The Effects on the Atmosphere of a Major Nuclear Exchange (1985) / Chapter Skim
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7 Atmospheric Effects and Interactions
7 Atmospheric Effects and Interactions
Pages 127-173
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From page 127... ...
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.
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From page 128... ...
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.
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From page 129... ...
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~.
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From page 130... ...
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.
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From page 131... ...
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.
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From page 132... ...
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 30°N 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.
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From page 133... ...
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.
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From page 134... ...
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.
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From page 135... ...
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)
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From page 136... ...
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.
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From page 137... ...
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)
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From page 138... ...
Figure 7.5 shows the time evolution of the optical depth for the NRC baseline, the fast-rainout excursion, and the TTAPS baseline. The total optical depth with contributions from dust as well as smoke is shown.
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From page 139... ...
2.5 3.0 FIGURE 7.4 Contr ibutions to total extinction optical depth for 2-km-thick layers at various times after hypothesized nuclear war smoke and dust injections.
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From page 140... ...
o . 0 = After 253 Days = After O Days = After 4 Days = After 29 Days = After 61 Days = After 1 28 Days ~ _ t ~ ' l 0.5 1.0 OPTICAL DEPTH (per 2-km layer)
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From page 141... ...
1 . ~ 1 .0 OPTICAL DEPTH (per 2-km layer)
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From page 142... ...
Vertical mixing of heat occurs in the model only by convective adjustment. Figure 7.6 shows the time evolution of near-surface temperature (actually the mean temperature of the lowest 2-km-thick atmospheric layer)
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From page 143... ...
The one-dimensional models do not provide reliable estimates of mean, or even typical, temperature changes over continental areas. Better estimates are provided by multidimensional models, also summarized in Table 7.3, though these too are subject to large uncertainties.
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From page 145... ...
145 o o ~o o O d' ~ o o fir fir cat kD 1 1 1 1 1 Q O Q O Q O Q O Q O z In z {Q A eQ ~ 0 oZ o ~ ~ ~o ~ ~ ~o ~ ~ :^ ~ ~ ~o o ~ ~ ~o ~ ~ ~o ~ o Cat GS ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~1 1 _ ~ 0 1 HI to 0 1 ~ ~ O ~ ~ O ~° o o o ~of ~ O 0 0 0 ~ ~ a)
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From page 146... ...
146 O ~ ~ O En ~ p: _ O ~ O £ on ~ P
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From page 148... ...
To illustrate this insensitivity, magnitudes, times, and altitudes of maximum temperature increases for several cases are given in Table 7.4. The essence of the temperature change tendencies indicated by the numbers in Tables 7.3 and 7.4 derives from the efficiency of the smoke particles as absorbers of solar radiation coupled with their inefficiency for absorption and emission of infrared radiation.
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From page 149... ...
In the limiting case of a globally uniform absorbing cloud in thermal equilibrium, with solar radiation absorbed above the altitude of thermal emission to space, the mean surface temperature would correspond approximately to the effective emission temperature of a planet in thermal equilibrium with the incident sunlight (about 255 K for the present earth) while the cloud temperature would be much warmer, the exact temperature depending on cloud thermal emissivity.
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From page 150... ...
The first problem is that they neglect the ameliorating effects of the nearly constant sea-surface temperatures on the climate over coastal areas and continental areas exposed to fresh flows of maritime air. This limitation would be most serious for the near-surface temperature changes, because the temperature increases due to absorption of solar radiation aloft would be likely to develop and persist over the oceans as well as over continents.
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From page 151... ...
, in which a 33 percent higher induced surface temperature drop occurs for a case without water clouds than for the corresponding case with normal clouds. In one attempt to account for these effects, MacCracken has used a two-dimensional meridional plane climate model to calculate temperature changes for particulate optical depths specified as meridional averages obtained from the Oregon State GCM (Gates and Schlesinger, 1977~.
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From page 152... ...
Extreme surface temperature changes produced by the model are quite localized, with extreme departures exceeding -40°C by day 40 in patches throughout the land-covered area of the northern hemisphere. Based on the one- and two-dimensional model results discussed previously, these results on day 40 are reasonable, given the very large initial injections and slow removal rates of this calculation.
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From page 153... ...
:40'33 :20 ~11,, 90 60 30 0 -30 LATITUDE (dog) -60 -90 FIGURE 7.8 Meridional temperature cross-section for the perturbed case, the control case, and the perturbed minus the control at t = 10 days from the CCM calculations for summer.
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From page 154... ...
The inclusion of scattering of sunlight and infrared absorption and emission, including the optical effects of injections of dust, would produce different predicted temperature changes. Multiple scattering and inclusion of dust would probably lead to somewhat lower near-surface temperatures, since the particulate cloud would have a higher albedo than the cloud assumed in the CCM simulation.
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From page 155... ...
The neglect of inferred absorption and emission due to the smoke in the CCM calculations could lead to exaggerated cooling rates, but detailed calculations show that this effect is not important at early times for the NRC baseline injections (Ramaswamy and Kiehl, 19841. On the other hand, it could be significant during winter at middle and high latitudes, since the contribution of thermal infrared radiation to the surface energy balance is more important relative to solar radiation during winter.
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From page 156... ...
The following phenomena are considered: fog, cloudiness and precipitation distributions, zonal mean winds, other large-scale wind systems, and ultra-high clouds. Ground Fog Under the influence of large-scale dust and smoke clouds, radiation fogs would form over land areas as the surface temperatures dropped below the dewpoint.
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From page 157... ...
and the two-dimensional model results of MacCracken do show increases in low-level cloudiness in the perturbed regime, but improved boundary layer formulations are needed for a more precise assessment of the role of fog. Because such fogs may be persistent and have ameliorative effects, the sensitivity of surface temperature changes over extensive continental areas to particulate lifetime could be more pronounced than indicated by the considerations of the preceding sections.
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From page 158... ...
These changes correspond to the temperature changes shown in the bottom panel of Figure 7.8. The main features are strongly enhanced west-to-east winds in the region normally occupied by the stratosphere at middle and high latitudes of the southern hemisphere and high latitudes of the northern hemisphere, and very strongly enhanced easterly winds above the subtropical {northern hemisphere)
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From page 159... ...
' 0; ll ~ _: _ H 20 1, _ 20 35 A -- \, I ,~ - -- ~-w~ , 90 60 30 0 -30 LATITUDE (deg) FIGURE 7.10 Zonal wind cross sections for the perturbed case, the control case, and the perturbed minus the control at t = 10 days from the CCM calculations for summer.
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From page 160... ...
~°~ ,, 1,, 20 ~ 15 I 111 0 I 5 O -60 -90 LATITUDE (deg) FIGURE 7.11 Meridional circulation mass flow stream functions from the NCAR CCM for the April (spring)
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From page 161... ...
Since the particulate heating becomes more effective at the lower air densities of higher altitudes, the intensity of the thermally driven meridional circulation would increase as the particles rose. This positive feedback arising from the coupling between transport and heating has been observed in the GCM calculation of MacCraken and Walton (1984)
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From page 162... ...
Thus, if smoke reaches the upper troposphere, there is a good possibility that bands or streamers would be separated from the main cloud mass and stretched into the southern hemisphere even if the enhanced meridional circulation described above does not operate. The separation of bands of smoke from the southern edge of the main cloud would be associated with the complementary process: injection of streamers of clear air northward.
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From page 163... ...
, 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.
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From page 164... ...
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.
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From page 165... ...
. 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.
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From page 166... ...
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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From page 167... ...
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 6°C for these events, although this is representative of the depression of the maximum rather than the daily mean temperature.
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From page 168... ...
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.
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From page 169... ...
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.
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From page 170... ...
(1984) Nuclear war: Illustrative effects of atmospheric smoke and dust upon solar radiation.
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From page 171... ...
Walton (1984) The effects of interactive transport and scavenging of smoke on the calculated temperature change resulting from large amounts of smoke.
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From page 172... ...
The short-term influence of the Mount St. Helens volcanic eruption on surface temperature in the northwest United States.
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From page 173... ...
A one-dimensional model describing aerosol formation and evolution in the stratosphere: II. Sensitivity studies and comparison with observations.
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