Simulation Analysis Research On Different Implementation Strategies Of Cirrus Cloud Thinning

To improve the ability to proactively deal with global climate change,scientists have proposed geoengineering that aims to cool the Earth artificially.Cirrus cloud thinning as a geoengineering approach that intervenes the radiation budgets has been received widespread attention.To fully exploit the globally cooling effect of cirrus cloud thinning,this study improved cirrus seeding method and conducts simulations using the Community Atmosphere Model version 5(CAM5).First,we compared the climate response of the seeding method used in the previous study(fixed seeding method)and the improved seeding method used in this study(flexible seeding method).The flexible seeding method refers to seeding with ice crystal particles(radius and number concentration)based on the atmospheric conditions,so that the homogeneous nucleation can be just suppressed.The simulation results show a global cooling effect of-1.36±0.18 W m^-2,which is approximately two-thirds of that from artificially turning off homogeneous nucleation(-1.98±0.26 W m^-2).However,simulations with fixed seeding ice nuclei particle number concentrations of 20 and 200 L^-1 show a weak cooling effect of-0.27±0.26 W m^-2 and warming effect of 0.35±0.28 W m^-2,respectively.This indicates that in terms of cooling potential and environmental safety,the flexible seeding method is better than the fixed seeding method.In addition,the surface downwelling shortwave radiation(i.e.,brightening effect)is increased(6.61 W m^-2)in the flexible cirrus seeding simulation.The sensitivity experiments are conducted to explore the cooling potential of the flexible seeding method.The globally cooling effect from seeding with different sizes of ice crystals have similar cooling effects(the difference is smaller than 0.2 W m^-2).Sensitivity experiments also show that cirrus seeding carried out at latitudes with large solar noon zenith angles yields a stronger global cooling effect than uniform seeding globally(the maximum cooling effect is-2.03±0.32 W m^-2).The cooling effect of flexible seeding method also differ(0.51 W m^-2)between high and low homogenous nucleation occurrence frequencies in the model,which is much weaker than the difference in the cooling effect from cirrus clouds(2 W m^-2).The results show that using different ice nucleation parameterizations can produce considerable globally cooling effects of stronger than-1 W m^-2.The above results provide a basis for further study of flexible seeding method in the future.Previous studies simplified cirrus cloud thinning experiments by increasing the sedimentation velocity of ice crystal particles.This study compares the climate response of this kind of highly idealized experiment with the flexible seeding.The results show that increasing the sedimentation velocity of ice crystal particles can produce the considerable globally cooling effect but cannot reproduce the climatic signals from optimal seeding method simulation.In summary,this study helps to further understand cirrus cloud thinning and provides a practical technical reserve for the cirrus cloud thinning.