331 Part 9. Tropical Cyclones 9.1 Introduction The tropical cyclone is known as the hurricane in the north Atlantic and eastern north Pacific, typhoon in the western north Pacific, and just cyclone in the Indian ocean and Australia. One of the more colorful names it has been given is ‘The Serpent's Coil1,’ a very appropriate name because from its coiled position the tropical cyclone strikes with a vengeance with winds in excess of 32 m/s (64 mi/hr), heavy rainfall, storm surges, and sporadic tornadoes. Born over warm, tropical, oceanic waters, the storm gains strength as it moves poleward and spins up into a cyclonically-rotating storm (counter-clockwise in the northern hemisphere) as it experiences the influence of the earth's rotation. At its maturity, the tropical cyclone is an inertially stable vortex whose circulations may extend to distances as much as 1600 km (890 mi) from the storm center. The highest sustained winds ever recorded in an Atlantic tropical cyclone occurred in Gilbert were 82.3 m/s (184 mi/hr) with a top gust measured at 199 mi/hr. Not only do tropical cyclones produce winds that can rival a tornado in strength, but also they can produce extensive damage over large areas and for sustained periods. 9.2 Structure of the Mature Tropical Cyclone A schematic of the three-dimensional structure of a mature tropical cyclone is shown in Figure 9.1. 1 Farley Mowat, 1985: The Serpent's Coil. Seal Books, McClelland and Stewart-Bantam Books, 222 pp.332 Figure 9.1. Schematic diagram of hurricane, showing low-level circulation and cloud types. The highest clouds, composed of cirrus and cirrostratus, occur at the tropopause, which is about 16 km. (From Stormfury, 1970: Project Stormfury Annual Report 1969, National Hurricane Research Laboratory, NOAA, AOML/Hurricane Research Division, Miami, FL, 20 pp.) Near the surface, warm, moist air spirals inward toward the center of low pressure. At radii greater than about 400 to 600 km from the center, this flow is divergent, and sinking motion extends throughout most of the troposphere. This warm, sinking air is dry and usually is free of deep convective clouds, as seen in the satellite photograph of Hurricane Becky, which occurred in 1974 (Figure 9.2). Inside a radius of about 400 km, the low-level flow is convergent and the associated lifting of the warm, humid air produces extensive cumulonimbus clouds and precipitation. In spite of the relatively uniform tropical environment in which the tropical cyclone develops, it is characterized by a number of important mesoscale features. The mesoscale structure consists of the eyewall, a generally circular ring of intense convection surrounding the often cloud free eye; a region of stratiform cloud333 Figure 9.2. Satellite photograph of Hurricane Becky, 1800 UTC 20 August 1974. (Anthes, R. A., 1982: Tropical cyclones. Their evolution, structure, and effects. Meteor. Monograph No. 41, American Meteorological Society, Boston, 208 pp.) and precipitation outside the eyewall; and spiral bands of convective clouds that assume various forms. Visible and infrared satellite imagery of four mature hurricanes shown in Figure 9.3 all show well-developed eyes and bands of clouds having variable structure.334335 Figure 9.3. Satellite imagery of the four hurricanes near the time of flights by the research aircraft: (a) GOES-2 visible view of Hurricane Anita at 2300 UTC 1 September 1977 in the western Gulf of Mexico. (b) SMS-2 IR picture of Hurricane David at 1530 UTC 30 August 1979 south of Puerto Rico. (c) SMS-2 visible picture of Hurricane Frederic at 1300 UTC 12 September 1979 south of Mobile, Alabama. (d) SMS-2 IR view of Hurricane Allen at 1430 UTC 5 August 1980 south of Hispaniola. (e) TIROS-N visible image of Hurricane Allen at 2115 UTC 8 August 1980 in the central Gulf of Mexico. (From Jorgensen, D. P., 1984a: Mesoscale and convective-scale characteristics of mature hurricanes. Part I. General observations by research aircraft. J. Atmos. Sci., 41, 1268-1285.)336 A schematic radar vertical cross section through the eyewall region is illustrated in Figure 9.4. The eye is depicted by generally sinking motions Figure 9.4. Schematic cross section depicting the locations of the clouds and precipitation, RMW, and radial-vertical airflow through the eyewall of Hurricane Allen on 5 August 1980. The slope of the cloudy region on the inside edge of the eyewall is based on radar minimum detectable signal analysis, aircraft altimeter readings, hand photography and observer notes. Darker shaded regions denote the location of the largest radial and vertical velocity. (From Jorgensen, D. P., 1984b: Mesoscale and convective-scale characteristics of mature hurricanes. Part II. Inner core structure of hurricane Allen. J. Atmos. Sci., 41, 1287-1311.)337 Figure 9.5. A horizontal radar depiction of rainbands. Low-level flow is also shown. (From Barnes, G. M., E. J. Zipser, D. Jorgensen and F. Marks, Jr., 1983: Mesoscale and convective structure of a hurricane rainband. J. Atmos. Sci., 40, 2125-2137.) and is relatively cloud-free except for some low cumulus clouds and thin middle level cumulus clouds. Between 30 and 35 km outward from the center of the eye is the eyewall that is characterized by rapid ascent of the air rushing in toward the storm center. The eyewall tilts radially outward with height at an angle of about 30 degrees from the horizontal and, along with it, so also does the radius of maximum winds (RMW). The strongest radar reflectivity is located several kilometers outside the radius of maximum winds, while the maximum updrafts lie several kilometers inward of the RMW. The region beyond the eyewall cloud is characterized by stratiform cloud and precipitation. The local regions of high radar reflectivity called338 bright bands, near 5 km height and outward from 45 km are due to melting of stratiform precipitation. Beyond the eyewall region, the predominant mesoscale features of the tropical cyclone are spiral bands of clouds and rainfall. The rainbands are typically 5 to 50 km wide and spiral inward toward the center over radial distances of 100 to 300 km. Cumulus clouds tend to form on the inside of the bands and move cyclonically (counter-clockwise in the northern hemisphere) around the storm center. The convective elements typically move outward from the storm's central regions during the formative