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Appendix D Drawings and Descriptions of Weather Phenomena The following drawings illustrate weather phenomenon features and, where applicable, show typical reflectivity patterns as displayed on weaker radars. The drawings are listed in the order Mat weather phenomena are presented in Tables 2-! and 2-2. HURRICANE 0 50 1 no km o 15 _ 67.5 \,. FVFWAI I I ~ ~ W~ ~100km Figure D-1 The hurricane eye wall. This is the area of tall cumulonimbus storms surrounding the eye of the storm. Heavy rain and very high winds occur in the eye wall. The "eye of the storm" (hurricane, typhoon) is the roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Based on data from Marks, 1990. 96
Weather Phenomena 97 MESOCYCLONE, HOOK ECHO HOOK ECHO ~ ~\ ' MESOCYCLONE R ~ / f j/: E A co / / , 08 km Figure D-2 Mesocyclone and hook echo. The mesocylone is a horizontal atmospheric rotation on a scale between 4 km and 400 km (Fujita, 1981). The hook echo is a pendant, curved-shaped region of reflectivity caused by precipitation being drawn into the cyclonic spiral of a mesocyclone (Davies-Jones, 1985). The hook echo is a fairly shallow feature, typically extending only up to 4 km in height. 10 A_ 1 1 1 1 0 10 20 30 SUPERCELL o 40 km ° MINI-SUPERCELL 30 km , 15- ul J C, A ce c, 10 In g 5 As Oh O - n A Figure D-3 Supercell and mini-supercell. The supercell is potentially the most dangerous convection storm type. The supercell may produce high winds, large hail, and long-lived tornadoes over a wide path. In its purest form the supercell consists of a single quasi-steady, rotating updraft that may have a lifetime of several hours (Weisman and Klemp, 1986). The radar-identified rotation typically has a diameter of 4 km to 12 km. The mini-supercell contains similar severe weather characteristics but the storm is significantly smaller in height and width. The diameter of the radar-detected rotation is 1 km to 8 km. This is a relatively new storm type whose existence has been confirmed by data from the new Doppler radars in the eastern half of the United States. Differentiating on a scale is useful here because of the greater difficulty in detecting these smaller rotations. Based on data from Burgess and Lemon, 1990; Davies-Jones, 1985 .
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Weather Phenomena 99 MICROBURST 3 E 2- n--/ ./ .' ' I\\-- 1 , ' SURFACE MICROBURST :): 0 1 2 3 4km / E 12 8- 4 - , __ O -_ , : ~-ooC ___ 4 > on - ----on HIGH RAINFALL MODERATE RAINFALL VERY LOW RAINFALL RATE RATE RATE Figure D-6 Microburst. The microburst is a strong down draft that induces an outburst of damaging winds on or near the ground. Damaging winds, either straight or curved, are highly divergent. The damaging winds extend over an area of less than 4 km and typically last only a few minutes. This scale of diverging winds has been found to be particularly hazardous to aircraft landing or taking-off. Microbursts can be associated with widely varying surface rainfall rates. Based on data from Fujita, 1981, 1985. MACROBURST sow ECHO W/,.. km A. MAcRosuRsT 50 C 0.~1 km ~ ~ ~ ~~ DEEP B. Figure D-7 Macroburst. The macroburst is similar to the microburst except the damaging winds extend over an area greater than 4 km and may last for tens of minutes. The term "downburst. includes both microbursts and macrobursts without reference to scale. Side A. Reflectivity signature-Several reflectivity patterns have been associated with macrobursts. One of the most common is the Bow echo" (depicted on the leR side of the figure), or region in a line of thunderstorms that bulges ahead of the line, and is associated with damaging surface winds, and occasionally tornadoes. Side B. Velocity signature-The velocity signature of a macroburst shows a broad pattern of approaching and receding velocities associated with the strong divergent storm outflow, depicted on the right side of the figure. Based on data from Fujita, 1981, 1985.
100 CONVECTIVE RAIN SCATTERED THUNDERSTORMS e-e t. J ,0 80' km 12 Y 9 6 Al 3 ~' SQUALL-LINE THUNDERSTORMS 0 80 I ~ J km .... Appendix D TIME EVOLUTION OF A THUNDERSTORM 0 5 10 TIME (min) 0 10 20 km I 1 1 Figure D-8 Convective rain. Convective rain is associated with convective clouds or cumuliform clouds characterized by vertical development in the form of rising mounds, domes, or towers. Based on data from Byers and graham, 1949; Burgess and Lemon, 1990. STRATIFORM RAIN, STRATIFORM SNOW '~ STRATIFORM RAIN 300 km MELTING LEVEL OR BRIGHT BAND Figure D-9 Stratiform rain. Stratiform rain is horizontally widespread in character, typically associated with macroscale fronts and pressure systems. Stratiform snow has the same features as Stratiform rain except precipitation is in the form of snow. Based on data from Weber and Wildrotter, 1981; Marks, 1990.
Weather Phenomena 101 LAKE EFFECT SNOW WIND DIRECTION Ontan~ 0 2 km Figure D-10 Lake-effect snow. Lake-effect snow is localized snow that occurs over and in the lee of lakes. Lake-effect snow is caused by relatively cold air flowing over warm water. In the United States, this phenomenon is most noted along the south and east shores of the Great Lakes during arctic cold-air outbreaks. HAIL ~ I >-~1 15 r ~ ~7 i--- 1-| ~ | C, too of j3 ~) STORM MOTION I 7ARKERREGIONSINDICATE l 5 1 ,sJII/ - III i MORE SEVERE DAMAGE l O 1l l HAIL DAMAGE REGION ~ if km,` HAIL RAIN Figure D-11 Hail. Hail is precipitation in He form of balls or irregular lumps of ice that is always produced by convective clouds, nearly always cumulonimbus. An individual unit of hail is called a hailstone. By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin may be classed as ice pellets or snow pellets. The figure on the left shows a large hail damage region, and He figure on the right shows a vertical cross-section of the hail shaft in a supercell storm. Based on data from Burgess and Lemon, 1990. CONVERGENCE LINE 2 n . 'me ~ \ ........... ~Ji Hi\ 1 1 1-5 km Figure D-12 Convergence line. The convergence line is a horizontal line along which horizontal convergence of the air flow is occurring. Common forms of convergence lines are sea-breeze fronts, cold-air outflows from thunderstorms, and macroscale fronts. The left side of the figure shows a convergence line ahead of a line storm caused by the pooled cold-air outflow from many storm cells in the line. The figure on the right shows a vertical cross-section of the leading edge of the convergence line. Based on data from Klingle et al., 1987.
