RADIATIVE-CONVECTIVE MODEL WITH AN EXPLICIT HYDROLOGIC-CYCLE .2. SENSITIVITY TO LARGE CHANGES IN SOLAR FORCING

The one-dimensional radiative-convective equilibrium model with an exp licit hydrologic cycle introduced in part 1 is used to study the sensi tivity of the model's atmosphere to large changes in the solar forcing , when various cumulus convection parameterizations are used. As shown by Simpson [1927], by Komabayasi [19671, and by Ingersoll [1969] when the concentration of the absorbing gas in the atmosphere is temperatu re dependent, equilibrium is impossible for values of the solar forcin g larger than a critical value. This result is referred to as a runawa y greenhouse. The cumulus convection parameterization schemes currentl y in use in global climate models (GCMs) employ different assumptions about moistening. This causes the critical solar forcing above which a runaway greenhouse occurs to be very sensitive to the cumulus convect ion scheme employed. Futhermore, using the microphysically based cumul us convection scheme proposed by Emanuel [1991], we show that the sens itivity of the equilibrium temperature to changes in the solar forcing depends crucially on the microphysics of cumulus convection. For fixe d cloud conditions, the critical solar forcing for a runaway greenhous e to occur is between approximately 1.22 and 1.49 times the global mea n value for the Earth, and for clear sky conditions, it is a few perce nt lower. The runaway greenhouse in the experiments with the mass flux schemes generally occurs more rapidly than in the experiments with th e adjustment schemes. In addition, the inability of the hard convectiv e adjustment scheme to produce an efficient vertical transport of mois ture, together with the saturation requirement for convection to occur , leads to the breakdown of the radiative-convective equilibria when o ther processes axe not available to provide the necessary vertical tra nsport of water vapor.