Clouds and Climate: Sensitivity of Simple Systems

Abstract

A one-dimensional radiative convective model is used to gage the influence of clouds on simple climate systems. The radiative transfer model is developed to accommodate in a systematic and consistent manner the optical properties of a hierachy of cloud types. Cloud albedo and emissivity relationships for both ice and water clouds are introduced. The model structure is highly sensitive to cloud height for all cloud types and particularly sensitive to water path for optically thin clouds. High thin clouds at low and middle latitudes in all seasons and all clouds at high latitudes in winter tend to warm the surface relative to the clear sky; all other clouds tend to cool the surface. The summer high-latitude effects are similar to the low- and middle-latitude situation. The model sensitivity is compounded by surface albedo effects. For a given cloud a critical surface albedo may exist at which the cloud transits from cooling the surface relative to clear-sky conditions to warming the surface when the surface albedo is increased. The critical albedos fall within the observed albedo range of real systems. The sensitivity of the model indicates that great care is necessary in developing parameterizations for the radiative structure of clouds and for surface albedos for use in climate models. It is argued that simple energy balance models (e.g., Budyko, 1969) will be of limited use in climate research unless a cloud amount-surface temperature relationship is established. For example, the cloud-surface temperature effect will tend to buffer the Budyko surface temperature-surface albedo feedback if cloud amount increases with decreasing surface temperature or is independent of surface temperature but will enhance the feedback if the cloud-surface temperature relationship were reversed. It is concluded that the establishment of a simple cloud amount-temperature relationship will remain an elusive goal.