Unit 11 Section 1 Computer Lab. Part 1: REMOTE SENSING OF THE EARTH SYSTEM BY SATELLITE

Transcription

1 Unit 11 Section 1 Computer Lab Part 1: REMOTE SENSING OF THE EARTH SYSTEM BY SATELLITE Educational Outcomes: Satellites orbiting the planet are ideal platforms for monitoring the Earth system from above and outside looking down. They are located far enough from the planet to see the large-scale features of the system, and yet have sensors capable of resolving relatively small-scale components or processes at work in the system. Different onboard sensors provide complementary views of Earth, which together provide us with comprehensive monitoring of the Earth system and its workings, including the global water cycle. A. Scientific Basis of Remote Sensing by Satellite Images produced by sensors aboard satellites are constructed from radiation that is measured electrically, converted to digital values, and downloaded to Earth stations. There, digital data are processed into images for interpretation. Most weather satellite sensors are designed to respond to two basic types of radiant energy emitted to space from the Earth system: visible light and infrared (IR) radiation. Visible light (0.4 to 0.7 um), originating from the sun, is reflected or scattered by Earth's surface and clouds, back to the satellite. Visible satellite images are essentially black and white photographs of the planet showing reflectivity of surfaces. Clouds, snow cover, and glaciers appear white to the sensor just as they do to our eyes. Surfaces such as evergreen forests and water bodies (e.g., ocean, lakes) reflect little sunlight back to space and therefore appear dark (i.e., gray or black). Visible images obtained by current geostationary weather satellites can resolve objects such as clouds as small as one kilometer across. The second principal type of onboard satellite sensor detects infrared (or heat) energy given off by surfaces within the sensor's field of view. The intensity and type of infrared energy emitted depends on the temperature of radiating surfaces transmitted through a clear atmosphere. Infrared radiation in the um range is used to measure surface temperatures. In this way, infrared images actually display patterns of temperature on a gray scale such that at one extreme dark gray is warm and at the other extreme bright white is cold. (A color scale is also used to depict temperature and some enhanced infrared images display two or more gray-scale sequences.) Land and water surfaces are usually warmer than clouds and appear darker on an IR image. High cold clouds are brighter white than low warm clouds. Infrared imagery shows the contrast in diurnal (day to night) temperature change between land and water surfaces. (That is, land warms more during the day and cools more at night.) The Earth system emits IR radiation day and night so that infrared images are always available. The infrared sensor on current geostationary weather satellites can distinguish features as small as 4 kilometers across. As we will see in Investigation 4B, a specific wavelength of infrared radiation (6.7 micrometers) is used to monitor atmospheric water vapor in the middle atmosphere. Conventional visible and infrared satellite imagery described above are obtained by so-called passive sensors, that is, sensors that respond to existing radiation. Another kind of sensor, so-called active sensors, emits certain types of radiation (e.g., gamma, microwave) and receives and interprets its reflected signal. Examples of this method of remote sensing are given in your WES text. B. Satellite Puzzle Satellites provide unique views of the Earth system. Sensors aboard satellites scan the Earth along narrow strips and measure signal strengths of reflected sunlight or Earth radiation (infrared) for small blocks within each

2 strip. Each block segment, known as a pixel, is the smallest picture element in the image. A series of numbers indicating pixel signal strength is transmitted to receiving stations on Earth where computers reassemble the values into lines of shaded (or colored) blocks. Lines are assembled sequentially to form an image. Weather satellite views seen on television weathercasts are examples of such images. This activity explores how satellite imagery is generated. Imagine that two different sensors scan the same scene and that both sensors detect infrared radiation. Scanning of the scene produced the two sets of data presented in the figure below. Grids to the right of each data set show the size of the pixels resolved by the two satellite sensors. One sensor is able to resolve pixels having a side-length that is one-half that of pixels detectable by the other sensor. The resolution of a satellite sensor refers to its capability of distinguishing individual objects in greater detail. A scale from 0 to 3 is used to represent the signal strength of radiation received by the satellite sensors. The value of 0 indicates no detection of infrared radiation whereas the value of 3 indicates the most intense infrared radiation. Reconstruct the scene based on the pixel values appearing to the left of each grid. Values are in the same relative positions as the pixels that they represent. First, use a pencil to fill in the Shading Key at the top of the worksheet as follows: 0 - no shading 1 -light gray 2 - medium gray 3 - black By referring to the Shade Key, shade in the left grid according to the data given. Then use the same procedure to fill in the right grid with the smaller pixels. 1. What was the image in the original picture? (Hint: a letter of the alphabet. Holding at arm's length may aid recognition.) Which of the two views exhibits greater detail (more resolved information bits)? Why? 2. Assume that the two satellite sensors used in this activity produce pixels that represent Earth-surface areas 2 kilometers and I kilometer on a side, respectively. Assuming that the lower 48 states are approximately 5,000 km east-west by 3,000 km north-south, how many pixels must be scanned to obtain full-area images by each of

3 the two sensors? 2 km by 2 km pixel-image: pixels 1 km by 1 km pixel-image: pixels Note: This activity is adapted, with permission, from Teacher's Guide: Weather Satellites, Everyday Weather Project, State University of New York, Research Foundation of State University of New York, C. What Can You See?

4 On the following pages, Figure I contains two satellite images centered on the lower 48states of the U.S. on 23 June 2000 at (1:15 p.m., CDT). The first image (la) is a visible light view and the second image (lb) is an infrared view.

5 3. In the visible image (Fig. la), clouds appear mostly white. Clusters of thunderstorm clouds (cumulonimbus) appear over much of Iowa and Florida. Compare the appearance of clouds with the relatively cloud-free surface of Lake Huron (the Great Lake northeast of Lower Michigan). Compared to cloud tops, the lake surface reflects [(more) (less) (about the same)] sunlight as cloud tops.

6 4. In Fig. I b, the infrared image, shades of gray represent temperature. (Based on their signature radiation, surfaces reveal their temperature.) Black or dark gray indicates relatively high temperature whereas bright white indicates relatively low temperature; medium shades of gray correspond to temperatures in between. In the lower portion of the atmosphere where most clouds occur (the troposphere), air temperature on average decreases with increasing altitude. Hence, from infrared images we can infer the relative heights of cloud tops: Warm low clouds are gray whereas cold high clouds appear bright white. In the infrared image, tops of the cumulonimbus clouds over Iowa are relatively [(cold) (warm)] and are located at relatively [(high) (low)] altitudes. 5. Based on the infrared image, the tops of clouds over much of Iowa exist at a [(higher) (lower) J altitude than the tops of clouds over southern Kansas. 6. Cloudiness that appears off the California coast on the visible image is not readily discernible on the infrared image. A possible explanation is that the cloud top temperatures depicted in the IR image are [higher than) (lower than) (about the same as)] the ocean surface temperatures in the same locations. 7. The speckled white pattern appearing on the visible image (e.g., over Arkansas) represents clusters of fairweather cumulus clouds. These clouds are products of convection currents driven by solar heating of Earth's surface. The sun heats the surface and the warm surface heats the overlying air. The less dense heated air is forced upward by surrounding cooler denser air. Water vapor in the rising convection current condenses into cumulus clouds that resemble puffs of cotton floating in the sky. Cumulus clouds are most likely to form during the day over relatively warm surfaces. On the visible image, cumulus clouds [(do)(do not)) appear over Lakes Erie and Ontario and [(do){do not)) appear over the adjacent land areas along their shores (e.g., south of Lake Ontario). From this observation, we conclude that the land surface temperature is [(lower than)(about the same as)] the surface temperature of the lake. 8. Compared to the near-midday IR image (Fig. 1b), an IR view of the same scene just after midnight on the same day would generally appear [(brighter) (darker)] in clear-sky land areas because of lower surface temperatures. Part 2: REMOTE SENSING OF THE EARTH SYSTEM BY SATELLITE Infrared satellite imagery depicts the temperature of surfaces in the sensor's field of view. Satellite remote sensing of the planet is a valuable tool for determining temperature patterns and comparing temperatures of different surfaces. Such information is useful in identifying the altitude of cloud tops (often an indicator of thunderstorm intensity) and the location of warm and cold ocean surface currents. Because of its unique thermal properties and abundance, water plays a key role in explaining temperature contrasts within the Earth system. The objective of this portion of the investigation is to become familiar with temperature measurements by infrared satellite imagery. This part of the investigation makes use of a special infrared-temperature applet. The applet uses composite infrared satellite images produced frequently in the 10.7 micrometer channel on GOES-East and GOES-West (geostationary) weather satellites positioned to "see" the area of the globe from the eastern Pacific eastward to the western Atlantic and from Central America northward to Central Canada.

7 Image 1 Examine Image 1,which displays a recent IR satellite image. The label along the bottom of the image specifies the date and time of the image. Following international convention, time is given as UTC, Universal Coordinated Time, the local time at the prime meridian (0 degree longitude) using a 24-hour clock. For example, 15:00 UTC corresponds to 11:00 a.m. Eastern Daylight Time (EDT), 10:00 a.m. Central Daylight Time (CDT), 9:00 a.m. Mountain Daylight Time (MDT), and 8:00 a.m. Pacific Daylight Time (PDT). 9. On the Image 1 map you have printed, the date is (day) (month) (year) and the UTC time is. This time is 3:30 a.m. EDT, 2:30 a.m. CDT, 1:30 a.m. MDT, and 12:30a.m. PDT 10. This IR satellite image utilizes a gray scale to display temperature. Note the cloud pattern associated with Topical Storm Ernesto along the northeast Florida and Georgia coast at the time of the image. Considerable cloudiness and moisture were moving northward into the mid-atlantic region ahead of Ernesto which was intensifying as it moved over the warm Gulf Stream waters. Also note the nearly circular cloud swirl associated with Hurricane John along Mexico's west coast. (At the time, John with maximum sustained winds of 125 miles per hour was rated a category 3 hurricane on the 5-point Saffir-Simpson Hurricane Intensity Scale.) Another swirl to the west of Hurricane John was Hurricane Kristy. Evaluation of the shades of gray on this map indicated the highest open-ocean temperatures (24.5 C) were detected in the water surfaces of the cloud-free areas of the Caribbean Sea southeast of Puerto Rico. Water surface temperatures were also in the mid 20s

8 Celsius in the Gulf of Mexico. The lowest temperature was?81 C found in top surfaces of the cloud cluster in Hurricane John. The highest temperature on the map corresponds to the *[(/_whitest_/)(/_darkest_/)]* area on the map. The lowest temperature on the map corresponds to the *[(/_whitest_/)(/_darkest_/)]* area on the map. Go to the Links Page, "Atmospheric Information" section, and click on "Infrared Surface Temperature Application." Note that loading this applet may take time, so please be patient. Once loaded, you will be viewing a very recent IR satellite image. Scroll to the area below the image and click on "click here" to get the image in a format suitable for printing (or you may click on "Printable Image" in the left border). Print a copy of the image by clicking on the "Print" command on your web browser. 11.On the map you have printed from the course website, the date is (day) (month) (year) and the UTC time is. Click on the "Back" command on your web browser to return to the interactive map. 12.Move your mouse pointer so that the cursor travels over the onscreen interactive image. Note that the cursor gives you the infrared temperature at the point of its arrow to the nearest half of a degree Celsius for each pixel by latitude and longitude. The highest temperature you found on the map is C associated with *[(/_land_/)(/_water_/)(/_cloud tops_/)]* and the lowest temperature you found is C associated with *[(/_land_/)(/_water_/)(/_cloud tops_/)]*. On the infrared image, the progression from darker to lighter gray shading is accompanied by a progression from *[(/_lower to higher_/)(/_higher to lower_/)]* temperatures. 13.In the portion of the atmosphere where most clouds occur, the air temperature almost always decreases with increasing altitude. The white and gray swirls on the IR image are produced by clouds at different altitudes. Use your cursor to investigate the temperature of cloud tops. Based on your investigation, brightest white indicates clouds with the *[(/_lowest_/)(/_highest_/)]* cloud top temperatures and therefore the *[(/_highest_/)(/_lowest_/)]* tops. 14 Determine the temperature of a cloud-free area of the Pacific Ocean just off the California Coast and a cloud-free area of the Atlantic Ocean off the coast of the Southeast U.S. (preferably east of Florida to the Carolinas) at about the same latitude. Write these temperature values along the edge of your printed map and draw arrows to their locations. The Atlantic surface water temperature typically is *[(/_higher than_/)(/_lower than_/)]* the Pacific surface water temperature. This temperature contrast reflects the presence of a *[(/_warm_/)(/_cold_/)]* ocean current off the California coast (the California Current) and a *[(/_warm_/) (/_cold_/)]* ocean current off the Southeast coast (the Gulf Stream). 15. Differences in thermal properties between land and water mean that land surfaces warm more during the day (and in summer) than do adjacent water surfaces. And land surfaces cool more at night (and in winter) than do nearby water surfaces. Use your cursor to investigate infrared temperatures of adjacent land and open water surfaces in cloud-free coastal areas such as along the Gulf of California (the water body separating Baja California and mainland Mexico). Mark these temperatures on your map. They show that at map time the ocean surface water temperatures are *[(/_higher than_/)(/_lower than_/)(/_about the same as_/)]* the temperature of the adjacent land surface.