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Gyula Molnar and Wei-Chyung Wang

Abstract

Cloud optical properties, in particular the optical thickness, affect the earth-atmosphere radiation budget, and their potential changes associated with climate changes may induce feedback effect. A one-dimensional radiative-forcing model was used to illustrate that the difference in the vertical distribution of the radiative forcing between C02 increase and changes of solar constant can result in a different τ feedback. Recently, Wang et al. carried out a general circulation model study of the climatic effect of atmospheric trace gases CH4, CFCS, and N2O, and the model results indicate that these trace gases provide an important radiative energy source for the present climate. Because the radiative-forcing behavior of CO2 is different from that of these other gases, the simulations also show that different radiative forcing can lead to quite different climatic effects. Consequently, increases in these trace gases may also induce different τ feedback than that due to CO2 increase. Since no study was attempted before to address this aspect, here a one-dimensional model is used to investigate the τ feedback associated with trace gases using an updated τ scheme that relates τ to cloud liquid water content through cloud layer latent heat flux. Because of the different changes in the τ vertical distribution the τ feedback is calculated to be a small negative value for a C02 increase, but much larger negative values for increases of trace gases. The strongest negative feedback is found for CFCs.

Similar experiments were also feedback conducted using a revised version of the Somerville and Remer τ scheme, which relates τ to cloud liquid water content through cloud temperature. The results indicate that the negative feedback for C02 increases for a single cloud layer becomes much smaller when multiple-layer clouds are used, mainly due to the compensating effect of changes in τ values between high and low clouds. Because this scheme assumes a strong functional dependence of the local temperature, the τ feedback is also found to be sensitive to model dimensionally. In addition, the strength and sometimes even the sign of the τ feedback calculated from both schemes depend on the vertical distribution of cloud cover for the control climate, indicating the complexity of cloud-radiation interaction Clearly, more observational and theoretical studies are needed to understand the cloud microphysics and their relation to large-scale climate variables.

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Robert M. MacKay, Malcolm K. W. Ko, Run-Lie Shia, Yajaing Yang, Shuntai Zhou, and Gyula Molnar

Abstract

In order to study the potential climatic effects of the ozone hole more directly and to assess the validity of previous lower resolution model results, the latest high spatial resolution version of the Atmospheric and Environmental Research, Inc., seasonal radiative dynamical climate model is used to simulate the climatic effects of ozone changes relative to the other greenhouse gases. The steady-state climatic effect of a sustained decrease in lower stratospheric ozone, similar in magnitude to the observed 1979–90 decrease, is estimated by comparing three steady-state climate simulations: I) 1979 greenhouse gas concentrations and 1979 ozone, II) 1990 greenhouse gas concentrations with 1979 ozone, and III) 1990 greenhouse gas concentrations with 1990 ozone. The simulated increase in surface air temperature resulting from nonozone greenhouse gases is 0.272 K. When changes in lower stratospheric ozone are included, the greenhouse warming is 0.165 K, which is approximately 39% lower than when ozone is fixed at the 1979 concentrations. Ozone perturbations at high latitudes result in a cooling of the surface–troposphere system that is greater (by a factor of 2.8) than that estimated from the change in radiative forcing resulting from ozone depletion and the model’s 2 × CO2 climate sensitivity. The results suggest that changes in meridional heat transport from low to high latitudes combined with the decrease in the infrared opacity of the lower stratosphere are very important in determining the steady-state response to high latitude ozone losses. The 39% compensation in greenhouse warming resulting from lower stratospheric ozone losses is also larger than the 28% compensation simulated previously by the lower resolution model. The higher resolution model is able to resolve the high latitude features of the assumed ozone perturbation, which are important in determining the overall climate sensitivity to these perturbations.

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