A Semianalytic Energy Balance Climate Model with Explicit Sea Ice and Snow Physics

Abstract

An energy balance climate model (EBCM) is presented having 1) a seasonal cycle; 2) surface-air, land-sea, and latitudinal resolution; 3) simulation of sea ice in terms of a number of explicit physical processes and in such a way that the sea ice fraction in any given zone changes continuously during the course of the seasonal cycle; 4) simulation of a continuously varying land-snow fraction in terms of explicit physical processes; and 5) a detailed treatment of surface and planetary albedo. A semianalytic solution is used which permits use of 6 day time steps, with very little dependence of the simulated climate on the choice of time step length for time steps of 1 to 6 days. Model sensitivity to internal parameter changes is investigated. The temperature response to a doubling of the drag coefficients for the vertical fluxes of latent and sensible heat is complex, and involves radiative constraints, the effect of stronger coupling to the large thermal inertia of the mixed layer, and ice and snow feedbacks. The model response to changes in the drag coefficient depends, in part, on the functional form of the parameterization of infrared emission to space, even when this parameter change has no direct or feedback effect on radiation. The effect on meridional heat fluxes of doubling the meridional diffusion coefficients is largely governed by radiative constraints; an important implication for EBCMs is that one cannot use model-simulated meridional heat fluxes to tune the diffusion coefficients or heat flux parameterization. The most important elements of the surface albedo parameterization for climate, in decreasing order of importance, are 1) the temperature dependence of ice and snow albedo, 2) the effect of partial vegetational masking of land snowcover, and 3) surface albedo zenith angle dependencies, with the latter having a negligible effect on planetary albedo. Removing the temperature dependence of ice and snow albedo leads to a doubling of both sea ice extent and thickness in the Northern Hemisphere, with smaller changes in the Southern Hemisphere. When both direct and diffuse beam surface albedos are altered, changes in planetary albedo are 60%?70% and 20%?25% the changes in surface albedo for clear and cloudy skin respectively. Because observed cloudiness tends to be large at high latitudes, zonally averaged planetary albedo changes are about 30% the size of surface albedo changes resulting from ice and snow feedbacks. The effect on temperature of partial masking of land snowcover by forests is found to be significantly smaller than obtained by others.