate that the surface air contains rather high levels of water vapor. Yet the water vapor image suggests that "the air is dry." Explain this apparent paradox.