Radiative effect of clouds on tropospheric chemistry in a global three-dimensional chemical transport model

Results using the linear assumption, which assumes linear scaling of cloud optical depth with cloud fraction in a grid box, show global mean OH concentrations generally increase by less than 6% because of the radiative effect of clouds. The OH distribution shows much larger changes (with maximum decrease of ~20% near the surface), reflecting the opposite effects of enhanced (weakened) photochemistry above (below) clouds.

The global mean photolysis frequencies for J[O¹D] and J[NO₂] in the troposphere change by less than 5% because of clouds; global mean O₃ concentrations in the troposphere increase by less than 5%. This study shows tropical upper tropospheric O₃ to be less sensitive to the radiative effect of clouds than previously reported (~5% versus ~20-30%). These results emphasize that the dominant effect of clouds is to influence the vertical redistribution of the intensity of photochemical activity while global average effects remain modest, again contrasting with previous studies.

Differing vertical distributions of clouds may explain part, but not the majority, of these discrepancies between models. Using an approximate random overlap or a maximum-random overlap scheme to take account of the effect of cloud overlap in the vertical reduces the impact of clouds on photochemistry but does not significantly change our results with respect to the modest global average effect.