INFLUENCE OF THE VERTICAL STRUCTURE OF THE ATMOSPHERE ON THE SEASONAL-VARIATION OF PRECIPITABLE WATER AND GREENHOUSE-EFFECT

By using satellite observations and European Centre for Medium Range W eather Forecasts analyses, we study the seasonal variations of the pre cipitable water and the greenhouse effect, defined as the normalized d ifference between the longwave flux emitted at the surface and that em ergent at the top of the atmosphere. Results show a strong systematic influence of the vertical structure of the atmosphere on geographical and seasonal variations of both precipitable water and greenhouse effe ct. Over ocean, in middle and high latitudes, the seasonal variation o f the mean temperature lapse rate in the troposphere leads to large se asonal phase lags between greenhouse effect and precipitable water. By contrast, the seasonal variation of the clear-sky greenhouse effect o ver tropical oceans is mainly driven by the total atmospheric transmit tance and thus by precipitable water variations. Over land, the season al variation of the tropospheric lapse rate acts to amplify the radiat ive impact of water vapor changes, giving a strong seasonal variation of the greenhouse effect. Over tropical land regions, monsoon activity generates a seasonal phase lag between surface temperature and relati ve humidity variations that gives a. seasonal lag of about 2 months be tween the surface temperature and the clear-sky greenhouse effect. Gen erally, the cloudiness amplifies clear-sky tendencies. Finally, as an illustration, obtained results are used to evaluate the general circul ation model of the Laboratoire de Meteorologie Dynamique.