GREENHOUSE SENSITIVITY EXPERIMENTS WITH PENETRATIVE CUMULUS CONVECTION AND TROPICAL CIRRUS ALBEDO EFFECTS

Results are presented from two versions of a global R15 atmospheric ge neral circulation model (GCM) coupled to a nondynamic, 50-m deep, slab ocean. Both versions include a penetrative convection scheme that has the effect of pumping more moisture higher into the troposphere. One also includes a simple prescribed functional dependence of cloud albed o in areas of high sea-surface temperature (SST) and deep convection. Previous analysis of observations has shown that in regions of high SS T and deep convection, the upper-level cloud albedos increase as a res ult of the greater optical depth associated with increased moisture co ntent. Based on these observations, we prescribe increased middle- and upper-level cloud albedos in regions of SST greater than 303 K where deep convection occurs. This crudely accounts for a type of cloud opti cal property feedback, but is well short of a computed cloud-optical p roperty scheme. Since great uncertainty accompanies the formulation an d tuning of such schemes, the prescribed albedo feedback is an interme diate step to examine basic feedbacks and sensitivities. We compare th e two model versions (with earlier results from the same model with co nvective adjustment) to a model from the Canadian Climate Centre (CCC) having convective adjustment and a computed cloud optical properties feedback scheme and to several other GCMs. The addition of penetrative convection increases tropospheric moisture, cloud amount, and planeta ry albedo and decreases net solar input at the surface. However, the c ompeting effect of increased downward infrared flux (from increased tr opospheric moisture) causes a warmer surface and increased latent heat flux. Adding the prescribed cirrus albedo feedback decreases net sola r input at the surface in the tropics, since the cloud albedos increas e in regions of high SST and deep convection. Downward infrared radiat ion (from increased moisture) also increases, but this effect is overp owered by the reduced solar input in the tropics. Therefore, the surfa ce is somewhat cooler in the tropics, latent heat flux decreases, and global average sensitivity to a doubling of CO2 with regard to tempera ture and precipitation/evaporation feedback is reduced. Similar proces ses, evident in the CCC model with convective adjustment and a compute d cloud optical properties feedback scheme, occur over a somewhat expa nded latitudinal range. The addition of penetrative convection produce s global effects, as does the prescribed cirrus albedo feedback, altho ugh the strongest local effects of the latter occur in the tropics.