A One-Dimensional Study of Possible Cirrus Cloud Feedbacks

Abstract Feedbacks involving cirrus clouds may Play an important role in modulating the response of the climate system to external forcing. A standard radiative-convective model with high vertical resolution (10-mb grid spacing in the upper troposphere) is used to investigate some of these feedbacks in response to changes in atmospheric carbon dioxide concentration. It is shown that such a high resolution (or, indeed, an even higher resolution) may be necessary to resolve the change in tropopause height following a doubling of carbon dioxide. The cirrus cloud is allowed to adjust its height and ice water content in response to the warming, and experiments are repeated 1) for fixed and moist adiabatic lapse rates, 2) with and without a prescribed relative humidity feedback, 3) at low as well as high vertical resolution, and 4) with the inclusion of a simple correction to account for the uncertain number of small ice crystals in cirrus clouds. In this model the ice water content feedback is found to be posi...