Cloud Reflectivity Modification - Reducing Cirrus Trapping of Outgoing Infrared Radiation

Reducing Cirrus Trapping of Outgoing Infrared Radiation

Synoptic cirrus clouds may be capable of modification to reduce their lifetime and hence their net positive radiative forcing (opposite to the low-cloud scheme), using seeding mechanisms distributed by airliners. It is believed that the synoptic cirrus in the upper troposphere are formed by homogeneous nucleation, resulting in large numbers of small ice crystals. If ice nuclei are introduced into this environment, the cirrus may instead form by heterogeneous nucleation. If the concentration of ice nuclei is tuned such that the resulting cloud particle density is less than for the natural case, the cloud particles should grow larger due to less water vapor competition and attain higher settling velocities. The net effect should be a reduced optical thickness for outgoing infrared radiation and a reduced cloud lifetime. The effects of this modification on the cloud radiative balance operate in different manner from that of the marine stratocumulus cloud brightening scheme. Instead of increasing the incoming shortwave reflectivity and lifetime of the cloud in order to increase the net radiative cooling effect, the outgoing infrared reflectivity and lifetime are reduced thereby decreasing the net radiative heating effect of cirrus. This authors claim that this scheme is sufficient to reverse the warming caused by a doubling of CO₂.