CLEAR-SKY NOCTURNAL TEMPERATURES FORECAST AND THE GREENHOUSE-EFFECT

Nocturnal evolution of air and soil temperatures are computed for clea r-sky situations. The model takes into account the soil heat conductio n and the atmospheric radiative transfers by using radiosonde data for temperature, water vapor, and ozone. Turbulent exchanges in the surfa ce layer are also taken into account. Simulations of several cases dis tributed around the year at a midlatitude station (Uccle, located in B russels) give very good results both at the ground and in the air clos e to the ground-the errors being usually less than 0.5 degrees C. This can be achieved thanks to Morcrette's accurate IR nux transmission fu nctions 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 sensi tivity of the results to CO2 changes. Due to close saturation of the p resent CO2-H2O bands, the dependence appears very weak: when progressi vely increasing CO2, the surface downward infrared flux tends to an as ymptotic value not too distant from the present one. The surface tempe rature dependence is further smoothed by the fact that soil conduction acts to dissipate the energy excesses reaching the surface. It appear s that changing the atmospheric water vapor content by 0.5 kg m(-2) ha s, on the surface temperature, the same effect as doubling CO2. Accord ingly, tb be convincing an analysis of the contribution to the greenho use effect of CO2 changes should, namely, restore the water cycle to a high degree of accuracy.