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7 Almo spheric Effects and Interactions OVERVIEW The dispersion, evolution, and effects of dust and smoke injected into the atmosphere from a major nuclear conflict involve a large set of interacting processes whose complexity precludes detailed quantitative prediction at the present time. The available tools include a variety of models, of which the most advanced are the general circulation models (GCMs) developed for application to studies of weather prediction and climate dynamics. In these models, pressure, temperature, wind, moisture, and cloudiness fields are represented with a horizontal resolution of a few hundred kilometers and at a number of tropospheric and stratospheric levels (see, for example, Gates and Schlesinger, 1977; Mahlman and Moxim, 1978; Washington, 1982~. Smaller scale processes such as microscale and mesoscale turbulence, convection, gravity waves, local topography, and land-sea circulations can only be treated parametrically. Nevertheless, several of these models provide realistic simulations of the present climate. For applications to the problem of atmospheric effects of dust and smoke from nuclear war, however, GCMs are deficient in several respects. Transport of trace gases and diurnal variations have been simulated in some GCM studies (Levy et al., 1980; Cess et al., 1984; MacCracken and Walton, 1984~. However, no existing GCM simulates the full physics of a radiatively active trace material where net heating effects drive the circulation while the distribution of material is itself continuously varying in response to the flow and to complex flow-dependent removal processes. Formulations of boundary layer processes in these models are necessarily somewhat crude because of the low spatial resolution. Some recent model calculations have included particulate transport and diurnally varying absorption of solar radiation by the particulates, but these calculations have thus far had very limited vertical resolution (Cess et al., 1984; MacCracken and Walton, 1984~. Perhaps most serious for the nuclear war particulate problem, the cloud microphysical processes that are primarily responsible for the removal of per ticulates from the atmosphere cannot now be included in detail in these models. Other more specialized models can be applied to aspects of the problem, for example: cumulus-scale and mesoscale circulation models, 127

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128 some with crude treatments of cloud microphysics, could be used to investigate specific processes that occur at scales smaller than that of the GCM grids. One-dimensional {vertical) radiative-convective models coupled to particle microphysical models have been used for detailed investigations of these critical processes, and, because of their computational efficiency, such models are extremely useful for sensitivity studies. Two-dimensional circulation models, though far less realistic than GCMs, can simulate the zonally symmetric components of the flow and the corresponding transport and radiative heating effects of nuclear per ticulates. Because they are relatively convenient computationally, they can be used for sensitivity studies, and therefore provide a valuable complement to Gems. Energy balance climate models (EBCMs) make up another class of relatively simple model that can be used to investigate radiative perturbations of surface energy balance and surface temperature (e.g., Sellers, 1973; Robock, 19831. Most such models deal only with the energy balance at the surface, and horizontal heat transport is modeled as a diffusive process with diffusion coefficients chosen to provide reasonable simulations of the present climate. Consequently, results from such models must be interpreted judiciously. The advantage of EBCMs is that, because of their computational efficiency and modeling of horizontal variations, they can be used to provide an indicator of the feedback effects of such relatively persistent climate factors as snow and ice albedo, sea ice cover, and sea surface temperature. Some of the principal results that are now available from one-, two-, and three-dimensional models are displayed in Tables 7.3 and 7.4. Of necessity, the results of simulations using models constitute the core of our knowledge of the likely atmospheric effects of smoke and dust from a nuclear war. In discussing these results, it is convenient to divide the problem into several subdivisions: early spread and evolution of the particulate clouds, direct optical effects, thermal effects as calculated by one-dimensional (vertical) models, thermal and circulation effects calculated by multidimensional models, and modification of circulation, cloudiness, and precipitation fields by the radiation perturbations induced by these particulate clouds. Several of these are rapidly evolving areas of research, and it should be clear that parts of this chapter may be superseded TV new developments in the near future. In the absence of observational analogs of the atmosphere as perturbed by nuclear war, observations of related, though inevitably very different atmospheric situations must be used. Several such partial analogs are discussed near the end of this chapter. The global-scale atmospheric perturbations associated with major volcanic eruptions and with plausible meteor impact events and their relationship to nuclear war scenarios are considered in the following chapter. EARLY SPREAD AND EVOLUTION OF PARTICULATE CLOUDS The area initially covered by the smoke plumes would depend on the number of fires, the cross-wind width of each fire, the average wind

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129 speed, the directional variability of the wind near the level of plume stabilization, the duration of the fires, and the overlap among fire zones. If urban fire plumes extended into the middle troposphere, they would be transported by winds whose average speeds are of order 20 m/s, so that fires of several hours duration would produce plumes several hundred kilometers in length. For this reason alone, it is reasonable that substantial fractions of Eurasia, North America, and the North Atlantic, would be covered initially by smoke plumes. Crutzen et al. (1984) have estimated that the initial area covered by smoke plumes would be between 1 x 107 km2 and 2 x 107 km2 for a scenario similar to the Ambio scenario (Ambio, 1982~. For the committee's 6500-Mt scenario with about 1000 urban mass fires, an initial coverage area (immediately following the phase of rapid burning and plume rise) of about 107 km2 seems to be reasonable. In a statically stable atmosphere subject to solar heating, local wind systems would develop in response to the differential heating associated with nonuniformities in smoke distribution, and these winds would tend to smooth out both the thermal perturbations and the smoke nonuniformities. Such forced circulation systems were found to be effective smoothing agents in a cumulus-scale circulation model with an initially nonuniform distribution of carbon black (Chen and Orville, 1977~. The committee is not aware of similar numerical experiments at larger scales, but there is good reason to believe that such wind systems would be effective at scales out to several hundred kilometers. This is the typical scale of the Rossby radius of deformation.* At this and larger scales, the effect of earth's rotation becomes important and would impose a structure that could partially restrict such thermally forced lateral spreading of the smoke. Nevertheless, many of the smoke-free holes originating over the North American and Eurasian continents between 30N and 70N latitude would be filled within the first 2 days. After about 3 days, under typical meteorological conditions, the major gap over the Atlantic in the 30N to 60N latitude belt would be largely filled and very likely would have drifted over Western Europe. Portions of the mid-latitude Pacific would also be covered. The speed of this further spreading would depend somewhat on season, being greater in winter and smaller in summer. Figure 7.1 shows a specific winter season realization of the smoke and dust distribution after 3 days, based on winds derived from the Oregon State GCM (Gates and Schlesinger, 1977), and a nuclear war smoke injection scenario somewhat similar to the baseline case (MacCracken, 19831. The initial injections for this case were 207 Tg soot and 118 Tg dust. The feedback between the radiatively induced perturbation to circulation and particulate transport was not included; it was, however, included in a more recent *The Rossby radius of deformation for mid-latitude disturbances driven by heating in the mid-troposphere is (N/f)H, where N ~ 10-2 s- is the frequency of buoyancy oscillations, f ~ 10-4 s-1 is the Coriolis frequency, and H ~ 0.7 x 104 m is the scale height (e.g., Holton, 1979~. Hence the Rossby radius is about 700 km.

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- 130 _ \ FIGURE 7.1 Hemispheric distribution of smoke-induced optical depth 3 days after a hypothetical nuclear exchange. (From MacCracken, 1983.) calculation using the Oregon State GCM, which produced quite similar results (MacCracken and Walton, 1984~. The spreading of smoke is probably underestimated in the calculation shown in Figure 7.1 because vertical wind shear has been neglected and the thermally forced smoothing and spreading of the smoke have not been taken into account. Nevertheless, smoke and dust cover much of the northern mid-latitude region. According to this calculation, there are large patches tof order 106 km2 in area) in which optical depth exceeds 20 at 3 days after the start of fires, but approximately 20 percent of the area of the hemisphere (about 40 x 106 km2) is already covered by smoke and dust with optical depth of 5 or more. The initial area covered by stratospheric dust, corresponding to the area occupied initially by stabilized nuclear clouds, would be much smaller, about 0.4 x 106 km2 for the baseline case. Although dust absorbs solar radiation far less efficiently than smoke, the heating per unit mass of air would still be significant at the lower densities of the stratosphere. Thus these clouds would also tend to spread laterally in response to their self-induced thermal circulation. Calculations of stratospheric dust cloud dispersion for a nuclear war scenario involving counterforce strikes show distributions qualitatively similar to that in Figure 7.1 when climatological mean midwinter winds are used (B. Yoon, private communication, 1983~.

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131 Dispersion would be faster with actual time-dependent winter winds, but during summer, spring, and autumn, zonal winds in the extratropical lower stratosphere are weaker, and dispersion would be correspondingly slower. Material injected above 18 km in midsummer would drift westward (e.g., Holton, 1975~. As discussed on pages 77 to 80, nuclear smoke clouds would be subject to early rainout and coagulation of particles during the initial plume rise phase, but the effectiveness of these processes would rapidly decrease after the clouds have stabilized and begun to spread out in horizontally stratified plumes. Crutzen et al. (1984), using a simplified model, found less than a factor of 2 increase of particle mode radius during the 30 days following the initial rapid rise phase of the fire plumes. Coagulation in slowly dispersing smoke clouds was also evaluated by Turco et al. (1983b). In a case intended to maximize the Brownian coagulation rate, they assumed initial plume coverage equal to that of the stabilized nuclear clouds (about 106 km2 for their baseline case); they also assumed slow horizontal diffusive growth such that coverage increased linearly with time, reaching 20 x 10~ km2 only after 20 days. For the reasons cited above, this spreading rate is unrealistically slow, but even with these extreme assumptions, average smoke particle radius was found to increase by only about 65 percent after 1 week. For spherical particles whose initial radii are <0.4 um having an imaginary refractive index (the absorption component of refractive index) of <0.1, such size increases cause a decrease in the absorption coefficient per unit mass of less than a factor of 2 (Bergstrom, 1973; Lee, 1983~. For smaller or more weakly absorbing particles and for infrared radiation, the effect of such a size change is smaller. As will be shown below, early temperature changes near the surface are not very sensitive to variations of a factor of 2 or less from the absorption coefficient per unit mass of the baseline smoke injections. This is because the baseline injection initially contains more than enough smoke to absorb almost all sunlight in the areas affected by the smoke cloud. The duration of direct thermal effects of the particulates is more sensitive to the absorption coefficient, however. In addition, factor of 2 reductions below the baseline in several quantities (e.g., initial injected mass and absorption coefficient) would affect even the early temperature changes. Thus coagulation and early rainout are very important and complex issues requiring additional research. Longer term chemical and physical modification, or "aging," of aerosols in the atmosphere is another area on which additional basic information is needed. Because elemental carbon is hydrophobic and unreactive in the atmospheric temperature range, this may be a slow process for soot, depending on coalescence with preexisting hydroscopic particles. When coalescence occurs, the resulting particles behave as hydroscopic particles and can grow further by adsorption of water (Ogren, 1982; Ogren and Charlson, 1983~. Because of the internally mixed elemental carbon, these composite particles would still be efficient absorbers of sunlight (Ackerman and Toon, 1981), but an increase in composite particle size due to aging could have an important effect on the ratio of absorption efficiencies at visible and infrared

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132 wavelengths. Since this ratio is an important factor controlling the influence of per ticulates on net radiation, the aging issue requires careful additional scrutiny. DIRECT OPTICAL EFFECTS Figure 7.2 displays the transmission of visible sunlight, including diffuse as well as direct radiation, as a function of smoke and dust opacities for particulates having the size and refractive index properties specified in the baseline case. Dust and smoke properties for the injections of the baseline case have been described and presented in Chapters 4 and 5 (readers are referred particularly to pages 27 to 32 and Table 5 .7 ~ . For convenience , the baseline in jection parameters are summarized in Table 7.1.* For these optical properties, light levels decrease very rapidly for smoke optical depths greater than one. When these light level reductions are combined with the extinction optical depths calculated by MacCracken (1983), and illustrated in Figure 7.1, the result is that light levels for much of the continental area north of 30N would be reduced below the limit of photosynthesis during the first week, and widespread dense patches of smoke would make seeing impossible for several days after the nuclear exchange. For the NRC baseline case, with smoke and dust assumed to be instantaneously dispersed to a uniform distribution over the 30N to 70N latitude belt, average light levels for the belt would be below those for a very cloudy day (about 10 percent of the normal clear sky illumination) for about 2 weeks after the exchange. This can be seen by comparing the total downward solar flux versus time for this case (Figure 7.3) with the transmission levels shown in Figure 7.2. The values shown in Figure 7.3 were calculated using the one-dimensional model of Turco et al. (1983a,b). As explained by these authors, this model combines a detailed radiative transfer model with a detailed particle microphysics model (Pollack et al., 1976; Toon et al., 1979; Turco et al., 1979; Ackerman and Toon, 1981; Pollack et al., 19831. There is an approximately exponential dependence of the total downward solar flux on smoke opacity when full allowance is made for multiple scattering, as shown in Figure 7.2. This is largely because of the high absorptivity of the smoke. As a consequence, a saturation effect occurs such that most of the solar flux is removed by a smoke optical depth as small as 2; further increases in smoke optical depth *Readers unfamiliar with radiative transfer theory may wish to consult Liou (1980), which describes the theory and computational approaches in detail. tThe abbreviation NRC is used to denote the committee's baseline and excursions; LLNL denotes Lawrence Livermore Laboratory (e.g., MacCracken, 1983), and TTAPS denotes Turco et al. (1983a,b).

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O Very Cloudy Day \ En .10 _ ~_ 6 \ \ 10-2 133 .75 .50 _ 25 \ Smoke \ _ Dust 10-4 10-6 10-8 1 o~ 1 0 Limit of Photosynthesis _ \ F ul I Moonl ight \ _ Limit of Human Vision \ - , , , , 1 ~, , it, 1 1 2 4 6 8 10 20 40 60 80100 OPTICAL DEPTH FIGURE 7.2 Fraction of incident solar radiation reaching the surface as a function of extinction optical depth for smoke and dust particulates with optical properties as in the NRC baseline case (Table 7.1~. Solar zenith angle of 60 is assumed. Diurnally averaged illumination depletions would be somewhat smaller at latitudes and seasons with smaller minimum zenith angles. These calculations use the radiative transfer algorithm of Pollack et al. (1976, 1983), in which full account is taken of multiply scattered radiation (cf. Pollack et al., 1983, and references therein for a fuller description). Note that the vertical scale is logarithmic. have relatively little additional effect on solar flux received at the surface. This saturation effect carries through to the temperature changes computed by one-dimensional radiative-convective models (Turco et al., 1983a) and energy balance climate models (Robock, 1984), since the degree of cooling at the surface predicted by these models is not very sensitive to variations in illumination at very low light levels, and these models do not allow for gaps and nonuniformity in the smoke. Because of the high absorptivity, smoke clouds produce much larger depletions of solar radiation than water clouds or dust clouds of comparable extinction optical depth. However, even for a relatively moderate depletion in surface illumination comparable to that produced by dense water clouds, smoke clouds would have a larger effect on the surface thermal balance than water clouds. This is because water clouds have a high ratio of infrared to visible opacity so that increased

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200 150 NRC Baseline '~ NRC Baseline. Hemisoheric 100 l 1 / 1/ 50 o / , 1~1 1 1 1 1 1 1 1 0 25 50 75 100 125 150 175 200 - / 134 , Hemispheric, Fast Rainout _ _ _ __ NRC Baseline, 30 -70 N TIME (days) FIGURE 7.3 Time evolution of the total downward solar flux at the surface for the NRC baseline (30N to 70N), the NRC baseline injections spread over 0 to 90N, and the NRC fast-rainout variant with injections spread over 0 to 90N. downward flux of infrared radiation can equal, or even exceed (on a 24-h basis), the depletion of solar flux. Such compensation between solar radiation depletion and infrared radiation enhancement would not occur for smoke clouds because of their low ratio of infrared to visible opacity, except in regions where the normal daily total of solar radiation is already very low, such as is the case very close to the polar twilight boundary during winter, or would be the case at very early times following a nuclear exchange in dense patches in which the optical depth reaches values of 20 or more. The corresponding saturation regime is not reached for dust until the optical depth of dust alone reaches a value of about 12 (see Figure 7.21. For this reason, among others, the thermal effect of dust is far more sensitive than that of smoke to the nuclear war scenario. Smoke opacity is initially well within the saturation regime for the baseline smoke emission given in Table 7.1--180 Tg spread over the 30N-70N latitude belt--and approaches the margin of the saturation regime only as this value is decreased by about a factor of 4 to ~40 to 50 Tg. For smoke injections below this level, saturation no longer applies, and the light reduction and temperature effects would decrease rapidly with

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135 TABLE 7.1 Properties of Injected Aerosols, NRC Baseline Case Dust (see pages 27 to 32) Smoke (see Table 5.7) Total injected mass (Tg) 15 180 Median particle radius rm rumba 0.25 0.10 Log normal dispersion ha 2.0 2.0 Refractive index (real part, 0.5 nm) 1.5 1.55 Refractive index (imaginary part, 0.5 um) 0.001 0.10 Extinction coefficient at 0.5 um (m2/g) 2.8 5.5 Absorption coefficient at 0.5 um (m2/g) 0.1 2.0 Infrared optical Wavelength-dependent Absorption only, properties basaltic glass (cf. cross section Pollack et al., 1973) 0.5 m2/g Vertical distribution 37% stratosphere Uniform mass per of injection 63% troposphere unit volume (see Table 4.1J between 0 and 9 km (see pages 73 to 76 and 83) Horizontal distribution Uniform in the Uniform in the of injection latitude belt latitude belt 30N-70N; none 30N-70N; outside none outside aParameters of the log normal size distribution; see page 62. decreasing injected mass. On the other hand, dust opacity approaches saturation only for rather extreme excursions that involve large numbers of surface bursts. For the dust optical properties and quantities of the baseline case, the extinction cross section of dust at 0.5 Am is 2.8 m2/g, and the corresponding extinction optical depths [lower limit (best estimate) upper limit for submicron dust in both troposphere and stratosphere] are [0.6 (0.9) 1.51 for dust uniformly spread around the 30N to 70N latitude belt. The extinction optical depths in the same

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136 belt with the added opacity due 32) are [1.3 (2.1) 3.31. - well below the saturation threshold for climatologically significant since most of this dust is in the stratosphere and has a long residence time. In the baseline case, only about 40 percent of the dust is initially injected into the stratosphere, but the remainder may have an anomalously long residence time in the upper troposphere if precipitation is suppressed because of the smoke (see below). Crutzen et al. (1984) have also estimated transmission versus time for the smoke cloud. They consider models with rainout removal times of 15 days and 30 days. For the 15-day rainout time, calculated solar illumination reductions to the 10 percent level persist for 10 to 14 days (the exact value depending on the assumed extent of forest fires) by which time a uniform cloud has dispersed to cover 60 percent of the northern hemisphere, whereas for the 30-day rainout time the reduction to 10 percent persists for about 14 to 24 days. According to their estimates, about one-half of the northern hemisphere will have been covered by the smoke cloud in 10 days, and about two-thirds of the hemisphere in 20 days. These reductions correspond quite well to the NRC baseline case despite differences in the scenarios and in the treatment of cloud dispersion and evolution. to the 8500-Mt dust excursion (see page The values for the 8500-Mt excursion, though dust, may nevertheless be THERMAL EFFECTS IN ONE-DIMENSIONAL MODELS General circulation models can provide the most detailed and reliable assessments of temperature changes associated with nuclear war; however, because of their complexity and computational requirements, they are not suitable for sensitivity studies in which parameters such as input scenarios and particulate removal rates are varied over wide ranges. Turco et al. (1983a,b) have carried out such sensitivity studies using the TTAPS one-dimensional model. In order to relate the results of the TTAPS studies to the current baseline and to the results of multidimensional modeling studies using the NRC baseline, the TTAPS model has been applied to the NRC baseline case and to two variations: a rast-ra~nout removal case, and a case in which the baseline smoke injection is uniformly distributed over the entire northern hemisphere. The TTAPS one-dimensional model (Turco et al., 1983b) calculates the microphysical evolution of particulates subject to coagulation, agglomeration, sedimentation, vertical eddy diffusion, surface deposition, and removal by parameterized rainout processes (see Turco et al., 1983b, and references therein--particularly Turco et al., 1979, 1981; Toon et al., 1979; Hamill et al., 1982--for details). In the NRC baseline case the smoke and dust clouds have been assumed to be uniformly distributed around the 30N to 70N latitude belt. The microphysical implications of this simplifying assumption have been discussed in previous sections of this chanter and in Chanter 5. Results are most sensitive to the particle process, which is parameterized as a linear loss mechanism with a height-dependent exponential lifetime. Since the particle lifetime increases rapidly ~. ~

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137 with altitude, there is a strong interaction between the altitude of initial smoke plume injection and the assumed vertical profile of rainout rate. AS described on pages 73 to 76 the committee has assumed that the smoke is distributed uniformly with altitude over the O to 9 km range, partly for simplicity in the absence of better information, and partly because it is the committee's judgment that the intensity of urban fires would tend to drive the plumes into the upper troposphere. If vertical mixing in the plumes is very rapid, it would tend to produce a uniform smoke mixing ratio rather than uniform smoke concentration. However, as will be seen below, the tendency to develop a uniform mixing ratio would probably decrease rapidly with time and would be strongly opposed by the increase in the rainout rate near the ground. The rainout removal rate profile assumed for the NRC baseline case is given in Table 7.2, where it is compared with the profile used in the TTAPS study. The TTAPS group chose baseline values designed to represent the rainout characteristics of the unperturbed atmosphere; for the NRC baseline case, these values have been modified so that faster rainout occurs in the lower troposphere (O to 5 km) and no rainout at all occurs above 5 km. These changes have been made in order to simulate possible effects of changes in static stability and cloudiness expected in the perturbed atmosphere (see pp. 156 to 158 below), and they are of course highly uncertain. Even in the absence of rainout, however, eddy diffusion acts in the model as an effective mechanism for removing particulates from the upper troposphere. Following Massie and Hunten (19811, the vertical eddy diffusion coefficient value 10 m2/s has been assumed for the NRC baseline case, as it was for the TTAPS calculations. This value gives a characteristic lifetime against dry deposition for particulates in the upper troposphere of about 40 days. Because the rainout time and its interaction with the initial vertical smoke distribution are so critical to the evaluation of climatic effects, a fast-rainout excursion has been considered, with rainout times given in the last column of Table 7.2. These high values of rainout rate are believed to provide a reasonable case bounding the smoke lifetime on the low side for the NRC baseline smoke injection. In this case, the smoke has been assumed to be dispersed over the entire hemisphere rather than over the 30N to 70N latitude band. The initial opacity for this case is nearly equivalent to that for smoke and dust spread over the 30N to 70N latitude band with half of the initial smoke and dust injections of the NRC baseline case. The TTAPS ~slow-rainout" case, with an effective removal rate about one-third as fast as their baseline case, represents a plausible bound to the smoke lifetime on the high side. This case is also compared with the NRC baseline. Figure 7.4 shows vertical profiles of the contributions to optical depth from smoke and dust in each 2-km layer for (a) the NRC baseline case, and (b) the fast-rainout excursion. For comparison, the TTAPS baseline is also shown (Figure 7.4c). In the TTAPS baseline case, the relatively rapid rainout removal assumed for the upper troposphere causes the center of mass of smoke to lower over time, while in both the NRC baseline case and the fast-rainout excursion, the rapid downward

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163 have significant radiative effects. Because ice crystals found in normal cirrus clouds tend to be of moderate size twith radii of several microns to a few tens of microns), and because ice is strongly absorbing in the infrared and reflective in the visible, normal cirrus generally has a larger influence on infrared radiation than on solar radiation. However, even a small increase in albedo due to such clouds would reduce the energy received by the atmosphere, so it is difficult to estimate the net climatic impact without detailed calculations. As an example, suppose that water vapor from the base of the convective region is mixed upward uniformly through the convective layer with a mixing ratio of 100 ppmv, a representative value for air originating near the 200-mbar level. With adiabatic cooling of the rising air, condensation could begin near or slightly below the 50-mbar level. If the cloud extends 1 km above the condensation level and most of the water vapor in the cloud layer condenses, the resulting cloud mass would be about 7 g/m2. The absorption cross section at 10-pm wavelength for spherical ice particles whose radii are a few microns or less is about 0.1 to 0.2 m2/g (Bergstrom, 1973~. Thus, in this example, an absorption optical depth of 1 for 10-pm radiation could develop for such a cloud. More work is needed to assess the significance of such ultra-high clouds. For example, if the absorbing particulate cloud moves upward, as a result of self-induced circulation or mixing, the infrared opacity of such an elevated cirrus layer would be correspondingly smaller. Longer Term Effects on Climate If nuclear war injections of smoke were as large as those of the NRC baseline case, longer term meteorological effects, extending beyond the time at which most of the smoke is removed from the atmosphere, might occur. Such effects could arise from changes in the distribution of snow, sea ice, and vegetation cover, which would cause changes in surface albedo, thermal inertia, and evapotranspiration potential. It is also possible that persistent changes in ocean current systems leading to changes in sea surface temperature distributions would be produced. The upward mixing of water vapor by convection to altitudes above 10 km could also have significant long~term climatic implications. Such possibilities are extremely difficult to evaluate, particularly because shorter term effects themselves are highly uncertain. However, Robock (1984) has recently attempted to assess some of these effects using an EBCM with snow and ice albedo feedback and sea ice thermal inertia and meltwater feedbacks included in the model (Robock, 1983~. Applying this model to the TTAPS scenario, he found depressed surface temperatures persisting but gradually ameliorating over several years in northern, middle, and high latitudes, primarily as a result of an increase in the surface covered by sea ice with a corresponding reduction in thermal inertia of the northern high-latitude oceans. An effect that could be significant but would favor warming of high-latitude surface temperatures is the depression of snow and ice

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164 albedo due to the fallout of smoke particles. If as little as 10 to 20 Tg of smoke particles was to fall out over the Arctic during the course of a few months and if the smoke particles were mixed with no more than the normal amount of snowfall, they could have a very significant effect on snow albedo (Warren and Wiscombe, 1984~. The actual importance of this effect is difficult to evaluate, however, since it depends on many detailed processes, such as the exact timing of smoke and snow fallout events, washout of smoke particles due to surface melting on snow or ice, and changes in the morphology of the snow or ice surfaces. Such longer term effects are difficult to investigate, but they should not be ignored. ANALOGS Of necessity the previous discussion relies heavily on model results, supplemented by occasional references to our understanding of how the undisturbed atmosphere behaves. Confidence in these results can be enhanced by examining natural situations where some of the key processes and their effects can be seen. Indeed, bare model results in the absence of such natural analog situations would be quite unconvincing to many observers. In this section several such natural analogs are examined. Arctic Haze Recent research has shown that there is a remarkable amount of aerosol pollution in the central Arctic, especially during spring (Patterson et al., 1982; Rosen and Novakov, 1983~. A major component of this pollution is a fine particle mode (particle mode diameter of about 0.4 um), which in turn is rich in soot carbon. This material has been detected near the surface and in layers at elevations as high as 5 km (Hansen and Rosen, 1984; Radke et al., 1984~. The particles in such elevated layers, following essentially quasi-isentropic trajectories,* must have originated at distant mid-latitude pollution sources, and they must in some cases have been in transit for many days. Thus the properties of these particles provide valuable information on the aging of carbonaceous particulates in the unperturbed atmosphere. Microscopic analysis and analysis of the optical properties of these particles indicate that the soot particles sometimes occur internally mixed in a nonabsorbing material, probably sulfate (A.D. Clarke, private communication, 1984~. The polluted layers also contain nonabsorbing *Heating can probably be neglected to first order in considering the transport of these particles, so that they would tend to move approximately on surfaces of constant specific entropy. Since these slope upward toward the pole, pollutants originating near the surface can reach the middle troposphere in the Arctic.

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165 particles unmixed with carbonaceous material so that the mean single scattering albedo of all particles varies around 0.86 (Clarke et al., 19843. This value is considerably higher than that of the postulated nuclear war smoke clouds, though nevertheless the polluted layers are quite strongly absorbing. In relating these aerosols to the smoke that could be produced by burning cities, it is important to keep in mind that the former are probably produced in pollution plumes that are rich in sulfur and not particularly black at the source; the smoke from burning cities is likely to be much blacker initially and throughout its life in the atmosphere. Elemental carbon several days removed from its sources has also been found to be an important component of the fine particle mode in the marine boundary layer over the Atlantic (Andreas, 1983~. Although highly variable, typical soot fractions of the fine particle mass were about 40 percent. Further experimental studies of the fine particle mode in regions remote from pollution sources should provide valuable information on the mechanisms, rates, and consequences of the aging of carbonaceous particles in the undisturbed atmosphere. This information is a necessary prerequisite to understanding the implications of soot aging for the consequences of nuclear war. Plumes from Large Forest Fires There are a number of accounts of observations of forest fire plumes at large distances from their sources (see Chapter 5~. Lyman (1918), for example, documents a case in which smoke from large fires in Minnesota darkened the sky over much of the northeastern United States and southeastern Canada. Shostakovitch (1925) gives a dramatic account of the obscuration persisting for more than a month due to the Siberian forest fires of 1915. Wexler (1950) provides a well-documented account of the plume from a large number of forest fires burning within a 40,000 km2 area of northwest Alberta and northeast British Columbia (although the extent of the area that actually burned is unclear from Wexler's account). Wexler describes events during the period September 24 to 30, 1950. Within 2 days of the beginning of the most intense phase of burning, the plume had reached Washington, D.C. Within 5 days, it had been observed over all of Canada except the far northeast and far west, over almost the entire United States east of the Mississippi River plus Minnesota and the Dakotas, and had stretched across the North Atlantic and had been observed throughout Western Europe from Portugal to Norway (Figure 7.12). At Washington, D.C., the smoke occurred in a layer between the 2.5 and 5 km altitudes bounded above and below by inversions, and was estimated by Wexler to have reduced the total incident solar radiation by as much as 54 percent. Associated with this reduction was a decrease in maximum temperature that Wexler estimated to be an average of 4C for 4 days. Smith (1950) quotes an estimate by Fritz that the maximum temperature was reduced by as much as 6C, with no compensating rise in minimum temperature. By the time the plume had reached England, it

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166 1 ~q 1 ~ of h In' FIGURE 7.12 The hatched area represents the region over which smoke was ~ . . . _ ~ _ . . ~ ~ observed from the western Canada forest fires of September 1950 (exclusive of observations from Western Europe). The boundary of this area is dotted where it is tentative. The darkened areas in western Canada are the areas in which the fires occurred, and the curves mark calculated trajectories for smoke reaching the vicinity of Washington, D.C., by September 24, two days after the most intense burning episode. (From Smith, 1950.) appears to have risen to an altitude range of 10 to 12 km (Bull, 1951) These incidents illustrate the rapid spread of fire plumes from relatively small areas. They also show that such plumes can have dramatic optical effects and can influence surface temperatures thousands of kilometers from the source. Such forest fire plumes are not necessarily highly absorbing for solar radiation, however. The reduction in solar radiation and the surface temperature decreases observed at Washington were probably due largely to reflection rather than absorption of sunlight by the cloud. As discussed in Chapter 5, urban fires are likely to produce much blacker smoke, and to produce much larger optical depths and reductions in solar radiation at the surface. .

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167 Early Plume from the Mount St. Helens Eruption The paroxysmal eruption of Mount St. Helens on May 18, 1980, produced a large plume of ash that spread rapidly across eastern Washington and into Idaho and Montana during the day following the eruption. Rapid daytime temperature decreases were observed beneath the plume. By comparing observed and forecast temperatures under the plume with those in the surroundings, Mass and Robock (1982) argued that the plume produced a drop in the maximum temperature of up to 8C. However, during the following night, as the plume drifted over Montana, increases in minimum temperature of about the same magnitude were observed. Evidently, the substantial reduction in solar radiation produced by the plume was compensated by a corresponding increase in the downward infrared radiation. The properties of the ash particles in this early volcanic plume were quite different from those of the smoke particles of the nuclear war scenarios. The Mount St. Helens ash particles had high single scattering albedos, and the size distributions had maximum diameters between 1 and 10 Em. The plume is estimated to have contained about 2 Tg of ash particles with diameters greater than 2 Em, but less than 10-2 Tg of particles with diameters less than 2 um (Hobbs et al., 1982), so it is not surprising that the plume was an effective emitter of infrared radiation at this stage of its evolution. These observations illustrate the rapidity with which such plumes can influence surface temperatures, and they serve to focus attention on the role of the ratio of infrared to visible absorptivity of particles in the nuclear war scenarios. Sahara Dust Plumes, the "Harmattan" Sahara dust carried over West Africa and the tropical Atlantic Ocean by northeasterly and easterly winds provides another natural analog for some facets of the nuclear war problem. Outbreaks of dust over the Atlantic can produce extinction optical depths of about 1 over areas of 106 km2 (Carlson and Caverly, 1977; Carlson and Benjamin, 1980). AS much as 8 Tg of dust may be involved in a large outbreak (Carlson, 1979), and strong heating occurs in the dusty layer. Suppression of convection has been noted when Sahara dust in the middle troposphere is transported over the tropical Atlantic. During the dry season in West Africa, the prevailing northeasterly wind, which is often laden with dust, is known as the "harmattan. n Brinkman and McGregor (1983) report harmattan events in Nigeria with optical depths up to 2 and associated reductions in daily mean total solar radiation of 28 percent. They also report temperature decreases of up to 6C for these events, although this is representative of the depression of the maximum rather than the daily mean temperature. Although these dust particles are probably generally much larger than the stratospheric dust particles and are both larger and more reflective than the smoke particles of the nuclear war scenarios, these observations show that such aerosols do have a rapid effect on surface

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168 temper atur es. They also show that such particles, even though less absorbing than smoke, produce elevated heated layers that can act to suppress convection. Martian Global Dust Storms It is now known that the planet Mars is subject to occasional global-scale dust storms in which dust spreads over most of the planet with mean optical depths of order 5. Martian dust is somewhat more absorbing at visible wavelengths than typical terrestrial dusts, so that the absorptivity for these situations is intermediate between values for nuclear war scenarios with dust only and those with both smoke and dust. Consequently, the scale of the associated optical perturbation is within the range of interest. These events produce temperature increases in the upper part of the dusty layer of order 80C over much of the planet. Temperature decreases at both subtropical and mid-latitude sites have also been observed in connection with these events (Martin and Kieffer, 1979; Pollack et al., 1979; Ryan and Henry, 1979~. The vertical profile of temperature changes associated with these events resembles that of the nuclear war scenarios except that the decrease in surface temperature is less. This is partly because Martian dust is much less absorbing in the visible than smoke, but, probably more important, it is because the "greenhouses effect is at most very weak on Mars, so that the ~antigreenhouse" effect at the surface due to the absorbing cloud is not very pronounced (see page 149~. These dust storms do not occur every Martian year. When they do occur, it is during southern hemisphere summer, Mars perihelion season, when dust generated locally in the summer subtropics is swept upward to great heights in the rising branch of the mean meridional circulation and then is swept rapidly poleward, reaching high latitudes of the opposite hemisphere within a few days (Haberle et al., 1982~. Proper phasing between dust injection and the meridional circulation is an essential feature of this phenomenon; dust injected into the normally subsiding branch of the tropical mean meridional circulation remains close to the latitude of injection. The analogy to the nuclear war scenarios should not be pressed too far. The total amount of material involved in the Martian dust storms is larger (Toon et al., 1977), but the particle sizes are larger so they are less efficient optically; precipitation processes are not active on Mars; and the global dust storms are driven by heating per unit mass of atmosphere that is larger than the largest reasonable values for the nuclear war smoke clouds. Nevertheless, Mars does provide a natural example of the thermal structure of an "antigreenhouse" atmosphere and of rapid meridional spread of per ticulates by an enhanced thermally driven meridional circulation.

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169 SU+ARY None of the natural situations described above bears a close resemblance to the atmospheric condition that is likely to prevail following a full-scale nuclear war. Nevertheless, each has elements that tend to support various conclusions drawn from the models. In sum then, the various model results in concert with a limited set of observations of related natural phenomena provide a basis for concluding that a nuclear war scenario like the NRC baseline case could produce large temperature decreases near the surface and temperature increases aloft for a period of weeks to months following the event (cf. the two- and three-dimensional model results summarized in Tables 7.3 and 7.4~. Moreover, rapid spreading of particulates into the tropics and even into the southern hemisphere is a real possibility. These conclusions are contingent upon the assumptions that a substantial fraction of the smoke particles produced by burning cities would survive early scavenging and coagulation, and that subsequent aging and scavenging processes would not remove submicron smoke particles distributed throughout the middle and upper troposphere at a removal rate* greater than about (2 weeks)~l. Because of optical saturation due to the high absorptivity of smoke, the climatic effects are likely to be insensitive to moderate changes in smoke or absorptivity about the baseline values. However, lower values of either of these quantities by a factor of about 4 would lie near the edge of the saturation regime, and climatic effects would decrease rapidly for large reductions. Climatic effects are also sensitive to the removal rate of smoke. If middle and upper tropospheric rates were as large as (1 week)~1 temperature perturbations would be considerably moderated although still significant (see the Fast rainout" used in Figure 7.6~. Improvements in the models are needed, particularly to investigate further the effects of realistic transport and dispersion of smoke and dust in the perturbed atmosphere, the infrared opacity of the smoke, diurnal and seasonal effects, and the possible roles of ground fog and stratus and of ultra-high clouds forming at the top of the convective layer that may be driven by absorption of solar radiation in smoke and dust clouds. Long-term effects arising from possible changes in the properties of the underlying surface also require further study. REFERENCES Ackerman, T.P., and O.B. Toon (1981) Absorption of visible radiation in atmosphere containing mixtures of absorbing and non-absorbing particles. Appl. Opt. 20:3661-3668. Aleksandrov, V.V., and G.L. Stench~kov {1983) On the modeling of the climatic consequences of the nuclear war. In Proceedings on Applied Mathematics. Moscow: Computing Center of the Academy of Sciences USSR. *Removal rate is defined in Table 7.2.

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170 Ambio (1982) Nuclear war: The aftermath. 11~2/3~:75-176. Andreas, M.O. (1983) Soot carbon and excess fine potassium: Long-range transport of combustion-derived aerosols. Science 220:1148-1151. Bergstrom, R.W. (1973) Extinction and absorption of atmospheric aerosol as a function of particle size. Contrib. Atmos. Phys. 46:223-234. Brinkman, A.W., and J. McGregor (1983) Solar radiation in dense Saharan aerosol in northern Nigeria. Quart. J. Roy. Meteorol. Soc. 109:831-847. Bull, G.A. (1951) Blue sun and moon. Meteorol. Mag. 80:1-4. Carlson, T.N. (1979) Atmospheric turbidity in Saharan dust outbreaks as determined by analysis of satellite brightness data. Mon. Weather Rev. 107:322-335. Carlson, T.N., and S.G. Benjamin (1980) Radiative heating rates for Saharan dust. J. Atmos. Sci. 37:193-213. Carlson, T.N., and R.S. Caverly (1977) Radiative characteristics of Saharan dust at solar wavelengths. J. Geophys. Res. 82:3141-3152. Cess, R.D. (1984) Nuclear war: Illustrative effects of atmospheric smoke and dust upon solar radiation. Unpublished manuscript. Laboratory for Planetary Atmosphere Research, State University of New York, Stony Brook. Cess, R.P., G.L. Potter, and W.L. Gates (1984) Climatic impact of a nuclear exchange: Sensitivity studies using a general circulation model. Paper presented at the 4th Session of the International Seminar on Nuclear War, Erice, Sicily. Aug. 19-24, 1984. Chen, C.-S., and H.D. Orville (1977) The effects of carbon black dust on cumulus scale convection. J. Appl. Meteorol. 16:401-412. Clarke, A.D., R.V. Charlson, and L.F. Radke (1984) Airborne observations of Arctic aerosol 4: Optical properties of Arctic haze. Geophys. Res. Lett. 11:405-408. Covey, C., S.H. Schneider, and S.L. Thompson (1984) Global atmospheric effects of massive smoke injections from a nuclear war: Results from general circulation model simulations. Nature 308:21-31. Crutzen, P., I.E. Galbally, and C. Bruhl (1984) Atmospheric effects from post-nuclear fires. Climatic Change (in press). Gates, W.L., and M.E. Schlesinger (1977) Numerical simulation of the January and July global climate with a two-level atmospheric model. J. Atmos. Sci. 34:36-76. Golitsyn, G.S., and A.S. Ginsburg (1984) Comparative estimates of the climatic consequences of Martian dust storms and possible nuclear war. Paper presented at the Conference, The World After Nuclear War, Oct. 31 to Nov. 1, 1983. Institute of Atmospheric Physics of the Academy of Sciences USSR. Haberle, R.M., C.B. Leovy, and J.B. Pollack (1982) Some effects of global dust storms on the atmospheric circulation of Mars. Icarus 50:322-367. Haberle, R.M., T.P. Ackerman, and O.B. Toon (1983) The dispersion of atmospheric dust and smoke following a large-scale nuclear exchange. Paper presented at the Fall 1983 Meeting of the American Geophysical Union, San Francisco, G.T. Wolff and R.L. Klimisch, eds. New York: Plenum. Pages 379-391.

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171 Hamill, P., R.P. Turco, C.S. Kiang, O.B. Toon, and R.C. Whitten (1982) On the formation of sulfate aerosol particles in the stratosphere. J. Aerosol Sci. 13:565-581. Hansen, A.D.A., and H. Rosen (1984) Vertical distributions of particulate carbon, sulfur, and bromine in Arctic haze and comparison with Barrow, Alaska. Geophys. Res. Lett. 11:381-384. Held, I., and A. Hou (1980) Non-linear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci. 37:515-533. Hobbs, P.V., J.P. Tuell, D.A. Hegg, L.F. Radke, and M.W. Eltgroth (1982) Particles and gases in the emissions from the 1980-1981 volcanic eruptions of Mt. St. Helens. J. Geophys. Res 87:11062-11086. Holton, J.R. (1975) The dynamic meteorology of the stratosphere and mesosphere. Meteorological Monographs, No. 34. Boston: American Meteorological Society. Holton, J.R. (1979) An Introduction to Dynamic Meteorology. 2nd ed. New York: Academic. 391 pp. Huang, H.-J., and D.G. Vincent (1983) Major changes in the circulation features over the South Pacific during FGGE, 10-27 January 1979. Mon. Weather Rev. 111:1611-1618. Joung, C.-H., and M.H. Hitchman (1982) On the role of successive downstream development in East Asian polar air outbreaks. Mon. Weather Rev. 110:1224-1237. Lee, K.T. (1983) Generation of soot particles and studies of factors controlling soot light absorption. Ph.D. thesis, Department of Civil Engineering, University of Washington, Seattle. Levy, H., II, J.D. Mahlman, and W.J. Moxim (1980) Three-dimensional tracer structure and behaviour as simulated in two ozone precursor experiments. J. Atmos. Sci. 37:655-685. Liou, K.-N. (1980) An Introduction to Atmospheric Radiation. New York: Academic. 392 pp. Lyman, H. (1918) Smoke from the Minnesota forest fires. ~Ion. Weather Rev. 46:506-509. MacCracken, M.C. (1983) Nuclear war: Preliminary estimates of the climatic effects of a nuclear exchange. Paper presented at the International Seminar on Nuclear War, 3rd Session: The Technical Basis for Peace, Ettore Majorana Centre for Scientific Culture, Erice, Sicily, Aug. 12-23, 1983. MacCracken, M.C., and J. Walton (1984) The effects of interactive transport and scavenging of smoke on the calculated temperature change resulting from large amounts of smoke. Paper presented at the International Seminar on Nuclear War, 4th Session, Erice, Sicily, Aug. 19-24, 1984. Mahlman, J.D., and W.J. Moxim (1978) Tracer simulation using a global general circulation model: Results from a mid-latitude instantaneous source experiment. J. Atmos. Sci. 35:1340-1374. Martin, T.Z., and H. Kieffer (1979) Thermal infrared properties of the Martian atmosphere. 2. The 15-pm band measurements. J. Geophys. Res. 84:2843-2852.

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172 Mass, C., and A. Robock (1982) The short-term influence of the Mount St. Helens volcanic eruption on surface temperature in the northwest United States. Mon. Weather Rev. 110:614-622. Massie, S.T., and D.M. Hunten (1981) Stratospheric eddy diffusion coefficients from tracer data. J. Geophys. Res. 86:9859-9868. Ogren, J.A. (1982) Deposition of particulate elemental carbon from the atmosphere. In Particulate Carton: Atmospheric Life Cycle, edited by G.T. Wolff and R.L. Klimisch. New York: Plenum. Ogren, J.A., and R.J. Charlson (1983) Elemental carbon in the atmosphere: Cycle and lifetime. Tellus 358:241-254. Patterson, E.M., B.T. Marshall, and K.A. Rahn (1982) Radiative properties of the Arctic aerosol. Atmos. Environ. 16:2967-2977. Pollack, J.B., O.B. Toon, and B.N. Khare (1973) Optical properties of some terrestrial rocks and glasses. Icarus 19:372-389. Pollack, J.B., O.B. Toon, C. Sagan, A. Suborners, B. Baldwin, and W. van Camp (1976) Volcanic explosions and climate change: A theoretical assessment. J. Geophys. Res. 81:1071-1083. Pollack, J.B., D.S. Colburn, F.M. Flasar, R. Kahn, C.E. Carlston, and D. Pidek (1979) Properties and effects of dust particles suspended in the Martian atmosphere. J. Geophys. Res. 84:2929-2945. Pollack, J.B., O.B. Toon, T.P. Ackerman, C.P. McKay, and R.P. Turco. (1983) Environmental effects of an impact-generated dust cloud: Implications for the Cretaceous-Tertiary extinctions. Science 219:287-289. Radke, L.F., J. Lyons, D. Hegg, P.V. Hobbs, and I. Bailey (1984) Airborne observations of Arctic aerosols. 1. Characteristics of Arctic haze. Geophys. Res. Lett. 11:393-396. Ramaswamy, V., and J. Kiehl (1984) Sensitivity of the radiative forcing due to large loadings of smoke and dust aerosols. Manuscript, National Center for Atmospheric Research, Boulder, Colo. (Submitted to J. Geophys. Res.) Robock, A. {1983) Ice and snow feedbacks and the latitudinal and seasonal distribution of climate sensitivity. J. Atmos. Scz. 40: 986-997.. Robock, A. (1984) Snow and ice feedbacks for prolonged effects of nuclear winter. Nature 310: 667-670. Rosen, H., and T. Novakov (1983) Combustion-generated carbon particles in the Arctic atmosphere. Nature 306:768-778. Ryan, J.A., and R.M. Henry (1979) Mars atmospheric phenomena during major dust storms as measured at the surface. J. Geophys. Res. 84:2821-2829. Schneider, E.K. (1983) Martian great dust storms: Interpretive axially symmetric models. Icarus 55:302-331. Sellers, W.D. {1973) A new global climate model. J. Appl. Meteorol. 12:241-254. Shostakovitch, V.B. (1925) Forest conflagrations in Siberia. J. Forestry 23:365-371. Smith, C.D., Jr. (19S0) The widespread smoke layer from the Canadian forest fires during late September 1950. Mon. Weather Rev. 78:180-184.

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173 Thompson, S.L. , V.V. Alekeandrov, G.L. Stenchikov, S.H. Schneider, C. Covey, and R.M. Chervin (1984) Global climatic consequences of nuclear war: Simulations with three-dimensional models. Ambio 13 {in press). Toon, O.B., and T.P. Ackerman {1981) Algorithms for the calculation of scattering by stratified spheres. Appl. Opt. 20:3657-3660. Toon, O.B., J.B. Pollack, and C. Sagan (1977) Physical properties of the particles comprising the Martian dust storm of 1971-1972. Icarus 30:663-696. Toon, O.B., R.P. Turco, P. Hamill, C.S. Ring, and R.C. Whitten (1979) A one-dimensional model describing aerosol formation and evolution in the stratosphere: II. Sensitivity studies and comparison with observations. J. Atmos. Sci. 36:718-736. Turco, R.P., P. Hamill, O.B. Toon, R.C. Whitten, and C.S. Kiang (1979) A one-dimensional model describing aerosol formation and evolution in the stratosphere. I. Physical properties and mathematical analogs. J. Atmos. Sci. 36:699-717. Turco, R.P., O.B. Toon, R.C. Whitten, J.B. Pollack, and P. Noerdlinger (1982) An analysis of the physical, chemical, optical and historical impacts of the 1908 Tunguska meteor fall. Icarus 50:1-52. Turco, R.P., O.B. Toon, T. Ackerman, J.B. Pollack, and C. Sagan {1983a) Nuclear winter: Global consequences of multiple nuclear explosions. Science 222:1283-1293. Turco, R.P., O.B. Toon, T. 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. Vincent, D.G. (1982) Circulation features over the South Pacific during 10-18 January 1979. Mon. Weather Rev. 110:981-993. Walter, B.A. (1980) Wintertime observations of roll clouds over the Bering Sea. Mon. Weather Rev. 108:2024-2031. Walton, J.J., and M.C. MacCracken (1984) Preliminary report on the global transport model Grantour. Unpublished report. Lawrence Livermore National Laboratory, Livermore, Calif. Warren, S., and W. Wiscombe (1984) Dirty snow after nuclear war. Nature (in press). Washington, W.M., ed. (1982) Documentation for the Community Climate Model (CCM) Version O. Boulder, Colo.: National Center for Atmospheric Research. (NTIS PB82-194192.) Wexler, H. (1950) The great smoke pall--September 24-30, 1950. Weatherwise (Dec.~:129-142. ,