The Effects of Moist Convection and Water Vapor Radiative Processes on Climate Sensitivity

Abstract

The primary interest of the present study is to examine the sensitivity of climate to radiative perturbations such as increases in CO2 and solar insolation for surface temperatures warmer than present day global averaged values (Ts> 290 K). The climate sensitivity, defined here as the change in Ts, is examined with the aid of a one-dimensional radiative-convective model. The solar insolation in the model is varied from 880 to 1840 W m?2 to obtain a wide range of Ts, from 255 to 325 K. We examine in detail the dependence of the computed ?Ts, on the following processes which are known to be important in the warmer regions (e.g., tropics) of the present day atmosphere: convective parameterizations (fixed lapse-rate, moist-adiabatic adjustment and cumulus adjustment); H2O vertical distribution; and H2O longwave radiative treatment. The climate sensitivity is shown to vary nonlinearly with Ts and to depend strongly on: (i) convective processes; (ii) H2O continuum absorption; and (iii) upper tropospheric (pressure, p<500 mb) relative humidity. With one major exception, for all the cases considered in this paper, the climate sensitivity increases with Ts, for Ts<300 K and decreases by more than a factor of 2 as Ts, increases from 300 to 325 K. Hence, for both fixed lapse-rate and moist lapse-rate models, our calculations clearly rule out the possibility (but for the exception noted below) of a runaway greenhouse effect. The one major exception is when the upper tropospheric relative humidity value is allowed to attain values of 50% as opposed to the traditionally assumed one-dimensional model profile in which the relative humidity decreases linearly with P from a value of about 80% at the surface to about 15% at about 200 mb. In the instance, when the upper tropospheric relative humidity is held fixed (as Ts changes) at 50%, the sensitivity seems to increase even when Ts exceeds 300 K. In order to facilitate theoretical interpretation of the numerical results, the climate feedback parameter ? is inferred from surface and from top-of-the-atmosphere energy balance considerations. The inferred ? illustrate the consistency between the two approaches of interpreting climate sensitivity.