An investigation of the global solar radiative forcing due to changes in cloud liquid water path

The instantaneous solar radiative forcing of the surface-atmosphere system associated with a change in the liquid water path (LWP) of low clouds has a significant space-time dependence, owing to the spatial and temporal variations in isolation, solar zenith angle, and surface albedo. This feature is demonstrated by considering globally uniform LWPs and LWP changes. Keeping cloud amounts fixed in space and time, we find that an increase in LWP imparts a distinct meridional gradient to the solar forcing, while the difference between summer and winter forcings introduces a seasonal variation at any given latitude. Relative to the global, annual mean (GAM) value (a negative quantity for an increase in LWP) the forcing is more negative at low latitudes throughout the year and during summer at the high latitudes (poleward of 40°). Thus even the simple assumption of a globally uniform LWP change does not yield a uniform forcing at all latitudes and/or times. While a specific globally uniform LWP increase can yield a global, annual mean radiative forcing that is opposite to but has the same magnitude as that for carbon dioxide increases, such a compensation in the forcing cannot be expected to be uniform with latitude or month. -from Authors