ALBERT

Description: This study investigates the role of aerosol microphysics in stratospheric sulfate aerosol changes after the 1991 Mt. Pinatubo eruption using an atmospheric general circulation model that is coupled interactively with a chemistry module and a modal aerosol microphysical module with three-modes. Our model can reproduce the global mean stratospheric aerosol optical depth (SAOD) observed by the Stratospheric Aerosol and Gas Experiment (SAGE) II during June 1991 – January 1993. The model underestimates the observed SAOD before the eruption and after January 1993. The model also underestimates the integrated backscatter coefficient (IBC) observed by ground-based lidar at Tsukuba, Naha, and Lauder. The modeled effective radius becomes larger (about 0.5 μ m) and agrees with the balloon-borne measurements at Laramie, Wyoming (41 ∘ N, 105 ∘ W). We further investigate effects of the inclusion of evaporation along with the condensation processes and the inclusion of van der Waals and viscous forces in the coagulation processes. The inclusion of evaporation along with the condensation processes reduces the global mean effective radius by up to 0.04 μ m and increases the global burden of stratospheric sulfate aerosols (about 15 % in late 1993). The inclusion of van der Waals and viscous forces in the coagulation processes increases the global mean effective radius by up to 0.06–0.07 μ m and decreases the global burden (15–30 % in late 1993). The effects of van der Waals and viscous forces differ between two schemes. However, we do not conclude which simulation is superior because all simulations fall within error bars.

Description: Results of comprehensive long-term simulations of surface all-sky and clear-sky ultraviolet (UV) radiation through 1960–2100 are presented. A new earth system model, MIROC-ESM-CHEM, is used for the simulation, which considers key processes that change the surface UV radiation: atmospheric dynamics and chemistry affecting ozone in the stratosphere and troposphere, aerosols and clouds in the troposphere, and changes in surface albedo with sea ice and snow cover. In contrast to previous assessments considering only the effect of long-term change in stratospheric ozone, the simulated long-term behavior of UV radiation in this study is strongly affected by other processes. In one of two simulations, all-sky UV radiation in the northern midlatitudes is projected to increase in the 21st century despite the expected recovery of the stratospheric ozone layer. Reductions in aerosols and clouds are expected to overcompensate for the effect of ozone recovery. The results are sensitive to the future socioeconomic scenario, describing GHG concentrations and emissions of aerosol and ozone precursors in the troposphere. The interannual variability of UV radiation associated with the 11 year solar cycle and local processes is also discussed.