Radiative effects of changing atmospheric water vapour

Abstract

We have used the radiative computation scheme of Dopplick to investigate the purely radiativeeffects resulting from variations in the total water vapour content in a clear atmospheric column.We consider a given water vapour profile uniformly scaled in the vertical by a factor over therange 0.2 to 2.4. For the case of varying water vapour but fixed temperature profile, we find thatadditional vapour produces greater cooling of the column as a whole while supplying energy tothe surface and reducing the net energy flux out of the top of the column. For the samechanges in vapour content, but atmospheric temperature varying so as to maintain constantrelative humidity, we find even greater relative cooling of the atmospheric column as a whole. butless effective energy supply to the surface and an actual increase in the net flux upward at thetop of the column.

Radiative heating rates were calculated for temperature and moisture profiles measured overthe Arabian Sea during the 1979 summer MONEX and show radiative cooling of the lowerlayer and energy flux to the surface associated with the presence of water vapour in the lowerlayers of the atmosphere which may be important in the evolution of this vapour-rich surfacelayer preceding the monsoon.