A POSSIBLE CHANGE IN CLOUD RADIATIVE FORCING DUE TO AIRCRAFT EXHAUST

Aircraft exhaust may reduce the crystal size in natural cirrus. This w ork investigates the change in cloud radiative forcing from such a siz e reduction by assuming a constant ice water content. A 1-dim model wi th radiative properties that depend on the mean crystal size is used t o compute the radiative transfer for an atmospheric column. The result s show that the negative shortwave cloud forcing is enhanced with smal ler crystals as they mainly increase the reflectivity of clouds. The c hange in the longwave cloud forcing is always positive although its ma gnitude depends strongly on the ice water path. The weighted sum of SW and LW cloud forcings depends on the mean crystal size, surface albed o and ice water content. It appears that there is a range of diameters between 15 and 25 mu m where the response to a reduction in crystal s ize is fairly insensitive. Below and above this range the change is ne gative or positive, respectively. In regions of dense air traffic the magnitude of the change in cloud forcing could be on the order of 0.3 W m(-2) under the assumption of a 20% decrease of the mean crystal siz e from about 30 mu m to 24 mu m. Aircraft exhaust thus has the potenti al to affect the climate but the results should be taken with caution as they are based on parameterized optical properties for cirrus cloud s.