Cirrus cloud susceptibility to the injection of ice nuclei in the upper troposphere

Due to their net warming effect, cirrus clouds play a crucial role in the climate system. A recently proposed climate engineering mechanism (CEM) intends to reduce high cloud cover by seeding cirrus clouds with efficient ice nuclei (IN) and therefore cool climate. Here, the susceptibility of cirrus clouds to the injection of ice nuclei in the upper troposphere is investigated in the extended Community Atmospheric Model version 5 (CAM5). Due to large uncertainties associated with the dominant ice nucleation mechanism in cirrus clouds, different control cases were simulated. In addition to pure homogeneous and heterogeneous nucleation, cases with competition between homogeneous and heterogeneous nucleation and different fractions of mineral dust active as IN were considered. Whereas seeding in the pure heterogeneous case leads to a strong warming due to overseeding, an optimal seeding IN concentration of approximately 18 l−1 was found for the other cases. For the optimal seeding concentration, a reduction in the net cloud forcing (NCF) of up to 2Wm−2 was simulated, corresponding to a strong cooling effect. To optimize the cooling and minimize the amount of seeding material, globally nonuniform seeding strategies were tested, with minimal seeding in the summer hemisphere and in the tropics. With seeding applied to less than half the globe, an even stronger reduction in the NCF was achieved. This suggests that the CEM could work for an atmosphere even with considerable heterogeneous ice nucleation and that the desired cooling could be obtained without seeding the entire globe.