Clear-Sky Nocturnal Temperatures Forecast and the Greenhouse Effect

Abstract

Nocturnal evolution of air and soil temperatures are computed for clear-sky situations. The model takes into account the soil heat conduction and the atmospheric radiative transfers by using radiosonde data for temperature, water vapor, and ozone. Turbulent exchanges in the surface layer are also taken into account. Simulations of several cases distributed around the year at a midlatitude station (Uccle, located in Brussels) give very good results both at the ground and in the air close to the ground-the errors being usually less than 0.5°C. This can be achieved thanks to Morcrette's accurate IR flux transmission functions and, for the surface temperature computation, by giving up the force-restore method for the benefit of Brunt's analytical solution. A by-product of the study is to use the model to investigate the sensitivity of the results to CO2 changes. Due to close saturation of the present CO2-H2O bands, the dependence appears very weak: when progressively increasing CO2, the surface downward infrared flux tends to an asymptotic value not too distant from the present one. The surface temperature dependence is further smoothed by the fact that soil conduction acts to dissipate the energy excesses reaching the surface. It appears that changing the atmospheric water vapor content by 0.5 kg m?2 has, on the surface temperature, the same effect as doubling CO2. Accordingly, to be convincing, an analysis of the contribution to the greenhouse effect of CO2 changes should, namely, restore the water cycle to a high degree of accuracy.