Sensitivities of the radiative forcing due to large loadings of smoke and dust aerosols

Abstract

Sensitivities of the optical properties and of the radiative perturbations induced by the microphysical characteristics of smoke and dust aerosols in the aftermath of a nuclear war are investigated. All optical calculations assume homogeneous spherical particles, prescribed by lognormal size distributions. A change in the mode radius of the aerosol size distribution between 0.1 and 1.0 μm, for a constant smoke loading, results in a tenfold decrease in the visible optical depth. Over the same size range the long‐wave optical depth increases from 1/30 of the visible optical depth for a mode radius of 0.1 μm to 1/2 for a mode radius of 1.0 μm. The direct radiative forcing is studied using a spectrally dependent, one‐dimensional radiative transfer model. The radiative heating is strongly influenced by the vertical distribution of smoke and dust aerosols. When compared with a constant density profile, the maximum in the heating rate, owing to a 3‐km scale height profile of smoke aerosols (visible optical depth ∼3), is reduced from 16° to 4°K/d and is more widely distributed in the troposphere. Solar absorption in the troposphere is reduced by increases in dust loadings in the stratosphere (a loading of 0.5 g/m2 results in a 30% reduction), while the planetary albedo is enhanced over the smoke‐only case (up to 150% for 0.5 g/m2). Perturbations to the long‐wave fluxes due to aerosols are modulated by the column amount of water vapor. The increase in the long‐wave flux at the surface, however, even for an order of magnitude enhancement in water vapor, is less than the decrease in the solar flux. A time‐marching, one‐dimensional radiative convective model, which uses an eddy diffusion approach to represent convective processes, is employed to study the temporal development of the thermal structure. The relative altitude of the maximum solar absorption to long‐wave emission determines the strength and vertical extent of the tropospheric inversion and the surface temperature response. For a nominal long‐wave smoke optical depth (0.2) the higher the altitude of solar absorption, the greater the reduction in the surface temperature. Thus the assumption of a constant smoke density profile between 0 and 10 km results in a surface cooling of 32°K after 20 days, while the assumption of a constant scale height profile of 3 km causes a cooling of 22°K. For an arbitrarily large long‐wave smoke optical depth (2.0) arising owing to the presence of large (>2 μm) particles in high concentrations in relation to the submicron sizes (a rare event in the unperturbed atmosphere) or owing to large smoke loadings, the altitudes of maximum solar absorption and long‐wave emission become identical. If this altitude is located near the surface, the surface temperature response becomes weak. Thus for a 3‐km aerosol scale height profile, the surface cooling is 3°K, while an extreme assumption of a 1‐km scale height profile causes no change in the surface temperature.