Influence of the vertical structure of the atmosphere on the seasonal variation of precipitable water and greenhouse effect

By using satellite observations and ECMWF analyses, we study the seasonal variations of the precipitable water and the greenhouse effect, defined as the normalized difference between the longwave flux emitted at the surface and that emergent at the top of the atmosphere. Over ocean, in middle and high latitudes, the seasonal variation of the mean temperature lapse rate in the troposphere leads to large seasonal phase lags between greenhouse effect and precipitable water. By contrast, the seasonal variation of the clear-sky greenhouse effect over tropical oceans is mainly driven by the total atmospheric transmittance and thus by precipitable water variations. Over land, the seasonal variation of the tropospheric lapse rate acts to amplify the radiative impact of water vaopor changes, giving a strong seasonal variation of the greenhouse effect. -from Authors