Sensitivity study of advection and diffusion coefficients in a two-dimensional stratospheric model using excess carbon 14 data.

Using the California Institute of Technology/Jet Propulsion Laboratory two-dimensional transport model, with transport coefficients taken from Yang and Tung (1989), we study the time evolution of excess carbon 14 in the stratosphere and the troposphere from October, 1963 to December, 1966. The model provides a satisfactory simulation of the observed data. Due to the impulsive nature of its source, initial distributions of excess carbon 14 exhibit large spatial gradients. This permits important constraints on the range of transport coefficients in the lower stratosphere to be derived. The standard model uses the circulation and eddy diffusivity of the year 1980. Large deviations (by factor of 2) from this standard transport are ruled out by our model. A self-consistently derived Kyy which is small (approximately 10(9) cm2 s-1) in tropical regions, but is larger (approximately 10(10) cm2 s-1) at higher latitudes is preferred. A Kzz as large as 1 x 10(4) cm2 s-1 would be inconsistent with the data. Excess carbon 14 is removed from the atmosphere with surface deposition velocities vS = 3 x 10(-3) cm s-1 and vN = 5 x 10(-3) cm s-1 in the southern and northern hemispheres, respectively. The last result is contrary to the current understanding that the oceans are the dominant sink for excess 14C.

Two-dimensional atmospheric transport and chemistry model: numerical experiments with a new advection algorithm.

Extensive testing of the advective scheme, proposed by Prather (1986), has been carried out in support of the California Institute of Technology-Jet Propulsion Laboratory two-dimensional model of the middle atmosphere. We generalize the original scheme to include higher-order moments. In addition, we show how well the scheme works in the presence of chemistry as well as eddy diffusion. Six types of numerical experiments including simple clock motion and pure advection in two dimensions have been investigated in detail. By comparison with analytic solutions it is shown that the new algorithm can faithfully preserve concentration profiles, has essentially no numerical diffusion, and is superior to a typical fourth-order finite difference scheme.

Enhancement of atmospheric radiation by an aerosol layer.

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of "photon trapping." We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogenous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20 degrees N latitude, summer, by as much as 45% at 2900 angstroms. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column.