Click for next page ( 37


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 36
S. . fores OVERVIEW* It is clear that nuclear explosions can ignite large-scale fires (Broido, 1960. In addition, it has been estimated that the smoke emissions from nuclear-initiated fires could produce major atmospheric perturbations (Lewis, 1979; Crutzen and Birks, 1982; Turco et al., 1983a,b). Only two nuclear explosions have ever occurred over populated areas (Hiroshima. Auoust 6. 1945. and Nagasaki. Auoust 9 . _ _ _ _ _ ~ ~ , , _ , _ _ , ~ , _ _ , , 1945); In each case, a clty-slzea conflagration resulted. At Hiroshima, a ~12-kt weapon caused a mass fire over an area of ~13 km2, essentially the entire central city (Ishikawa and Swain, 1981~. At Nagasaki, where high terrain shadowed large regions of the city from direct irradiation by bomb light, a ~20-kt device burned ~7 km2 (Ishikawa and Swain, 1981~. It is difficult to extrapolate the effects of these two isolated events, which involved <40-kt total yield, to the possible effects of a global nuclear exchange involving 6500 Mt. Nevertheless, a logical sequence of steps can be taken to obtain estimates of the areal extent and particulate emissions of fires initiated in a full-scale nuclear war: 1. Review historical fire experience to assess the probability of ignition and spread of large fires. 2. Define the effectiveness of nuclear explosions for initiating fires in urban and forest settings. *In the text, the following symbols are used: a, approximately equal to; ~, of the order of; , greater than or of the order of. tFor the purposes of this report, large-scale fires can be classified as "mass fires, n in which many individual fires burn simultaneously over a large area, "conflagrations, n in which the fire is most intense along a line of propagation, "firestorms, n in which the entire area of the fire burns intensely and strong winds blow inward from all directions, and "fire whirls, n in which a firestorm plume develops an unusually strong vorticity. 36

OCR for page 36
37 3. Determine the burdens and distributions of combustible materials around potential nuclear targets. 4. Evaluate data on the quantity and physical properties of smoke generated by common fuels. 5. Consider mass fire dynamics to determine the likely heights and rates of injection of the smoke. 6. Describe a scenario for the locations, yields, and heights of nuclear detonations (See Chapter 3, The Baseline Nuclear Exchangers. 7. Combine the foregoing information to estimate the total quantity and optical characteristics of nuclear war smoke emissions. These topics are discussed in subsequent sections of this chapter. On the basis of such an analysis, an approximate equation can be written that emphasizes the important factors that enter into the estimation process, E = YfA0m0fbe x 101, where E is the total smoke emission (in grams), Yf is the total explosion yield (in megatons) in air bursts that effectively ignite fires, AD is the average area ignited by each megaton of yield (in square kilometers per megaton), my is the average loading of flammable materials (in grams per square centimeter), fb is the fraction of my burned, and is the mean smoke emission factor (grams of smoke per gram of material burned). The factor of 101 converts square kilometers (Ao) to square centimeters. The key parameter values that apply to the baseline nuclear war scenario are given in Table 5.1. The total smoke emission calculated for the baseline case is ~180 Tg (1 Tg = 1012 g ~ 106 metric tons), or ~0.7 g/m2 averaged over the northern hemisphere. Since the specific extinction (scattering plus absorption) coefficient of many smokes at visible wavelengths is ~5.5 m2/g, the hemispherical average optical depth* in this case is ~4. Of course, if the smoke were confined to the northern mid-latitude zone, the optical depth would be ~2 to 3 times larger, or ~8 to 12. A more detailed discussion of these estimates follows. The optical and climatic effects of the smoke are discussed in Chapter 7. PRESENT-DAY SMOKE EMISSION AND REMOVAL It is estimated that the current global smoke emission to the atmosphere is ~200 Tg/yr (Seller and Crutzen, 1980; Turco et al., 1983a,c). The graphitic carbon fraction is about 5 to 10 percent by *The optical depth is a dimensionless quantity that determines the light transmission properties of a layer of gas or aerosols. If the layer has an optical depth T. e ~ is the fraction of a beam of light perpendicularly incident on the layer that suffers no scattering or absorption in passing through the layer. The total light transmitted consists of the direct light plus a scattered (diffuse) component.

OCR for page 36
38 TABLE 5.1 Baseline Nuclear War Fire and Smoke Parametersa Parameterb Urban Fires Forest Fires Yf (Mt) 10001000 AD tkm2/Mt)250250 me (g/cm2)42 fb g/g)C 0.75 0.20 0.02 0.03 aExcursions from the baseline case, and uncertainties in the baseline parameters, are discussed in the text. bYf is the effective ignition yield in megatons, An is the average ignition area per megaton, my is the burden of combustibles per unit area, fb is the fraction of the combustibles burned, and ~ is the net smoke emission factor per unit of fuel, assuming in the case of urban fires that 50 percent of the smoke is promptly scavenged and removed from the plumes mainly as black rain. n CThe smoke consists of 20 percent graphitic carbon (soot) by mass, and 80 percent transparent oily compounds. NOTE: Urban fire smoke emission: Eu = 150 x loll g Forest fire smoke emission: Ef = 30 x 1012 g Total smoke emission: Et = 180 x 1012 g mass. The primary sources of smoke are agricultural burning, fossil fuel combustion, and wildfires. The important characteristics of background smoke emissions that distinguish them from "nuclear" fire emissions are as follows: 1. The smoke emission factors are low in relation to the quantity of fuel burned, because most of the burning takes place under controlled conditions. 2. The overall graphitic carbon component is low, because most of the smoke is generated during the prescribed combustion of natural cellulosic materials. 3. Almost all of the smoke is injected into the lowest 1 km of the atmosphere, because the sources are small in horizontal scale and/or total power. 4. The smoke emissions occur in diverse locations throughout the course of a year, which prevents significant concentrations from building up. 5. The average atmospheric lifetime of the smoke is < 10 days (Ogren, 19821.

OCR for page 36
39 As a result of these factors, the average background concentration of airborne graphitic carbon is typically only ~ 0.1 ug/m3, and its integrated vertical absorption optical depth is <0.01 (Charlson and Ogr en, 1982; Turco et al., 1983c). Over a period of about 1 month, background smoke emissions would be negligible in comparison with the estimated smoke emissions of a nuclear war (Turco et al., 1983a,b; Crutzen et al., 1984~. Removal of smoke and soot from the atmosphere occurs mainly through precipitation scavenging. Smoke particles have sizes of about 0.1- to 0.5-pm radius, at which sedimentation is negligible and dry deposition is very inefficient (Slinn, 1977; Sehmel, 1980~. In the background atmosphere, soot is usually found as a minor component of hydroscopic sulfate aerosols. This suggests removal by efficient scavenging of the hydroscopic aerosols in and below clouds (Radke et al., 1980; Ogren, 1982; Turco et al., 1983c). The arctic haze that forms in winter and spring is known to contain soot (Rosen and Novakov, 1983~. The haze is (relatively) highly absorbing because of the soot (Patterson et al., 19821. The seasonal conditions that lead to the formation of the winter polar vortex create a stable air mass with low precipitation in which carbon emissions produced by combustion can remain suspended for several months. This demonstrates that under some meteorological conditions, particularly with the suppression of precipitation, smoke and soot can have an extended atmospheric lifetime. Generally speaking, it is expected that smoke from nuclear-initiated fires would have a longer atmospheric lifetime than background smoke (notwithstanding prompt scavenging in the fire plumes), because of its greater heights of injection. This point is expanded in subsequent sections. HISTORICAL FIRE EXPERIENCE Human experience with mass fires and firestorms includes urban conflagrations triggered by natural disasters (e.g., earthquakes), wartime city fires initiated by incendiary and nuclear bombing, massive wildfires and forest fires, and field experiments with large-scale fuel beds (Carrier et al., 1982~. Although few of these experiences are directly applicable to the nuclear war problem, all contribute to a general understanding of the properties and behavior of large-scale fires. Earthquakes Earthquakes have started urban conflagrations by breaking gas lines, exposing stored fuels, shorting electrical circuits, breaching open fires, and hampering effective firefighting. Particularly striking examples of large fires induced by earthquakes occurred in San Francisco in 1906 and Tokyo in 1923. A nuclear blast wave would have

OCR for page 36
40 similar impact and, in combination with the thermal light pulse, would represent a much greater fire threat than an earthquake. World War II The World War II saturation bombing of German and Japanese cities provided ample evidence that mass fires can be readily ignited in urban settings. The nuclear explosions over Hiroshima and Nagasaki are discussed later. The conventional bombing of cities such as Hamburg, Dresden, Darmstadt, and Tokyo produced intense fires over many square kilometers and, in some instances, triggered firestorms. From anecdotal evidence, it is known that thick, dark plumes rose from these fires to altitudes of 6 to 12 km. Within the fire zones, almost all the buildings were gutted and all combustible materials consumed. Such experiences show that, when many simultaneous fire ignitions occur among closely spaced structures and firefighting capability is suppressed, mass fires are likely to develop. Occasionally, massive urban conflagrations, such as the Great Chicago Fire of 1871, are touched off by single ignitions (Kerr, 1971~. Although such fires are not typical, they are symptomatic of the hazardous fire conditions that exist in many crowded urban centers. Forest Fires Plummer (1912), Ayers (1965), and F.E. Fendell (in Appendix 5-1), among others, have reviewed the largest forest fires of the past 160 years in which areas up to 20,000 km2 were blackened. The conditions under which these catastrophic fires developed included long drought, low humidity, and high winds (e.g., Plummer, 1912~. Clearly, such conditions are not common over large areas of the northern hemisphere during most of the year (Chandler et al., 19631. However, for the analysis of nuclear-induced fires, three general types of fire danger conditions should be distinguished: (I) fires are difficult to ignite and do not spread if ignited; (II) fires are readily ignited, but their spread is limited by factors such as humidity, moisture, topography, winds, and firebreaks; and (III) fires readily ignite and spread uncontrollably over large areas. Historical catastrophic forest fires are exclusively of type III. By contrast, most nuclear forest fires would probably be of type II. Historical fires are characterized by a limited number of ignition points, perhaps one ignition for each 50 to 500 km2 burned (Ayers, 1965~. Nuclear explosions, by contrast, can ignite forest debris instantly over a large area, with numerous ignition points developing into moderate size fires (although the probability of extensive fire spread outside of the original burning zone would be much lower--see below). The great Tunguska meteor, which fell over Siberia on June 30, 1908, provides a very rough indication of the effects that might be produced by a high-yield nuclear explosion over a forest. The Tunguska

OCR for page 36
41 event was equivalent, in terms of the blast wave, to a ~10-Mt detonation at 8-km altitude (Krinov, 1966~. (As noted below, high-yield nuclear bursts have smaller incendiary efficiencies than low-yield bursts.) Roughly 16QO km2 of Siberian forest was flattened. Eyewitness accounts describe burning falling trees" and widespread fires. A series of Russian scientific expeditions to the fall site concluded that several major fires had broken out in the central zone of devastation and burned for 5 days. From the description of the charred remains, it appears that bark and many small branches were stripped from the trees and burned, to an extent not usually observed in natural fires of that area. Experimental Fires Experimental large-scale fires have been used to study fire development and plume dynamics. Among these experiments are the Flambeau series (Martin, 1974; Palmer, 1981), the Euroka fires (Williams et al., 1970), and the Meteotron events (Desserts, 1962; Church et al., 19803. However, because the extent of these fires was only about 103 to 105 m2, extrapolation of the results to city-size fires is difficult. Of particular interest here is the height of the smoke plume in a large fire. In the experiments noted above, the plume aspect ratio (i.e., the plume height divided by the fire diameter) was always >>1, and the plumes often formed vortices penetrating to heights >1 km. (The plume aspect ratio cannot be simply scaled to larger fires. The dependence of plume height on fire size and intensity, and extrapolations to city-sized fires are discussed in later section.) The Flambeau experiments also led to the definition of a set of conditions for firestorm genesis that has been widely accepted (FEMA, 19821. The conditions include a fuel loading of >4 g/cm2, a building density of >20 to 30 percent, a fire area of >3 km2, initial fires in >20 percent of the buildings, and ambient winds of <10 km/in (Baldwin, 1968; Martin, 1974~. However, these conditions are still controversial, as they have never been tested on an appropriate scale. Moreover, in view of the atmospheric effects being considered here, it is not clear that firestorms and very intense mass fires need to be differentiated, except perhaps to refine the estimation of smoke injection altitudes (see below}. IGNITION OF NUCLEAR FIRES Thermal Phenomena In a nuclear air burst at low altitude (<10 km), about 30 to 40 percent of the energy is released as an intense pulse of visible light; about 45 to 55 percent of the energy is converted to blast pressure waves; and about 15 percent is contained in prompt and delayed nuclear radiation (Glasstone and Dolan, 1977, hereafter GD77~. Most of the

OCR for page 36
100 N - C~ - LL ~ 10 CD o X LL Ad U] N 0. \ ~ ~ \ 2 ~2 _ 1 1 1 1 , t1 1 \1 1 i 1 ~1 1\ 5 10 20 50 100 HORIZONTAL DISTANCE (km) FIGURE 5.1 Maximum radiant exposures versus ground range from a 1-Mt air burst (detonated below several kilometers altitude) as a function of the ground level visibility. The radiant exposures scale roughly with the yield in megatons. (From Kerr et al., 1971) bomb light is emitted within a few seconds for megaton yield explosions, and in less than a second for kiloton-size bursts (GD777. For a 1-Mt low air burst, Figure 5.1 shows the thermal fluences (in calories per square centimeter incident on a surface normal to the line-of-sight through the burst point) as a function of distance from ground zero, and for various atmospheric Risibilities. With a 1-Mt explosion and normal Risibilities (>10 km), the 20-cal/cm2 thermal fluence contour lies about 7 km from the explosion hypocenter, versus 9 km in a perfectly transparent atmosphere. With a 100-kt explosion, atmospheric transmission, for Risibilities of >5 km, has little effect on radiant exposures where fluences exceed 20 cal/cm2. Lower Risibilities restrict the range at which nuclear thermal effects are important. Oblique incidence of the bomb light on exposed surfaces also reduces the effective fluence. On the other hand, cloud and surface reflections enhance the radiant fluxes in localized regions.

OCR for page 36
43 As sunlight, focused by a lens, can ignite flammable materials, so can the thermal emissions of a nuclear explosion (Glasstone, 1957; Miller, 19621. Ignition data obtained during atmospheric nuclear test detonations and by laboratory experimentation are summarized in Table 5.2. At a specific thermal fluence, small nuclear explosions are generally more efficient at igniting fires than large explosions because the thermal pulse has a shorter duration and larger peak intensity (in addition, there is a lower probability of significant atmospheric attenuation over the shorter ranges involved). Newspaper, brown paper, cotton cloth, and dried plant material can be ignited by 10 cal/cm2 from a <1-Mt explosion. The perimeter of the Hiroshima fire zone roughly coincided with the 10-cal/cm2 contour. At Nagasaki, in directions unobscured by hills, the conflagration zone also extended roughly to the 10-cal/cm2 limit. In the application of nuclear weapons against ~soft. targets (e.g., urban and industrial targets), peak overpressures* of >5 psi (pounds per square inch) are often used to define the zone of assured destruction (GD77~. The 5-psi contour circumscribes an area of ~1.4 km2/kt for a 1-kt explosion (at the optimum height of burst), ~0.30 km2/kt for a 100-kt explosion, and ~0.14 km2/kt for a 1-Mt explosion. The corresponding areas enclosed within the 20-cal/cm2 thermal irradiance contours (GD77) are ~0.30 km2/kt, ~0.30 km2/kt, and ~0.25 km2/kt, respectively (in the 1-Mt case, the atmospheric visibility is assumed to be 20 km). In estimating the potential fire areas for nuclear air bursts, the committee has chosen an average ignition area of 0.25 km2/kt (250 km2/Mt) for individual explosions, which is roughly consistent with 5-psi overpressures and 20 cal/cm2 thermal fluences at the limits of the ignition region, under normal conditions of atmospheric transmission. These areas are quite conservative in relation to the areas burned at Hiroshima (~1 km2/kt) and Nagasaki (~0.35 km2/kt). The question of overlap of ignition zones for closely spaced detonations, and the total potential fire area in a full exchange, are discussed in a separate section of this chapter. Close to the hypocenter of a nuclear explosion, the thermal energies are much larder than 20 cal/cm2. Within the 30-cal/cm2 contour (about 150 km for a 1-Mt explosion), substantial quantities of natural and synthetic organic and cellulosic materials would be instantly pyrolized, and the combustible vapors ignited in a massive "flashover" fire. The rising fireball would then draw the flames and smoke toward the stem of the nuclear cloud, establishing the conditions for accelerated burning and, in some cases, the core of an incipient firestorm. For surface and subsurface nuclear detonations, the potential thermal effects are greatly reduced (although the dust and prompt radioactive fallout effects are increased). The bomb light from a *The term "overpressure" refers to the incremental static pressure above ambient atmospheric pressure (about 14.7 pounds per square inch at sea level) caused by the passage of the explosion wave.

OCR for page 36
44 TABLE 5.2 Approximate Radiant Exposures for Ignition of Various Flammable Materials for Low Air Bursts Radiation Exposurea (cal/cm2) Effect on35 1.4 20 Material Color Materialkt Mt Mt Household Tinder Materials Newspaper, shredded Ignites4 6 11 Newspaper, dark Ignites5 7 12 picture area Newspaper, printed Ignites6 ~15 text area Crepe paper Green Ignites6 9 16 Kraft paper Tan Ignites10 13 20 Bristol board, 3 ply Dark Ignites16 20 40 Kraft paper carton, Brown Ignites16 20 40 used (flat side) New bond typing paper White Ignites24b 30b 50b Cotton rags Black Ignites10 15 20 Rayon rags Black Ignites9 14 21 Cotton str ing Gray IgnitesLob 15b 21 b scrubbing mop (used) Cotton string Cream Igniteslob lob 26b scrubbing mop (weathered) Paper book matches, Ignites11b 14b 2ob blue head exposed Excelsior, ponderosa Light Ignites__c 23b 23b pine yellow Outdoor Tinder Materialsd Dry rotted wood Ignites4b 6b 8b punk (fir) Deciduous leaves Ignites4 6 8 (beech) Fine grass (cheat) Ignites5 8 10 Coarse grass (sedge) Ignites6 9 11 Pine needles, brown Ignites10 16 21 (ponderosa)

OCR for page 36
45 TABLE 5.2 (continued) Radiation Explosurea (cal/cm2 ~ Effect on 35 1.4 20 Material Color Material kt Mt Mt Construction Materials Roll roofing, mineral surface Ignites c >34 >116 Roll roofing, smooth Ignites _ c 3077 surface Plywood, Douglas fir Flaming 9 1620 during exposure Rubber, pale latex Ignites 50 80110 Rubber, black Ignites 10 2025 Other Materials ~- Aluminum aircraft Blisters 15 3040 skin (0.020 in. thick) coated with 0.002 in. of standard white aircraft paint Cotton canvas sandbags, dry filled Coral sand Siliceous sand Failure 10 18 32 Explodes (popcorning) Explodes (popcorning) 15 27 47 11 19 35 aRadiant exposures for the indicated responses (except values marked with a superscript b, see footnote b) are estimated to be valid to +25 percent under standard laboratory conditions. Under typical field conditions, the values are estimated to be valid within +50 percent with a greater likelihood of higher rather than lower values. bIgnition levels are estimated to be valid within +50 percent under laboratory conditions and within +100 percent under field conditions. CData not available or appropriate scaling not known. dRadiant exposures for ignition of these substances are highly dependent on the moisture content.

OCR for page 36
46 surface detonation is more effectively shadowed by buildings, terrain, and other obstructions than is the light from an air burst (Miller 19621. The crater ejecta may also cover nearby fuel and smother incipient fires. In a subsurface explosion (where an armored penetrating warhead is used) the thermal pulse is substantially attenuated (GD77~. Moreover, the base surge (caused by ejected material falling back upon the crater) could snuff out small fires and cover the fuel near the explosion site. Nevertheless, in a surface burst, it is still likely that primary thermal (and in cities, secondary blast-induced) fires would occur out to the -2-psi overpressure contour (i.e., over an area of about 150 km2 for a 1-Mt detonation; GD77~. In buried explosions the situation is more complicated because both ground shock and air blast could contribute to secondary fire ignitions in cities. In any case, the present baseline scenario specifies air bursts against all urban and industrial targets, with only 30 percent (1500 Mt) of the remaining bursts detonated on the surface. The fire effects of multiple nuclear detonations over cities and forests are complex and undetermined. Smoke from the fires of initial bursts could block subsequent thermal flash effects in some cases. Delayed bursts would probably spread existing fires, however, particularly by generating strong surface winds and convective plume activity. Closely spaced explosions over forests could greatly enhance the probability of fire ignition and spread. The problem of multiburst phenomena has not yet been adequately treated in the nuclear effects literature. Urban Ignition Some evidence that nuclear explosions are unique in their ability to ignite mass fires is offered by the Hiroshima and Nagasaki experiences. One crude estimate of the average energy release rate places the Hiroshima fire among the least intense of the mass fires of World War II (Martin, 1974~. Nevertheless, centripetal winds characteristic of a firestorm apparently developed, and the fuel consumption within the fire zone was nearly complete (GD77; Ishikawa and Swain, 1981~. Some of the factors that affect nuclear fire genesis in cities are summarized in Table 5.3. Even though the blast wave that follows the thermal pulse could extinguish many of the primary thermal radiation fires, a substantial number of these ignitions would continue to burn. Idealized field tests to determine the efficiency of fire extinction by pressure waves are contradictory, and often little or no effect is observed (Wiersma and Martin, 1973; OTA, 1979; Backovsky et al., 19821. In fact, in one study, the blast dispersal of burning curtain fragments through a room was a major factor in fire development (Goodale, 19711. In addition, the blast ignites many secondary fires and creates conditions (Table 5.3) that strongly favor the growth and spread of the surviving fires. Overall, blast would appear to encourage mass fire development. The evidence from Hiroshima and

OCR for page 36
96 Wiersma, S.J., and S.B. Martin (1973) Evaluation of the Nuclear Fire Threat to Urban Areas. Report AD779-340. Menlo Park, Calif.: Stanford Research Institute. 131 pp. Williams, D.W., J.S. Adams, J.J. Batten, G.F. Whitty, and G.T. Richardson (1970) Operation Euroka: An Australian Mass Fire Experiment. Report 386. Maribyrnor, Victoria, Australia: Defense Standards Laboratory. Wolff, G.T., and R.L. Klimisch feds.) (1982) Particulate Carbon: Atmospheric Life Cycle. New York: Plenum Press. 411 pp. Woodie, W.L., D. Remetch, and R.D. Small (1983) Fire spread from tactical nuclear weapons in battlefield environments. PSR Note 566 Santa Monica, Calif.: Pacific Sierra Research Corp. 53 pp. Wright, H.A., and A.W. Bailey (1982) Fire Ecology, United States and Southern Canada. New York: John Wiley and Sons. .

OCR for page 36
97 APPENDIX 5-1: OBSERVATION OF PLUME HEIGHTS AND ASH TRANSPORT IN LARGE FIRES, by F.E. Fendell Plume Heights The altitude achieved by a plume over a maintained source of buoyancy depends largely on the strength of the source (heat released per unit time), the stratification and humidity of the ambient air, the strength of the crosswind (if any), and the size of the region of exothermicity. Rarely are all the desired inputs known for a single event. As a reference, one of the more dramatic persistent plumes of the last quarter century was that associated with the creation of Surtsey off Iceland. An effective heat source estimated at 1011 J/s (with upflow at the base of roughly 120 m/s) was initiated at 7 A.M. on November 14, 1963 (the energy release rate was equivalent to about 250 kt every 3 h). By 10:30 A.M. the plume was at 3.5 km; by 3 P.M., at about 6.3 km; and by the next day, at over 9.3 km (i.e., to the height of the tropopause near Iceland). Vapor columns rose from neighboring sites on the sea to 2.5 km, and ash-laden steam burst upward to 0.6 km in a gigantic, ink-black column (Bourne, 1964; Thorarinsson and Vonnegut, 1964; Thorarinsson, 19661. As another reference, the series of artificial convection experiments conducted at the Centre de Recherches Atmospheriques Henri Dessens, on the Lannemezan plateau in the French Pyrenees, entailed 105 fuel oil burners deployed in a three-arm spiral within a 140 m x 140 m square (the Meteotron). The heat release rate was about 109 J/s for 20 to 30 min (a total energy release of about 0.5 kt), and the plume reached 1 to 2 km {Benech, 1976; Church et al., 19801. Plumes of most small-scale fires reach only a few kilometers into the troposphere. The black plume of a 101 J/s oil fire that persisted for days near Long Beach, California, rose to 4 km (Henna and Gifford, 1975~. The convection column associated with the bombing of Leipzig in World War II, an event severe enough to give 15 m/s ground-level radial inflow at 4 km from the center and 34 m/s closer in, rose to only 3.9 km (Broido, 1960~. The first thousand-bomber raid by the British in World War II (on Cologne, on May 30-31, 1942) produced a column of smoke that rose to 4.5 km (and hung as a huge pall at daybreak) (Barker, 19651. Taylor et al. (1973) reported a brushfire near Darwin River, Australia, on September 10, 1971, in which the ambient temperature fell almost linearly from 301 K at ground level to 268 K at 6 km. Whereas the plume rose to 3 to 4 km for a heat release rate of 1011 J/s, during a 10- to 15-min interval the plume advanced to 5.8 km when the heat release rate doubled. A small cloud above the plume was sucked down into it 10 min after this rapid additional ascent. However, the fuel loading for this case was about one-tenth that in portions of the American Pacific Northwest, which has the highest loadings in the continental United States. Thus one is motivated to examine severe burning events more closely. Of the acreage burned in the United States annually, 95 percent comes from 2 to 3 percent of the total number of fires; these exceptional fires tend to occur in dry, hot, windy weather, can jump

OCR for page 36
98 rivers and lakes, and decay only with wind shifts, the arrival of precipitation, and/or the exhaustion of fuel. Thirteen fire complexes in the recorded history of North America have each taken 4000 km or more. Twelve thousand square kilometers were burned by the Maramichi and Maine fires of 1825, the North Carolina fire of 1898, and the Idaho and Montana fires of 1910; the Alaskan fires of 1957 consumed 20,000 km2. Fire complexes in Michigan in 1871, in Wisconsin in 1894, and in Washington and Oregon in 1910 each burned 8000 km2. Southern states lead the national fire statistics annually in both frequency and area burned; however, natural decomposition is slower in the North and fuel loads accumulate, so while the number of fires is fewer, with droughts come holocausts. As for extremes in spread rate, an 1887 Texas grass fire spread 26 km in 2 h, and crown fires propagating at 16 km/in have been recorded (Pyne, 1982~. At one time the August 1933 fire in Tillamook County, Oregon, was regarded as the most intense in recorded American experience. On August 24, 1933, hurricane-like winds arose, and 800 km2 were burned in 20 h. The plume, which had reached 3 km (Holbrook, 19431, pierced an inversion, and the smoke column reached 11.1 to 12 km, near-tropopause-level altitude (Pyne, 19821. In recent years, several events perhaps comparable in intensity to the Tillamook fire have been recorded. The Sundance fire in the northern Idaho area of Pack River and McCormick Creek advanced 14.5 km and burned 200 km2 from 2 to 11 P.M. on September 1, 1967. The energy release rate is estimated at 5 x 105i J/s, and the convection column reached 10.7 km, even though a 32- to 80-km/in wind was blowing (Anderson, 1968~. The peak rate was achieved during saturation spotting in a valley somewhat sheltered from the wind. A fire at an Air Force bombing range in North Carolina in 1971 was characterized by a crosswind of 32 km/in, a heat release rate of 1.2 x 1011 J/s, and a plume height of 4.6 km. A fire in the Sierra National Forest on July 16, 1961, burned 20 km2 in 5 h, and a convective column rose to 6 to 9 km. The so-called Mack Lake fire in the Huron National Forest, Michigan, on May 5, 1980, burned 100 km2 in 6 h; though the highly bent plume rose to only 4.6 km in the intense crosswind, the heat release rate has been estimated at 1.6 x 1011 J/s. However, the highest free-burning-fire heat release rates are associated with firestorms, the exceptional heat-cyclone consequences of massive incendiary air raids on urban targets during World War II. The rareness of these events is evidenced by the fact that the U.S. Strategic Bombing Survey characterizes only four firestorms (Hamburg, Kassel, Darmstadt, and Dresden) arising from the 49 major German cities subjected to incendiary bombing (SPRI, 1975~. No firestorm arose as a consequence of fifteen massive incendiary raids from March to June 1945 on Osaka, Kobe, Nagoya, Tokyo, or Yokohama, although the atomic bombing of Hiroshima produced a firestorm. At Hamburg, during the raid on July 27-28, 1943, a cumulonimbus- cloud-like plume with an anvil top, of 3-km thickness, rose to 10 km (Ebert, 1963; Morton, 1970) in a near-adiabatic lapse rate in the lowest few kilometers of the troposphere; this altitude was ascribed by a meteorologist 6 km away, although Brunswig (1982, page 245) ascribes

OCR for page 36
99 a height of only 7 km. Thick black smoke reached 6.9 km in half an hour after the onset of bombing; later-arriving crews reported severe turbulence, and some aircraft returned to base soot-covered (Middlebrook, 1981; Musgrove, 1981~. Large black greasy raindrops fell along the outskirts of the fire (Caidin, 1960~. Smoke and dust blotted out the sky for 30 h after the attack; the sun was not seen by Hamburg residents the next day (Rumpf, 19631. Dresden was subjected to two massive raids on February 13-14, 1945, though stratocumulus clouds caused a total overcast above 3 km for most of the night, and strong winds persisted. In these raids, 12.4 km2 were 75 percent destroyed, and an additional 4 km2 were 25 percent destroyed, by fires that persisted 7 days and 8 nights. A firestorm occurred in a quarter circle of 2.2-km radius around the time of the raid. At daybreak on February 14, the city was obscured by a column of yellow-brown smoke filled with lifted flotsam; this column appeared particularly dark up to 4.8 km. Sooty ash showered downwind as far as 29 km for several days (Irving, 19651. Smoke Obscuration There are accounts of smoke so thick from Pacific Northwest forest fires that navigation on the Columbia River and other inland waterways was brought to a standstill in 1849 and 1868. An instance of sun obscuration is given by the Peshtigo fires (October 8-9, 1871), in which 5000 km2 were burned along both banks of the Green Bay. The sun was obscured for 320 km, and gloom persisted, even at noontime, for a week {Holbrook, 19431. Paper lofted from Michigan crossed Lake Huron and landed in Canada. On August 20, 1910, some 1750 separate fires in Idaho and Montana blew up and 12,000 km2 were burned, such that the sun was blotted out (Holbrook, 1943~. However, the time scale for reduced daytime visibility was days, not weeks. References Anderson, H.E. (1968) Sundance Fire: An Analysis of Fire Phenomena. Research Paper INT-56. Ogden, Utah: Intermountain Forest and Range Experiment Station, Forest Service, U.S. Dept. of Agriculture. Barker, R. (1965) The Thousand Plan. London: Chatto and Windus. Benech, B. (1976) Experimental study of an artificial convection plume initiated from the ground. J. Appl. Meteorol. 15:127-137. Bourne, A.G. (1964) Birth of an island. Discovery 25 (Aprill:16-19. Broido, A. (1960) Mass fires following nuclear attack. Bull. Atmos. Sci. 16~10~:409-413. Brunswig, H. (1982) Feuerstrum uber Hamburg. Stuttgart: Motorbuch Verlag. Caidin, M. (1960) The Night Hamburg Died. New York: Ballantine.

OCR for page 36
100 Church, C.R., J.T. Snow, and J. Dessens (1980) Intense atmospheric vortices associated with a 1000 MW fire. Bull. Am. Meteorol. Soc. 61(7):682-694. Ebert, C.H.V. (1963) The meteorological factor in the Hamburg fire storm. Weatherwise 16~2~:70-75. Hanna, S.R., and F.A. Gifford (1975) Meteorological effects of energy dissipation at large power parks. Bull. Am. Meteorol. Soc. 56~1~:1069-1076. Holbrook, S.H. (1943) Burning an Empire. New York: Macmillan. Irving, D. (1965) The Destruction of Dresden. New York: Ballantine. Middlebrook, M. {1971) The Battle of Hamburg. New York: Charles Scribner's Sons. (1970) The physics of fire whirls. Fire Res. Abstr. Rev. Morton, B.R. 12:1-19. Musgrove, G. (1981) Operation Gomorrah--The Hamburg Firestorm Raids. New York: Jane's. Pyne, S.J. (1982) Fire in America--A Cultural History of Wildland and Rural Fire. Princeton, N.J.: Princeton University Press. Rumpf, H. (1963) The Bombing of Germany. New York: Holt, Rinehart, and Winston. Stockholm Peace Research Institute (1975) Cambridge, Mass.: MIT Press. Incendiary Weapons. Taylor, R.J., S.T. Evans, N.K. King, E.T. Stephens, D.K. Packham, and R.G. Vines (1973) Convective activity over a large-scale bushfire. J. Appl. Meteorol. 12:1144-1150. Thorarinsson, S. (1966) Surtsey, the New Island in the North Atlantic. Reykjavik, Iceland: Almenna Bokafelagio. Thor arinsson, S., and B. Vonnegut (1964) Whirlwinds produced by the eruption of Surtsey volcano. Bull. Am. Meteorol. Soc. 45~8~:440-444.

OCR for page 36
101 APPENDIX 5-2: WATER IN NUCLEAR CLOUDS Clouds produced by nuclear explosions and by the fires they ignite can hold large quantities of water. The injection of this water into the upper air layers, and the consequences of the injection, are discussed in this appendix. Explosion Clouds Nuclear explosion clouds hold water that is vaporized and engulfed by the fireball. Surface bursts over deep water are expected to be relatively rare in a nuclear exchange and will be neglected (based on Pacific test data <3 x 106 tons of condensed water per megaton of yield are expected in the stabilized clouds (Gutmacher et al., 1983~. Subsurface ocean bursts do not generate high-altitude clouds (Glasstone and Dolan, 1977~. Surface bursts over land can raise about 3 x 105 tons/Ml of soil to the stabilized cloud height. About an equal amount of groundwater and mineralized water of hydration might be assumed. The fireball also entrains ambient water vapor as it rises through the lower troposphere. Adopting a fireball expansion rate such that dR/dz ~ 0.2 (that is, the increase in the fireball radius is about one-fifth of the height traversed), and a U.S. Standard (1976) mid-latitude water vapor profile, the entrained water vapor could vary from <1 x 105 to about 1 x 106 tons/Ml for a surface burst, depending on surface humidity. Accordingly, an average stabilized- cloud-water content of 1 x 106 tons H2O Mt is generous. The water concentration in stabilized nuclear clouds would be <1 g/m3, which is generally too small to cause precipitation but large enough to form an optically thick (ice) condensation cloud. As the nuclear cloud disperses, the ice particles would either settle out or evaporate. Air bursts above about 2 to 3 km would hold <1 x 105 tons of H2O per megaton. The total water injected by explosion clouds in the baseline exchange would almost certainly be less than 6000 Tg. Most of the water would be deposited in the troposphere. Fire Plumes There are three sources of moisture for fire plumes: water of combustion, evaporated surface water, and entrained water vapor. Most combustible materials generate <1 g-H2O/g-burned. Thus, in the present baseline exchange, up to 8500 Tg of H2O would be produced directly by fires, and could disperse with the plumes. Even if 1 cm of water were evaporated over the entire fire area in the baseline scenario, only 5000 Tg of additional water would enter the plumes; the actual amount would be much less, of course. Entrainment of ambient humidity into the plume, particularly at ground level where air is often efficiently sucked into the fire, could add >1 g-H2O/g-burned. At Hiroshima, a crude estimate suggests that about 10 g-H2O/g-burned

OCR for page 36
102 were entrained due to the high humidity at the time of the fire (R.P. Turco, private communication, 1984~. However, most of this water fell as precipitation (the "black rainy. Due to condensation and precipitation, only a limited quantity of water can remain suspended in the fire plumes and be carried long distances. This quantity is assumed to be S g H2O/g-burned, which consists primarily of moisture drawn into the fire near the ground. The total fire plume water injection in the baseline exchange may then be estimated as 40,000 Tg. The water is injected uniformly between 0 and 9 km (as is the smoke in the baseline case), or about 4000 Tg/km of altitude. Note that the injection represents primarily a redistribution of water vapor from the boundary layer into the free troposphere--as occurs during natural convection--because very little "new. water vapor is introduced by the combustion process. The water concentration (condensate plus vapor) in the stabilized high-altitude plumes of large fires is expected to be about 1 g/m3, based on the analysis of the water budget of a fire plume discussed above, air inflow rates obtained from plume theory, and direct measurements in fire cumulus cap clouds (L. Radke, private communication, 1984~. The onset of condensation in the convective column of a fire may occur above the level expected for condensation in surface air lifted adiabatically, due to the added heat of combustion and the entrainment of dry ambient air aloft (Taylor et al., 1973~. Low surface humidity, induced precipitation and entrainment of dry air can all limit the water concentration in fire plumes. The column abundances of water in fire clouds could be 1000 to 5000 g/m2, compared to about 10,000 g/m2 in natural cumulus clouds and 10 to 100 g/m2 in cirrus clouds. An upper limit to the water injection by fires in a nuclear conflict is in the vicinity of 500,000 Tg. This figure assumes that the initial fire plumes occupy a volume of 1017 m3 (about one-tenth of the volume of the northern hemisphere mid-latitude troposphere), all of the air in the plumes originates in the surface layer and holds an average of 5 g H2O/m3, and no rainout occurs. Obviously, these circumstances are highly unlikely. Water Perturbation Table 5.2-1 gives the average ambient profile of water vapor at mid-latitudes. The global troposphere holds roughly 107 Tg of water vapor and the stratosphere, about 3000 Tg. If all of the water in nuclear explosion clouds were confined to the mid-latitude stratosphere, H2O concentrations could increase by a factor of <10 there. Because the stratosphere normally is very dry, with a relative humidity of only 1 to 5 percent, and injected smoke and dust clouds can be heated by solar and infrared radiation, any condensed water would soon evaporate as the individual explosion clouds dispersed. A factor of 10 increase in stratospheric H2O would affect ozone photochemistry and the infrared radiation balance of the stratosphere. The

OCR for page 36
103 TABLE 5 . 2-1 Amb lent Atmospher ic Water Vapor a Equivalent Cumulative H2O Altitude Water Vapor Air Global H2O Mass up to the Interval Mixing Ratiob Density2 in the Layer Top of Layer (km) (ppmm) (kg/m3) (Tg) (Tg) 0-0.5 0.5-1.5 1.5-3.0 3.0-5.0 5.0-7.0 7.0-9.0 9.0-11. 11.-13. 13.-15 15.-17. 4686 3700 2843 1268 554 216 43.2 11.3 3.3 3.3 1.225 1.112 1.007 0.8194 0.6601 0.5258 0.4135 0.3119 0.2279 0.1665 1.4x106 2.1x106 2.1x106 l.Ox106 3.7x105 1.1x105 1.8x104 3500 750 550 1.4x106 3.5x106 5.6x106 6.6x106 7.0x106 7.1x106 7.1x106 7.1x106 7.1x106 7.1x106 he aCondensed water, which may reach concentrations of 10 g/m3 (5 x 106 Tg globally in a 1-km-thick layer), is neglected. U.S. Standard Atmosphere (1976) Midlatitude Mean Model. The water vapor mixing ratio is given in parts per million by mass (ppmm). Local water vapor fluctuations typically exceed 10 percent. photochemical effect of the H2O, however, would probably not be any more important than the photochemical effect of the explosion-generated NOX. The radiation perturbations are discussed below. The fire plume water injection of about 4000 Tg/km up to 9 km is typically <1 percent of the ambient water vapor at any level in this height interval. The total fire H2O injection is <0.5 percent of the global water vapor burden, and represents about 45 min of the normal global atmospheric water budget. The maximum perturbation could occur in the 7- to 9-km layer, where the average mid-latitude water vapor burden could increase by about 20 percent. If all of the fire water were put into this layer at northern mid-latitudes, the water burden would increase by about 0.20 g/m3, or about 400 g/m2. However, most of this water originates in lower regions of the atmosphere; the redistribution of water is likely to be less significant than an increase in the total water burden of the atmosphere. The improbable "upper limits water injections discussed in the previous section would lead to more substantial effects. Nevertheless, in view of the large ambient quantities of water vapor in the atmosphere, and the indirect water vapor perturbations to be discussed below, even the maximum credible water injections by fires could turn out to be of secondary interest.

OCR for page 36
104 CO2 Perturbation Carbon dioxide injections by nuclear fires are much less important than water injections. Because CO2 is uniformly mixed throughout the troposphere and stratosphere (at about 340 parts per million by volume), the transfer of air between different altitude levels by nuclear explosions and fires has little effect on the CO2 distribution. The global atmosphere holds about 3 x 106 Tg of CO2. Nuclear fires could generate about 1 x 104 Tg of CO2, roughly the amount produced in 1 year from fossil fuel combustion. Carbon dioxide is transparent in the visible spectrum, does not condense, and has only a limited infrared opacity (Liou, 1980~. On the other hand, CO2 perturbations could result from indirect disturbances in the global biospheric carbon cycle in the aftermath of a nuclear war (a subject that is not pursued in this report). Effects of Water Injections The water injected into the upper atmosphere with dust and smoke can have a number of important effects: 1. Modification of the photochemistry of ozone (see the previous discussion and Chapter 6~. 2. Scavenging and washout of dust and smoke particles (see the discussion in Chapter 5~. 3. Perturbation of the visible and infrared radiation balance by the condensed and vapor states of water. During the first week after the start of a nuclear war, the localized explosion clouds and fire plumes could hold significant quantities of condensed water. The visible and infrared opacities of these clouds could be very large (>>1~. Light levels below the clouds would be very low, particularly when heavy soot loadings are present. The infrared energy balance of the clouds would be complex, and some degree of thermal blanketing could result. Nevertheless, without solar insolation, the ground should still tend to cool. A strong greenhouse effect is not likely (at least in the case of smoke plumes) because solar absorption and heating would occur above most of the infrared opacity of the clouds (see Chapter 7~. In daylight, the smoke clouds would warm up rapidly, possibly inducing strong vertical and horizontal mixing of the cloud tops and edges, and perhaps causing some of the condensed water to evaporate. At night the clouds would cool by infrared emission, and subsidence might occur. The turbulence created by these heating and cooling cycles would be confined primarily to the upper cloud layers where precipitation is less probable. The major effect might therefore be to accelerate the dispersion of the smoke clouds. Some of the extended fire plumes would hold sufficient water to form thick cirrus anvils.

OCR for page 36
105 These cirrus could greatly increase the albedo above the smoke plumes, but would also hold in upwelling infrared radiation. The large-sca~e advection and spreading of smoke clouds by self-induced heating has been studied on different size scales. Chen and Orville (1977) investigated cumulus-scale convection of carbon-black clouds. R. Haberle et al. (private communication, 1983) and M. MacCracken (private communication, 1984) simulated the motions of hemispherical-scale soot clouds. In each case, the same general behavior was predicted. The clouds tended to rise and spread horizontally at a faster rate than would be expected if only ambient air motions were acting. Direct observations of large sooty smoke clouds reveal the same behavior (Davies, 1959~. Thus it is expected that some of the energy absorbed in the dust and smoke clouds would be converted into the kinetic energy of winds, which eventually dissipates as frictional heat. Within about 2 weeks, the nuclear dust and smoke clouds would be sufficiently dispersed that their infrared opacities would be quite small (<1~. The atmosphere could then approach the radiative regime analyzed by Turco et al. (1983a,b), Crutzen et al. (1984), and others, in which the infrared properties of the injected nuclear debris are less important than the visible properties. Water vapor, particularly in the stratosphere, can affect the infrared radiation balance of the atmosphere. It has been estimated, for example, that a five-fold increase in stratospheric H2O (with all other factors unchanged) would eventually lead to a 2C surface warming (e.g., Manabe and Wetherald, 1967~. However, in the perturbed atmosphere, even this modest effect is unlikely to occur, because the surface temperatures and infrared radiation fluxes of the lower atmosphere would already be greatly reduced. Indirect Water Perturbations Changes in surface air temperatures, winds, and atmospheric stability would disturb the "normal" hydrological cycle. Such disturbances could be more important than the primary water injections of the explosions and fires. Among the hydrological perturbations that might develop: 1. Increased low-level storminess and precipitation near ocean-continent margins, induced by exaggerated sea-land temperature contrasts. 2. Formation of widespread ground fogs over continents due to rapid radiative cooling of surface air. 3. Suppression of deep convection and upper-level precipitation caused by soot-induced heating of the upper troposphere. 4. Decrease in general cloudiness above several kilometers altitude as a result of warming and reduced relative humidity. 5. Reduction in the global water vapor burden associated with a general decrease in surface air temperatures. 6. Increase in water vapor concentrations above several kilometers altitude due to the enhanced moisture capacity of the heated air.

OCR for page 36
106 It is not likely that all of these effects would occur. A partial discussion of the possibilities is given in Chapter 7. Further research into these problems will be necessary to determine their importance. References Chen, C.-S., and H.D. Orville (1977) The effects of carbon black dust on cumulus-scale convection. J. Appl. Meteorol. 16:401-412. Crutzen, P.J., C. Brahl, and I.E. Galbally (1984) Atmospheric effects from post-nuclear fires. Climatic Change, in press. Davies, R.W. (1959) Large-scale diffusion from an oil fire. Pages 413-415 In Atmospheric Diffusion and Air Pollution, edited by F.N. Frenkiel and P.A. Sheppard. New York: Academic Press. Glasstone, S., and P.J. Dolan (eds.) (1977) The Effects of Nuclear Weapons. Washington, D.C.: U.S. Department of Defense. 653 pp. Gutmacher, R.G., G.H. Higgins, and H.A. Tewes (1983) Total mass and concentration of particles in dust clouds. Rep. UCRL-14397. Livermore, Calif.: Lawrence Livermore Laboratory. 22 pp. Liou, K.-N. (1980) An Introduction to Atmospheric Radiation. New York: Academic Press. Manabe, S., and R.T. Wetherald (1967) Thermal equilibrium of the atmosphere with a given distribution of relative humidity. J. Atmos. Sci. 24:241-259. Taylor, R.J., S.T. Evans, N.K. King, E.T. Stephens, D.R. Packham, and R.G. Vines (1973) Convective activity above a large-scale brushfire. J. Appl. Meteorol. 12:1144-1150. Turco, R.P., O.B. Toon, T.P. Ackerman, J.B. Pollack, and C. Sagan (1983a) Nuclear winter: Global consequences of multiple nuclear explosions. Science 222:1283-1292. Turco, R.P., O.B. Toon, T.P. Ackerman, J.B. Pollack, and C. Sagan (1983b) Global Atmospheric Consequences of Nuclear War. Interim Report. Marina del Rey, Calif.: R&D Associates. 144 pp. U.S. Standard Atmosphere (1976) Washington, D.C.: U.S. Government Printing Office.