Click for next page ( 82


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 81
7 INTRODUCTION Thunderstorm Origins, Morphology, and Dynamics EDWIN KESSLER NOAA National Severe Storms Laboratory Thunderstorms involve rapid vertical rearrangement of deep air layers. Large processes promote and shape vertical and horizontal air motions, and processes within storms control development of rain, hail, and strong local winds. Espy (1841) first presented the con- cept that thunderstorm circulations are driven largely by the heat latent in water vapor, released during con- densation. During 1946-1947, a U.S. federally funded project investigated thunderstorms in Ohio and in Flor- ida. Findings of that project (Byers and graham, 1949) were the first accurate description of thunderstorm de- tails, and project methods were a principal part of the foundation of subsequent studies. A modern compre- hensive and abundantly illustrated textbook was pre- pared by Ludlam (1980~. Thunderstorm processes are presented in much detail, with many references, in Vol- ume 2 of a recent three-volume treatise on thunder- storms (Kessler, 1985), and the lightning process is de- tailed elsewhere in this volume. VERTICAL CONVECTION AND THE GREENHOUSE EFFECT Vigorous vertical air currents and thunderstorms are a consequence of excessive warmth and moisture at low 81 altitudes. In a global sense this stratification of proper- ties stems largely from the radiative properties of atmo- spheric constituents. The atmosphere is largely trans- parent to solar radiation, which is concentrated in the yellow region of the spectrum. Most of the solar energy not reflected by clouds passes through the atmosphere unabsorbed and is converted to heat at the ground or in surface waters. The absorbing surface also radiates, but because its temperature is much lower than the Sun's, terrestrial radiation is concentrated in infrared wave- lengths. Atmospheric carbon dioxide, water vapor, and some other trace constituents are significant absorbers of much of the outgoing infrared emission, so this outgo- ing radiation is substantially blocked and further heats the air at low altitudes. The average temperature at low altitudes provides long-term equality between the rate at which heat is carried away and the rate at which it is received. Re- moval processes include some radiative losses through spectral windows between absorbing bands of carbon dioxide and water vapor and transport of heat by air parcels in motion. This latter process is known as con- vection. Heat gained at the surface is carried by convec- tion to altitudes where the radiative process is more ef- fective because there is less absorbing medium above. Figure 7.1 shows that, globally, thunderstorms are most frequent in low latitudes where a larger surplus of heat

OCR for page 81
82 FIGURE 7.1 Annual number of thunder- storm days. Adapted from World Meteoro- logical Organization (1953~. 150 120 (due to radiative imbalance) is transported aloft by con- vection. The process of vertical convection within an air mass contrasts with a global process driven by horizontal temperature differences on a very large scale. Thus, equatorial regions are warmer than polar regions, and there are various circulation regimes that function to re- duce this temperature contrast. These circulations, pri- marily horizontal, associate with some vertical air mo- tions as well, and they tend sometimes to enhance and sometimes to diminish the conditions that favor the in- tense vertical circulations of thunderstorms. THE SIZE OF THUNDERSTORMS About 85 percent of the atmospheric mass is con- tained in the lowest 13 km this is the layer that gener- ally participates in the vertical circulation of a typical thunderstorm. The typical horizontal dimension of a thunderstorm is similar to its vertical dimension. On the other hand, the storms of winter, essentially embodying horizontal air motions, respond to variations of temper- ature in the horizontal plane extending over hundreds or even thousands of kilometers; these storms are corre- spondingly larger horizontally than vertically. The vertical circulation of air in a thunderstorm is a result of density differences. When the air is relatively warm, it is relatively expanded and weighs less than a parcel of the same size in its surroundings. The warm parcel is subject to pressure forces; these vary with height according to the weight of air in the large envi- ronment, and they are not fully balanced by the warm parcel weight. The imbalance is represented by a net upward force, Archimedean buoyancy, which causes the warm parcel to rise. As it rises, the air ahead of it EDWIN KESSLER 70p 30 TO 30 _ fi~ 70 , , , << 1 ~ ~ ' ' ' ' 1 80 150 120 90 60 30 0 30 60 90 120 150 180 must move out of the way. The greater the horizontal extent of a rising parcel, the farther the displaced air must move and the more resistance there is to the par- cel's motion. Very large buoyant parcels cannot rise rap- idly because of the large volume of air that must be si- multaneously displaced horizontally. Thus, vertical motions are faster in smaller rising parcels. On the other hand, the smallest buoyant parcels tend to be more eroded by diffusive processes operative on their bounda- ries. In the final analysis, more definite conclusions as to scale rest largely on empirically determined characteris- tics of diffusion and consideration of equations of three- dimensional motion. Figure 7.2 shows a zone of severe thunderstorms ex- tending hundreds of miles across the central United States. The large horizontal extent of the zone is related to the large scale of a disturbance engendered primarily by horizontal temperature contrasts. Along the zone, numerous boundaries between individual thunder- storms are evident. They mark the dimensions of local storms driven primarily by vertical temperature con- trasts, locally enhanced by processes intrinsic to the larger-scale disturbance. Conditioning the Air for Overturning A proclivity for overturning is enhanced as the lower atmosphere is warmed. This occurs daily after sunrise, and afternoons are often marked by puffy clouds indica- tive of rising thermal currents. Seasonally, temperature rises as day length increases, and as the Sun rises higher in the sky, so spring and early summer usually present more frequent and more vigorous showers and thunder- storms than does fall. In other words, the thundery weather that tends to be frequent around the vernal

OCR for page 81
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS FIGURE 7.2 Photograph in visible light from synchronous Meteorological Satellite SMS-1, 6 May 1975, 1405 CST. A major squall line harboring tornadoes lies across the central United States. Lakes Superior, Huron, and Michigan are in upper right. Photo courtesy of NOAA, National Environmental Satellite Service. equinox in mid-latitudes is stimulated by the relative coolness aloft that remains from winter. A rising air parcel encounters decreasing air pressure and expands. This expansion represents work done on the environment and is accompanied by a decline in temperature. A rising parcel continues to rise as long as it remains warmer than its environment-this means that the ambient temperature must decline with height as fast as or faster than the temperature of the rising parcel declines as its altitude increases. A shower or thunderstorm occurs only when there is a sufficient de- cline of ambient temperature with height in a deep layer. There would be no rain without moisture in the air,

OCR for page 81
84 but moisture represents energy as well a large amount is latent in water vapor and enters the atmosphere as surface waters evaporate. Most of the air that harbors thunderstorms is conditioned during days or weeks over warm seas. During the westward journey of an air mass in the North Atlantic trade-wind zone, for example, the depth of the moist layer increases daily and is often about 2000 m thick when the air turns northward and enters the United States. The low-altitude moist layer may be surmounted by a very dry layer. This stratifica- tion is produced even as heat and moisture are supplied from below, while weak subsidence, associated with the same subtropical high-pressure area that engenders the trade winds, maintains dry air at middle levels, pro- tected from incursions of moisture from below by a tem- perature inversion. Dry air aloft may also be produced by other processes. For example, precipitation that ac- companies ascent of air on the windward side of a mountain range leaves the air drier on the leeward side. A warm temperature and large quantity of water vapor in an air mass at low altitudes and only a small amount of water vapor aloft, is a significant combination (known as a connectively unstable condition) whose po- tency is most realized when a large-scale disturbance causes generalized horizontal convergence and rising air motion. In such a case, the moist air at low altitudes cools less rapidly than the air above because the low- level air gains the heat latent in the water vapor as that vapor condenses. A result is that the temperature de- cline with height increases, i.e., the lapse rate becomes much steeper. Then rising parcels within the air mass and overturning within the air mass can become a vio- lent process. CONDENSATION AND PRECIPITATION IN RELATION TO AIR MOTION As an ascending air parcel cools below the tempera- ture at which its vapor is saturated, condensation occurs on nuclei, usually motes of sea salt, nitrates, or sulfates, numbering typically about 1000/cm3 over land. There is an initial selection or activation process whose details depend on the nature and number of the nuclei and on the ascent rate, reflected in extent of vapor supersatura- tion. Following activation, only the selected nuclei con- tinue to grow and the balance evaporate. Owing to dif- ferences in initial size and composition, growth of the different nuclei proceeds at different rates. If ice coexists with supercooled liquid water, i.e., liquid at tempera- ture below the normal freezing point, there is growth of the ice particles by deposition of vapor diffused from the liquid particles, which tend to evaporate. Brownian motion, turbulence, and differential fall speeds among EDW I N KESS LER the particles all contribute to coalescence and further broadening of the size distribution. After some tens of minutes, unless reduced in size by evaporation in dry air mixed into the cloud from its environment, some of the particles have appreciable fall speeds. Whether ice or liquid, they now begin to sweep out the smaller particles in their paths and fall ever more rapidly to the ground. Convective precipitation is typically intermittent. Its fluctuating character at a place is related not only to the passage of discrete shower cells overhead but also to a dual effect of the condensation process. The condensa- tion process releases latent heat, which tends to reduce air density and stimulate the updraft, but the condensa- tion process also burdens the rising air with the weight of the thousandfold-denser condensate particles, which may be water or ice. Unless the updraft speed is mark- edly stronger than the fall speed of precipitation parti- cles, precipitation accumulates in the updraft until the positive effects of thermal buoyancy are quite over- whelmed. Thus an initial updraft usually becomes a downdraft after about 30 minutes, as indicated in Fig- ure 7.3. The downdraft can be quite strong, especially when the weight of precipitation is augmented by a cooling that accompanies evaporation of cloud and pre- cipitation into dry air mixed into the cloud at middle levels. Such downdrafts necessarily become divergent horizontally as they near the Earth's surface, where they represent a significant threat to aviation. In some severe storms, the weight of accumulated condensation products contributes to a splitting process that leads to separate storms, moving to the left and right of the direction of the vertically averaged wind. In the northern hemisphere, the rightward-moving storms are more likely to harbor tornadoes; damaging hail is often found in left movers. E -12 c o 4,, 8 o D ~ 4 ._ I CU M U L US MATURE DISSIPATING ~;tftilIT)~\l /: ~ ~,'',//,~D,~;,, ,; '," ~ In,/' ;_1~. Did;_ I at 1 6km FIGURE 7.3 Three stages in the life cycle of an ordinary thunder- storm cell, as observed by the Thunderstorm Project. After Byers and Braham (1949).

OCR for page 81
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS STRONG CONVECTION Development of a severe convective system depends on a stratification of temperature, moisture, and hori- zontal wind that produces extreme buoyancy forces tending to accelerate the motion of both ascending and descending air parcels. A steepening of the temperature- lapse rate is not alone sufficient, because there are many situations in which the energy of overturning can be dis- sipated in minor events about as fast as it becomes avail- able through solar heating or other processes. Regimes of frequent events of weak to moderate intensity charac- terize air masses that are moist and weakly unstable through great depths. Usually a combination of the following conditions is associated with extreme convection in middle latitudes: (a) The airmass is connectively unstable. That is, there is a considerable lapse rate of temperature, and moisture is abundant only at low altitudes. (b) A disturbance in the larger-scale flow (e.g., a short-wave trough in the westerly current aloft, often having a marked low-pressure system at the Earth's sur- face) provides generalized lifting, which causes the con- vective instability to become realized. (c) The moist lower layer is separated from the dry zone by a stable layer or even a temperature inversion, which inhibits early overturning within the airmass and premature loss of potential energy. (d) There usually is differential temperature advec- tion (warm air advection at low altitudes and, occasion- ally, cold air advection aloft). (e) A marked increase of the wind with height has a dual enhancing effect. First, warm air from convective towers is carried rapidly downstream by the strong winds aloft, preventing its local accumulation aloft and permitting longer duration of local convection; second, variation of the wind with altitude can cause the up- draft column to slope with height and contribute to an organization of the flow that enables the intrinsic cool- ness of the air at middle levels to be manifested in overall storm energy. In brief, the precipitation formed and carried to great heights in strong updrafts can descend into intrinsically cold middle-level air, hastening that air's descent. In this case, where precipitation does not descend in the updraft in which it was formed, the storm may acquire a quasi-steady character. The conditions described above are often associated with weather-map features like those shown in Figure 7.4. Figure 7.5 shows a schematic vertical cross section through a quasi-steady severe storm that might form un- der the conditions depicted in Figure 7.4. The more de 85 / ~N~ \ Upper Ridge ~km ~ ~ `~- l o 200 , ~ FIGURE 7.4 Idealized sketch of a middle-latitude weather situation especially favorable for development of severe thunderstorms. Thin lines denote sea-level isobars around a low-pressure center with cold and warm fronts. Broad arrows represent low-level jetstream (LJ), polar jet (PJ), and subtropical jet (SJ). The LJ advects moisture-rich air from subtropical regions to provide the basic fuel for convection. Se- vere storms (hatched area) are most likely to start near I and gradually shift toward the east while building southward. Severe thunderstorms also occur with many variations on this basic theme. From Barnes and Newton (1985). tailed plan view of such a storm (Figure 7.6) illustrates two downdrafts. The forward-flank downdraft in the rain area is largely an effect of the weight of condensa- tion products; the rear-flank downdraft is thought to owe its existence in part to a barrier effect on the ambi- ent winds produced by ascent of air from low altitudes in a shearing environment. Another cause of the rear- flank downdraft is evaporative cooling of air, dry and intrinsically cold at heights of 3 to 4 km, by precipitation descending into it from greater altitudes. Finally, Brandes (1984) proposed a mechanism by which this rear-flank downdraft would be stimulated by formation of a tornado or other vortex at low altitudes. Figures 7.5 and 7.6 show asymmetries that are critical features of persistent severe-traveling-storm complexes; Figure 7.3, in contrast, presents more symmetrical fea- tures, especially in its first and third frames. South of the severe-storm center in Figure 7.6, where a mesocyclone and possibly a tornado are located, a line of convective clouds (the flanking line) marks the intersection of air descended from middle levels with warm air rushing to

OCR for page 81
86 km mb 16 - 100 14 _ 12 ~ 200 10 _ - 300 8 _ 6 500 4 _ - 700 2 _ 0 _ 1000 EDWIN KESSLER l 1 ~ ~ . ~ 0 10 20 30 40 km ~J~ 1~g =,,, PRECI P ': `' at. PRECI ~ ~ ~ ~ ALOFT ', - ~ ~ ~ ~ MID-LEV ~: ~Hi_ , ~ ~U_) PREC`p, ~ INFLow57 \ ~ \) Em \MDT-7~ FIGURE 7.5 Profile of squall-line thunderstorm (based on series of radar observations, fivefold vertical exaggeration) as it passed Oklahoma City on May 21, 1961. Winds (full barb, 5 m/see; pennant, 25 m/see) are plotted relative to squall-line orientation; a shaft pointing upward is parallel to the squall line (azimuth 225~. In sounding behind storm, balloon passed through anvil outflow, and winds shown are not representative of the environment. Arrows indicate main branches of airflow relative to squall line that was moving toward right at 11 m/sec. On the right, dashes outline the supposed air plume; the radar-detected cloud plume at lower elevations consists of small precipitation particles that have partly fallen out of the air plume while drifting downwind from the storm core. Simplified from Barnes and Newton (1985~. ward the storm along the ground. At the Earth's sur- face, the intersection is called the gust front. It can itself be hazardous to aircraft because of remarkable turbu- lence and wind shears, and its horizontal winds are sometimes strong enough to do considerable damage to trees and buildings. HAIL At temperatures below the melting point of ice, solid and liquid phases can coexist, but the liquid phase is metastable and starts to freeze with release of latent heat in the presence of a suitable nucleus or on contact with r an ice surface. Growth of hail to large sizes occurs when strong and enduring updrafts, with temperature below the melting point of ice, bear liquid cloud particles in addition to some icy motes. The liquid particles start to freeze when contacted by the ice particles, and the latter thereby grow; they descend to the ground when their fall speed exceeds the rising speed of the enveloping air current. The release of latent heat that attends freezing causes a rise of temperature toward 0C; a growing hailstone thus tends to be somewhat warmer than its environ- ment. (This condition of relative warmth is associated with an important electrification process treated in Chapters 9 and 10, this volume.) In the presence of much liquid water and little supercooling, the hailstone may enter a stage of wet growth, i.e., one wherein all the impacting liquid is not frozen immediately but be- comes absorbed into a previously developed porous structure or is somewhat shed. In this growth regime, the frozen material exhibits a clear structure of large crystals, readily distinguishable from each other under polarized light. At sufficiently low temperatures and water contents, liquid cloud and small raindrops im- pinging on the growing hailstones freeze so quickly that air bubbles remain entrapped within and between the globules. The variable growth process is revealed by the concentric translucent or opaque layers and nearly transparent zones that appear in hailstone sections (Fig- ure 7. 7) . The magnitude and distribution of vertical and hori- zontal air currents and the associated content of super- cooled liquid water determine the growth and trajec- tory of hailstones. If the updraft speed has a maximum

OCR for page 81
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS ,~ STORM MOTION N \ ~\ BOUNDARY OF RAIN EMS< \ \AND REFLECTIVITY Ad\ FEEDER CELLS ~ ' \\ \ \ ~j ~ GUST FRONT ~ \ \\ - O 5 \ FIGURE 7.6 Schematic plan view of a tornadic thunderstorm at the surface. The heavy solid line encompasses radar echo. The wavelike gust front structure Is depicted by a solid line and frontal symbols. Surface positions of the updraft are finely stippled; forward-flank downdraft (FED) and rear-flank downdraft (RF13) are coarsely stip- pled; arrows represent associated streamlines (relative to the ground). Likely tornado locations are shown by encircled Ts. From Davies- Jones (1985~. value of 30 m/see, and the cloud content is 4 g/m3, a hailstone can grow to about 7.5 cm in diameter before arriving at the ground, 25 minutes after being selected for such growth by having fortuitously attained a larger size than its neighbors. The rapid ascent of small particles in sufficiently strong updrafts gives insufficient time for the growth of any of them to large size. The development of hail in such strong updrafts may depend on the insertion into the updraft of hail embryos formed nearby and cycled into the updraft column by virtue of their descent from higher levels into horizontally convergent regions be- low. Major tornadoes are usually accompanied by large hailstones. However, we find that tornadoes are usually absent from the storm class that includes the most dam- aging hailstorms. Doppler-radar observations of the air- flows show that the hailstorm updrafts are not so strong as updrafts in tornadic storms, but the updrafts in major hailstorms cover a substantially larger area. 87 TORNADOES Major tornadoes are most often associated with thun- derstorms of a type illustrated in cross section in Figure 7. 6, and they are identified by a rapidly rotating funnel- shaped cloud that marks the condensation boundary of in-spiraling air at low altitudes undergoing adiabatic expansion and cooling. Tornadoes are most severe and least uncommon in the United States, but they occur oc- casionally in India? Australia and New Zealand, South Africa, Argentina, Japan, and several countries of west- ern Europe. In Mississippi, the state in the United States most subject to tornado deaths and damage, statistics on annual damage and storm frequency suggest that about 1/1000 to 1/300 of the area is affected by tornadoes each year, with tornado winds of 50 m/see or more and signif- icant damage to structures. The maximum rate of pres- sure change may be between 50 and 100 mbars/sec, and the maximum wind of a major tornado is about 100 m/ sec. The frequency and intensity of these storms and the areas visited vary from year to year in association with irregular departures from seasonal norms of other quan- tities such as temperature and moisture. Radar data show that tornadoes start at middle levels (about 5 km), to the rear side of pre-existing cyclonic circulations about 3 to 10 km in diameter, and develop downward and upward on a time scale of about half an hour. At the ground, tornadoes appear on or near a boundary between rising warm moist air, within which the release of latent heat of condensation is the storm's principal source of energy, and air descending from middle levels where it is intrinsically cold, and cooled sensibly by the evaporation of precipitation into it. The most critical dynamical aspect of tornadoes in- volves the concentration of rotation within them. Vari- ous investigations during the past 10 years have estab- lished that two processes have direct importance in this concentration. The air's angular momentum is a conse- quence of the Earth's rotation and of various weather systems. The conservation of angular momentum ac- companying horizontal convergence and ascent of air has long been appreciated. Such conservation is mani- fested, for example, by the increased rotation rate as a skater's arms are brought in from an outstretched posi- tion. The second process, more recently detailed, in- volves the rotation or twisting of horizontal voracity to- ward the vertical plane. Horizontal vorticity is represented by the vertical variation of the horizontal wind (vertical wind shear), already cited as significant for severe-storm development through its role in facili- tating removal of condensation products from the up- draft. The twisting process is commonly effective on an

OCR for page 81
88 FIGURE 7.7 Cross section through stone. Alternating clear and opaque layers mark growth under different conditions of temperature and rate of accretion of liquid water. Photo courtesy of Charles Knight and Nancy Knight, National Center for Atmo- spheric Research. :'' .. . Aft- At.. ~ i: ail-;<, ~ ~i^ t~ ~r. .j,:. `,5.! updraft boundary when the winds at low altitude rela- tive to the storm motion veer markedly with height. The properties related to these processes vorticity, circulation, and angular momentum are interrelated through considerations of area and distance. Thus the circulation in a region is the average velocity along the closed curve that defines the region times the length of the curve. This is the same as the average component of vorticity normal to the same region times the area of the region. The angular momentum of a particle or air par- cel is measured about a reference point as the distance from the point times the velocity normal to the line con- necting the particle and reference point. CONCLUDING REMARKS The thunderstorm entity is a result of thermodynami- cal, microphysical, and electric processes. All processes interact and must be observed contemporaneously in or- der to be well understood. The summary presented in the foregoing pages represents remarkable progress in understanding, a result of public investment in a fo EDW I N KES S LER ~. J~ . I- -:.', A, He ^ Hi.. _. my. _ {.. ~ ". - E ~.- ~s .... :, .. .. .. ; ^.., > cused use of new tools during the past two or three dec- ades. Meteorological radar, artificial satellites, and marvelous new technologies for data processing, com- puting, and communicating have all been critically im- portant aids and have facilitated impressive new meth- ods for early identification of storms, dissemination of information about their location, movement and inten- sity, significant reduction in death rates from tornadoes, and marked decline in the rate of weather-related air- craft accidents. Now there are a host of new methods for study of the lightning process. As past is prologue, we confidently expect these new aids to contribute much toward clarification of the function of electric processes in severe-storm evolution and toward diminishing the still significant lightning hazard. ACKNOWLEDGMENT The author thanks Evelyn Horwitz, who typed this paper through several drafts, and Lindsay Murdock, who improved the paper editorially.

OCR for page 81
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS REFERENCES Barnes, S. L., and C. W. Newton (1985). Thunderstorms in the synop- tic setting, in Thunderstorm Morphology and Dynamics, E. Kessler, ea., 2nd edition, Univ. of Oklahoma Press, Norman, pp. 75-112. Brandes, E. A. (1984). Relationship between radar derived thermody- namic variables and tornadogenesis, Mon. Weather Rev. 112, 1033- 1052. Byers, H. R., and R. R. Braham (1949). The Thunderstorm, Report of The Thunderstorm Project, U.S. Weather Bureau, U.S. Govern- ment Printing Office, Washington, D.C., 287 pp. . 89 Davies-Jones, R. P. (1985). Tornado dynamics, in Thunderstorm Mor- phology and Dynamics, E. Kessler, ea., 2nd edition, Univ. of Okla- homa Press, Norman, pp. 197-236. Espy, J. P. (1841). The Philosophy of Storms, Charles C. Little and James Brown, Boston, Mass., 552 pp. Kessler, E., ed. (1985). Thunderstorm Morphology and Dynamics, 2nd edition, Univ. of Oklahoma Press, Norman, 415 pp. Ludlam, F. H. (1980). Clouds and Storms, Pennsylvania State Univer- sity Press, University Park, Pa., 405 pp. World Meteorological Organization (1953~. World Distribution of Thunderstorm Days, WMO no. 21, part 2, Geneva, Switzerland, 71 pp. and maps.