Two‐dimensional simulations of possible mesoscale effects of nuclear war fires: 1. Model description

The two-dimensional mesoscale meteorological model and the aerosol model described in the companion paper by Giorgi [this issue] are used to investigate mesoscale effects induced by atmospheric injections of purely absorbing smoke from nuclear war fires. Simulations are carried out for different fire types (city center, suburban, and forest fires), aerosol loadings, particle properties, and atmospheric conditions. We analyze three effects which develop on spatial scales of 10–500 km and time scales of 1–2 days and can be important for assessments of environmental impacts of nuclear war: (1) smoke-induced formation of clouds and precipitation and efficiency of smoke removal; (2) smoke vertical transport; and (3) surface cooling induced by the smoke absorption. In convectively unstable and moist environments the low-level uplifting induced by the smoke heating can initiate convective precipitation. In the absence of substantial moisture sources from the fires, precipitation develops mostly at the edges of the smoke plumes and is associated with the inhomogeneities in the smoke distribution, since these allow low-level smoke heating. When the smoke is dispersed by the atmospheric winds and attains a more homogeneous distribution, most of the heating takes place at more stable higher altitudes, the smoke shielding stabilizes the lower troposphere and precipitation formation is hindered. Wet removal dominates dry removal processes and its efficiency depends on the properties and vertical distribution of the injected aerosol. In a wide variety of experiments, the fraction of the total injected smoke mass removed during 48 hour simulations varied from 3 to 20% for injections from suburban fires, from 10 to 20% for forest fires, and about 1% for city center fires. It is also shown that substantial moisture sources from the fires can significantly enhance precipitation formation and removal. Early after injection, when the individual smoke plumes have not yet merged together, self-lofting in the upper regions of the plumes and cloud updrafts in the presence of smoke-induced deep convection carry out most of the smoke vertical transport. At later stages, when the smoke is progressively dispersed by the atmospheric winds, the smoke heating generates a region of relatively weak stability in the upper troposphere overlying a midtropospheric inversion. As a result, upper tropospheric smoke is efficiently uplifted, while lower tropospheric smoke is trapped near the surface. In our simulations, after 48 hours of model time, 34–37% of the smoke mass is transported above 280 mbar for city center fires and 5–12% for suburban fires (with 25–30% above 500 mbar). In the case of forest fires, 80–90% of the smoke remains trapped below 500 mbar. For vegetated surfaces, the canopy foliage temperature is more sensitive to depleted insolation than the ground temperature (by up to several degrees) and as such could be a better indicator for assessments of ecological impacts of nuclear war. For dry environments and large smoke loadings, our calculated canopy foliage coolings reach 25–30 K for the peak daytime temperature and 6–7 K for the minimum nighttime temperature. They decrease for increasing atmospheric water vapor, soil moisture, and surface layer winds. We give an example of conditions that can lead to foliage frosting but not ground freezing during the quick-freezing episodes predicted by recent general circulation model studies. We finally discuss how our results can be used in the design of injection scenarios for general circulation model simulations.