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OCR for page 127
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
OCR for page 128
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
OCR for page 129
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 30°N and 70°N 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 30°N to 60°N 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.
OCR for page 130
-
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~.
OCR for page 131
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
OCR for page 132
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 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. For the NRC baseline case, with smoke and dust assumed to be
instantaneously dispersed to a uniform distribution over the 30°N to
70°N 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).
OCR for page 133
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
OCR for page 134
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 (30°N to 70°N), the NRC baseline injections
spread over 0° to 90°N, and the NRC fast-rainout variant with injections
spread over 0° to 90°N.
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 30°N-70°N
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
OCR for page 135
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
30°N-70°N; none 30°N-70°N;
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
30°N to 70°N latitude belt. The extinction optical depths in the same
OCR for page 136
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 30°N to 70°N 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
~. · ~
OCR for page 137
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 30°N to 70°N latitude band. The initial
opacity for this case is nearly equivalent to that for smoke and dust
spread over the 30°N to 70°N 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
OCR for page 163
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
OCR for page 164
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.
OCR for page 165
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 4°C for
4 days. Smith (1950) quotes an estimate by Fritz that the maximum
temperature was reduced by as much as 6°C, with no compensating rise in
minimum temperature. By the time the plume had reached England, it
OCR for page 166
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 8°C. 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 6°C 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
OCR for page 168
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 80°C 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.
OCR for page 169
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.
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,
Representative terms from entire chapter:
solar radiation
