The Life of a Cyclone

Transcription

1 Unit 2 The Life of a Cyclone In this unit, you will learn The stages in the life of a tropical cyclone. How, where, and why tropical cyclones die. How scientists track tropical cyclones. How to determine the intensity of a tropical cyclone. NASA/GSFC Three satellite images superimposed on the same map show the development of Hurricane Georges on September 25,

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3 Warm-up 2.1 Tropical cyclone a type of low-pressure system originating over tropical or sub-tropical waters with cyclonic surface wind circulation. Hurricane and typhoon are regionally specific terms for a strong tropical cyclone. Media Viewer Web site If you have any data frame of the ddtc_unit_2.mxd project open in ArcMap, you can click the Media Viewer button and choose Observing Tropical Cyclones from the media list to open this Web page in your browser. Who or what is NOAA? NOAA is the National Oceanic and Atmospheric Administration. It is a government agency that conducts research and gathers data about the oceans, atmosphere, space, and sun. NOAA warns of dangerous weather, charts our seas and skies, guides our use and protection of ocean and coastal resources, and conducts research to improve our understanding and stewardship of the environment. Part of the U.S. Department of Commerce, NOAA provides services through five major organizations. National Weather Service National Ocean Service National Marine Fisheries Service National Environmental Satellite, Data and Information Service NOAA Research and special program units Observing tropical cyclones In terms of human casualties, the 1900 Galveston Hurricane was the worst natural disaster in U.S. history. Why was this hurricane so destructive? Part of the answer is that, in 1900, forecasters had to make predictions based almost exclusively on direct observation. The skies, the tides, and the changing winds and air pressure were their only clues that a hurricane was coming. Unfortunately, by the time they realized it, the storm was nearly upon them. The forecasters had little time to warn the residents to evacuate or to seek shelter. Today s forecasters use sophisticated tools to locate and track tropical cyclones well before they make landfall. By modeling tropical cyclones on computers, researchers are improving their ability to accurately predict storm tracks. Using satellites and other modern tools, forecasters observe and measure cyclones remotely. To learn about both direct and indirect methods of observing tropical cyclones, point your Web browser to the following NOAA Web site: NOAA hurricanes.noaa.gov/prepare/observation.htm On the NOAA Web page, click on each of the technologies to learn more about it. Find out how each tool helps scientists monitor these dangerous storms. In Table 1, list the name of each technology and tell whether it provides a direct or an indirect method of observation. Table 1 Modern weather-observation technologies Number Name of technology Direct or indirect? Observing tropical cyclones 35

4 1. What are the two main types of satellite imagery used in tropical cyclone forecasting? 2. Radiosondes and dropsondes help us understand the structure of tropical cyclones by measuring and transmitting information about which atmospheric properties? 3. What is the most direct method of measuring wind speeds within a tropical cyclone? 4. Which two important properties of tropical cyclones are measured using radar? 36 Observing tropical cyclones

5 Investigation 2.2 Tracking Hurricane Georges Tropical cyclones do not start out fully developed. Like people, tropical cyclones also experience stages of growth. Unfortunately, instead of mellowing out, hurricanes generally get more dangerous with age. This investigation explores the development of a single hurricane, Georges (pronounced zhorzh), that affected Atlantic and Caribbean coastal communities in September Launch ArcMap, then locate and open the ddtc_unit_2.mxd file. Refer to the tear-out Quick Reference Sheet located in the Introduction to this module for GIS definitions and instructions on how to perform tasks. In the Table of Contents, right-click the Tracking Georges data frame and choose Activate. Expand the Tracking Georges data frame. Universal Time (UT) The times listed in the Hurricane Georges IR Images layer group are in 24-hour Universal Time format. Abbreviated UT, this is the time at 0 longitude, or the prime meridian. The U.S. Atlantic Coast is 5 time zones west of the prime meridian, subtract 5 hours from Universal Time to get local time for the Atlantic Coast. NOAA Figure 1. False-color image of Hurricane Georges. The highest (coldest) cloudtops, shown in orange and yellow, indicate where upward convection is strongest. This data frame shows the path, or track, of Hurricane Georges as it swept across the Atlantic and the Caribbean. Hurricane Georges is among the top 25 most deadly cyclones in history. The large number of casualties occurred as Georges passed directly over several Caribbean islands. An eye in the sky Turn on the Hurricane Georges IR Images layer group. This layer group includes seven layers, labeled by date and time, that show satellite images of Hurricane Georges at different points in its journey across the Atlantic Ocean. At this point, only the 9/15 10:19 UT layer should be turned on. The hurricane s clouds were not bright colors like those you see on the screen and in Figure 1. These false colors represent the temperatures of the cloud tops recorded by the satellite s infrared sensor (Figure 1). Grays and whites represent the warmest temperatures, whereas greens, yellows, and reds represent the coldest (Figure 2 on the following page). Temperature decreases with altitude, so colder temperatures indicate higher cloud tops. As the upward convection in a storm intensifies, clouds rise higher into the atmosphere. Thus, the cloud temperatures tell us about the strength of the convection, which in turn indicates the energy of the hurricane. Beginning at the bottom of the list, turn on each of the seven hurricane image layers in order. Tracking Hurricane Georges 37

6 Increasing altitude and decreasing temperature Higher cloud tops = Cooler temperatures = Stronger convection red orange yellow green blue violet white gray black Lower cloud tops = Warmer temperatures = Weaker convection Figure 2. Relationship between color, cloud temperature, and height in false-color satellite images. 1. On what date did Hurricane Georges make landfall on the U.S. mainland? How many days did it take Hurricane Georges to travel across the Atlantic Ocean before landfall? 2. Convert the number of days from question 1 into hours. 3. In what direction did Georges travel as it crossed the ocean? Now you will measure the distance Georges traveled and use it to find the average rate of travel, or speed of the storm. Using the Measure tool, click on each of the storm centers, in order, starting with the September 15 storm center. Double-click on the last storm center (September 27) to finish the measurement. The total distance Georges traveled (Total) is reported in the status bar at the bottom left of your screen. (Your Total will be different.) 4. How far (in km) did Georges travel? 38 Tracking Hurricane Georges

7 5. Use this distance and the number of hours you calculated in question 2 to find Georges average speed (in km/hr) as it crossed the Atlantic Ocean (speed = distance time). 6. An average person walks at a speed of about 6.5 km/hour, and runs at about 10 km/hour. Compare these rates with the average speed you calculated for Hurricane Georges. 7. If a major tropical cyclone is spotted 500 km offshore, how much warning time might a coastal community have before the storm strikes? (time = distance speed) On the path of a killer Next you will learn about the countries affected by Georges as it traveled across the Atlantic. Look at the Storm Track layer, which shows the path of the center of the hurricane as it crossed the Atlantic Ocean. Turn the Hurricane Georges IR Images layers on and off as needed to better see the track. To learn the names of countries, islands, and states along Georges path, click the Identify tool. In the Identify Results window, select the Countries layer from the drop-down list of layers. Next, click on a country or island and read the Name of the country in the Identify Results window. If you need to identify U.S. states, select the States layer from the drop-down list of layers in the Identify Results window. 8. Identify these landmarks associated with Hurricane Georges as it crossed the Atlantic Ocean: a. Hurricane Georges formed south of this group of islands off the west coast of Africa:. b. Georges passed directly over three large Caribbean island countries and one U.S. territory. These are,,, and. c. The first U.S. state affected by Georges heavy rainfall and strong winds was. d. The U.S. state where the eye of Hurricane Georges first made landfall was. Tracking Hurricane Georges 39

8 Close the Identify Results window. Hurricane formation Click the QuickLoad button. Select Data Frames, choose Formation and Movement, and click OK. Turn on the Formation Points layer. Massive storms Remember that each hurricane track represents the path traveled by a large and dangerous storm. Each storm affected an area far wider than the thin lines you see. Each point in this layer indicates where a storm system first showed the typical features of a tropical cyclone in the Atlantic between 1950 and Notice that the points are not evenly distributed. 9. On Map 1 identify the two areas where hurricanes tend to form in the Atlantic Ocean. Map 1 North Atlantic Ocean hurricane-formation regions Turn off the Formation Points layer. Turn on the End Points layer. Look for areas where hurricanes appear to lose strength and die. 10. Identify and label all the areas on Map 1 where hurricanes tend to weaken and die out. Turn off the End Points layer. Click the QuickLoad button. Select Spatial Bookmarks, choose Atlantic, and click OK. Turn on the Hurricane Tracks layer. The tangled lines are the paths traveled by Atlantic hurricanes from 1950 to Each path was defined by recording the location of a storm s center at regular intervals and then connecting the dots. Before the existence of weather satellites, storm locations were determined by aircraft and by reports from ships at sea. Next, you will discover the number of hurricanes that have crossed the Atlantic over the past 55 years. 40 Tracking Hurricane Georges

9 How many hurricanes? Every hurricane is a unique event with its own personality. This may be one reason why it seems fitting to give hurricanes human names. Still, most hurricanes go through similar life cycles. To determine how many hurricanes occurred between , select the Hurricane Tracks layer. Click the Open Attribute Table button. Read the total number of hurricanes from the bottom of the table window (Your total will be different from the one shown below): Read total here. 11. How many Atlantic-basin hurricanes occurred between 1950 and 2005? 12. What is the average number of hurricanes that occur each year in the Atlantic Ocean basin? (total number of hurricanes years) Close the attribute table. Global wind patterns and hurricane tracks Next you will look at wind patterns and determine their influence on hurricane movement in the Atlantic Ocean and the Gulf of Mexico. Turn off the Hurricane Tracks layers. Turn on the Wind Patterns layer. Prevailing winds surface winds that generally flow in the same direction for long time periods. In general, winds over the U.S. mainland blow from the southwest, and are called the prevailing westerlies. The arrows in this layer indicate the direction of prevailing winds over the Atlantic Ocean. 13. In what direction do the prevailing winds over the Atlantic Ocean appear to be moving near the equator? 14. In what direction do the prevailing winds over the Atlantic Ocean appear to be moving near 40 N latitude? 15. Do the prevailing winds over the Atlantic appear to be rotating? If so, are they rotating clockwise or counterclockwise? Turn off the Wind Patterns layer. Turn on the Hurricane Tracks layer. Select the Hurricane Tracks layer. Tracking Hurricane Georges 41

10 Using the Select Features tool tracks., click on one of the hurricane One or more hurricane paths will be highlighted. Repeat this several times at random locations and try to identify any general patterns in hurricane movement. 16. Draw the general pattern(s) of hurricane movement on Map 2. Be sure to include arrows to indicate the direction of movement. Map 2 Atlantic hurricane-movement patterns Click the Full Extent button to view the entire map. Click the Clear Selected Features button. Turn on the Global Tropical Cyclone Movement layer. 17. Describe the relationship between the paths of Atlantic tropical cyclones and global wind patterns. Quit ArcMap and do not save changes. 42 Tracking Hurricane Georges

11 Reading 2.3 Classifying tropical cyclones An average tropical storm system lasts about 12 days. If conditions are favorable, it may progress through four stages of development. 1 Tropical disturbance A tropical cyclone begins as a low-pressure area with warm, moist air rising from the ocean surface. As the moisture condenses, clouds and precipitation form. If the system sustains itself for 24 hours, it is called a tropical disturbance. Winds in a tropical disturbance blow at up to 36 km/hr (22 mph), but the system has very little rotation. NASA/GSFC Eye wall Central eye Rain bands 2 Tropical depression As convection intensifies and the surface pressure decreases further, the tropical disturbance begins to rotate. When the winds reach speeds over 36 km/hr (22 mph), the system is classified as a tropical depression. 3 Tropical storm If a tropical depression intensifies, with wind speeds increasing to 63 km/hr (39 mph) or higher, it is called a tropical storm, and is assigned a name. The eye of the storm may become visible, and the storm begins to resemble a tropical cyclone in many ways. Figure 1. Tropical cyclone structure. Eye a region of relatively calm, clear air that forms as rising air from the thunderstorms converges and sinks in the center of the storm. Eye wall a ring of tall thunderstorms surrounding the eye where the strongest winds and rain are found. Rain bands long bands of thunderstorms that spiral inward toward the eye wall, also known as feeder bands. Naval Research Laboratory, Monterey, CA 4 Tropical cyclone When wind speeds exceed 117 km/hr (73 mph), the storm is classified as a tropical cyclone or hurricane. Distinct bands of thunderstorms rotate around the eye of the storm. The eye is an area of calm surrounded by the eye wall, where the winds reach their maximum speed. Edouard tropical cyclone Fran tropical storm Gustav tropical depression Hortense tropical disturbance Figure 2. Tropical cyclone family portrait. This unique satellite image taken on August 31, 1996 shows systems at each of the four stages described. Hurricane Edouard, Tropical Storm Fran, Tropical Storm Gustav (soon to be downgraded to a tropical depression), and Tropical Disturbance #8 (which eventually developed into Hurricane Hortense). Classifying tropical cyclones 43

12 1. If a tropical storm is downgraded to a tropical depression, what does this mean about the wind speeds? 2. Fewer than 10 percent of tropical disturbances evolve into tropical cyclones. Based on what you have learned about tropical cyclones, why do you think this percentage is so low? Knot a measure of speed, used in air and water travel, equal to 1 nautical mile per hour. (A nautical mile 1.15 U.S. statute, or land miles.) km/hr 1.15 mph The word knot comes from an early device used to measure a ship s speed. The device consisted of a rope with regularly spaced knots tied to a weighted, wooden float called a log. The pilot threw the log overboard, and counted the number of knots that ran out while a sand glass emptied. This gave the speed of the ship in nautical miles per hour. Mb stands for millibar, a metric unit of atmospheric pressure. One mb is 1/1000 of a bar, and is equal to 100 Pa (pascals). Standard atmospheric pressure at sea level is mb (29.92 in Hg). Intensity scales Forecasters responsible for monitoring tropical cyclones have developed scales to describe a storm s potential for destruction. The U.S. uses the Saffir-Simpson Hurricane Scale, shown below. This scale ranks hurricanes in five categories from 1, the weakest, to 5, the strongest. Table 1 Saffir-Simpson Hurricane Scale Saffir- Simpson Category Maximum sustained wind speed Minimum surface pressure Storm surge mph km/hr knots mb ft m Tropical < 39 < 63 < depression Tropical storm H > H H H H5 > 155 > 250 > 135 < 920 > 18 > 5.5 Why rank hurricanes? The categories of the Saffir-Simpson and similar scales used around the world estimate the potential for flooding and other damage from tropical cyclones. Wind speed is related to the surface pressure in the center of the hurricane. The maximum wind speed and air pressure reflect the storm s energy and destructive potential. Category 3 5 hurricanes are considered to be major, and are capable of causing tremendous damage. 44 Classifying tropical cyclones

13 NASA/GSFC Path Strongest winds Highest surge Figure 3. For Atlantic storms, the surge is usually located in the right front quadrant, where the storm s rotation and its forward motion add to produce the strongest winds. This satellite photograph shows Hurricane Andrew approaching the Louisiana coast on August 25, Measuring wind speed The most meaningful way to measure wind speed is to calculate the average surface wind speed over a period of time. It is then reported as the maximum sustained surface-wind speed. The main difference between intensity scales used by the U.S. and other countries is the length of time over which the wind speed is averaged. The World Meteorology Organization recommends using a 10-minute average, whereas U.S. forecasters continue to use a 1-minute average. Storm surge Storm surge can be the most destructive aspect of a tropical storm (Figure 3). Storm surge is a mound of water piled up on the leading edge of a tropical cyclone by strong winds blowing across the ocean surface. As the surge washes ashore, it temporarily raises sea level over wide areas of the shoreline. Storm intensity and damage The Saffir-Simpson Hurricane Scale provides an estimate of the damage potential of hurricanes based on wind speeds and storm-surge heights. Use the Saffir-Simpson Scale to answer the following questions. 3. As a storm grows from a tropical depression to a Category 5 hurricane, how do the surface pressure and the wind speed change? 4. Storm surge adds to the normal tides for a section of coastline. How would the time of day affect the severity of the storm surge striking coastal areas? 5. How high would sea level be if an H3 storm made landfall at high tide and the normal tides range ±2 m from mean sea level? 6. How high would sea level be if the same H3 storm made landfall at low tide? Classifying tropical cyclones 45

14 40 35 $ 36.2 Damage in billions of dollars $ 7.1 $ 3.5 $ 5.7 $ H1 H2 H3 H4 H5 n = 3 n = 4 n = 15 n = 5 n = 4 Storm category (n = number of storms) Figure 4. Bar graph depicting the average monetary damage inflicted by the top 31 most damaging Atlantic hurricanes of varying strength that made landfall between 1950 and Use Figure 4 to answer the following questions. 7. In general, what happens to the damage caused by hurricanes as their intensity increases? 8. About how many times more destructive, in terms of dollar damage, is an average H5 hurricane than an average H1 hurricane? Logically, property damage should increase with hurricane intensity. However, this chart shows that the most destructive H1 hurricanes caused more damage than the most destructive H2 and H3 hurricanes. 9. Describe a situation where a H1 hurricane could strike land and cause more damage than more intense storms. 46 Classifying tropical cyclones

15 Investigation 2.4 Monitoring cyclone growth path strongest winds highest surge In an earlier investigation, you examined the path of Hurricane Georges across the Atlantic Ocean. In this investigation, you will use measurements of the wind speed to classify the intensity of the hurricane at each point along its path. Launch ArcMap, then locate and open the ddtc_unit_2.mxd file. Refer to the tear-out Quick Reference Sheet located in the Introduction to this module for GIS definitions and instructions on how to perform tasks. In the Table of Contents, right-click the Tracking Georges data frame and choose Activate. Expand the Tracking Georges data frame. Turn on the Hurricane Georges IR Images layer group. Turn on all the layers in the Hurricane Georges IR Images layer group. Click the Identify tool. In the Identify Results window, select the Storm Centers layer from the drop-down list of layers. Next, click the Georges 9/15 storm center. The Identify Results window provides information about the conditions at the center of the developing cyclone. 1. Record the pressure in millibars (mb) and wind speed (km/hr) for September 15th in Table 1. Table 1 Development of Hurricane Georges Hurricane Georges image date Characteristic How to find 9/15 9/17 9/18 9/20 9/23 9/27 Pressure mb use Identify tool Wind speed km/hr Category Rotation use Identify tool Saffir-Simpson Scale look at images 2. Use the pressure and wind speed on 9/15 to determine the Saffir- Simpson category using the table on page 44 and enter it in Table 1. Figure 1. Hurricane Andrew approaches the Louisiana coast on August 25, Early in the development of a tropical cyclone, the storm begins to rotate. Look carefully at the image for evidence of rain bands, a central eye, or any other visual clue that the storm has begun to spin. Monitoring cyclone growth 47

16 3. Write either N (none), F (faint), or S (strong) for the storm s rotation in Table 1. Repeat this process by using the Identify tool remaining storm centers, and complete Table 1. to click on the Answer the questions below based on Table 1 and your observations. 4. When did Georges first reach hurricane status? How did its appearance change from the previous image? 5. When was Georges at peak intensity? 6. What was Georges maximum wind speed at its peak intensity? Using the Measure tool Close the Identify Results window. Use the Measure tool to measure the diameter of Hurricane Georges at its peak, when its wind speeds were the highest (see sidebar). Read the diameter (Total) in the status bar. (Your Total will be different from the one shown below.) Position the measuring tool cursor on one side of the hurricane, then click and drag across to the opposite side of the storm. Read the distance in the status bar. Double-click to stop measuring. 7. How wide (in km) was Georges at its peak intensity? 8. How did Georges structure change after it passed over Haiti and the Dominican Republic? 9. Look at the 9/29 image of Georges. How did the storm s appearance change? What caused this change? 48 Monitoring cyclone growth

17 Area locator Selecting areas handle 1 To select an area, click its border using the Select Elements tool. Handles and a solid or dashed line appear when the area is selected. Above, Area 3 is selected. Catastrophic storms The final part of this exercise looks at the life cycles of the most powerful hurricanes, using the 80 Category 4 and 5 hurricanes that have occurred between 1950 and These storms had wind speeds greater than 210 km/hr (130 mph) and were capable of catastrophic damage. As hurricanes develop and move across sea and land, their energy, wind speed, and potential for damage can change. In this section, you will examine some of these changes and identify where hurricane intensity appears to increase or decrease. Click the QuickLoad button. Select Data Frames, choose Hurricane Intensity, and click OK. Click the QuickLoad button. Select Spatial Bookmarks, choose Atlantic, and click OK. This data frame shows the storm tracks for all of the Category 4 and 5 hurricanes between Differences in the width and color of the tracks indicate changes in the wind speed as the storms progressed from Tropical Storm to a Category 4 or 5. Now you will examine the relationship of wind speed to storm location in more detail. The hurricanes in each of the five outlined areas have unique characteristics. Select the Catastrophic Hurricanes ( ) layer. Select the outline of Area 1 by clicking on its boundary line using the Select Elements tool (see Area locator in sidebar). Handles (small boxes) and a dashed line appear around the boundary to indicate that it has been selected (see Selecting areas in sidebar). Click the Select By Graphics button. Hurricane tracks that fall within the outline will be highlighted. Click the Statistics button. In the Statistics window, calculate statistics for only selected features of the Catastrophic Hurricanes ( ) layer, using the Wind speed (km/hr) field. Click OK. The average wind speed for Category 4 and 5 hurricanes in Area 1 (Coast of Africa to Mid-Atlantic) between is reported in the Statistics window as the Mean. 10. In Table 2 on the following page, record the average wind speed for the Area 1, and whether the area is over land or over water. Round to the nearest whole number. Monitoring cyclone growth 49

18 Table 2 Average Atlantic tropical-cyclone wind speeds by region Area 1 Coast of Africa to Mid-Atlantic 2 Mid-Atlantic to U.S. East Coast 3 Caribbean and Gulf Coast 4 Eastern U.S. 5 North Atlantic Average wind speed km/hr 75 Over land or Over water? Close the Statistics window and repeat the selection and statistics procedures for the other four areas to complete Table 2. Close the Statistics window. Click the Clear Selected Features button. Turn on the August SST layer. The August SST layer shows the average sea-surface temperature, in degrees Celsius, for the month of August. Use the sea-surface temperature layer and Table 2 to answer the following questions about Atlantic hurricanes. 11. Describe the location(s) and, if available, the sea-surface temperature where the wind speeds are the highest. 12. Describe the location(s) and, if available, the sea-surface temperature where the wind speeds appear to diminish. 50 Monitoring cyclone growth

19 13. Briefly describe how wind speed (and thus storm energy) changes a. when hurricanes move over warm, tropical water. b. when hurricanes move onto land. c. when hurricanes move over colder water. d. when hurricanes move over the Gulf of Mexico. Quit ArcMap and do not save changes. Monitoring cyclone growth 51

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