The Tropospheric Lapse Rate and Climatic Sensitivity: Experiments with a Two-Level Atmospheric Model

Abstract

The sensitivity of both moist and dry versions of a two-level primitive equation atmospheric model to variations in the solar constant is analyzed. The models have fixed surface albedos, fixed cloudiness and a zero heat flux lower boundary condition, and are forced with annual mean solar fluxes. An attempt is made to understand the response of the static stability in these model atmospheres and the importance of these changes in stability for the climatic responses of other parts of the system. In the moist model, the static stability increases in low latitudes but decreases in high latitudes as the solar constant increases, resulting in considerable latitudinal structure in the sensitivity of surface temperatures and zonal winds. In the dry model the stability decreases at all latitudes as the solar constant increases. It is argued that this decrease in stability in the dry model, through its effect on isentropic slopes and the supercriticality of the flow, is responsible for the observed large increases in eddy energies and fluxes. Parameterization schemes for the eddy heat flux are critically examined in light of these results.