Determining Where and When Ice Formation Processes in Cirrus Clouds are Favorable for Cirrus Cloud Thinning

As stated in the 2023 United Nations Environment Programme report “One Atmosphere: An independent expert review on Solar Radiation Modification research and deployment”, “A less studied approach is cirrus cloud thinning (CCT), which aims to decrease the amount of high cirrus clouds that trap infrared radiation emitted by Earth. The idea is that injecting ice-nucleating particles would increase the sedimentation rate of the ice crystals that compose these clouds. This would thin the clouds and allow more infrared radiation to escape to space. The feasibility of CCT is uncertain, in part because of the larger uncertainties associated with the ice nucleation processes in high clouds.”

CCT research at the Desert Research Institute has targeted this uncertainty, namely the relative roles of heterogeneous and homogeneous ice nucleation (henceforth het and hom) in cirrus cloud formation, and the dependence of cirrus clouds strongly affected by hom (i.e., hom cirrus) on temperature, latitude, season, and surface type (land vs. ocean). This knowledge is important since CCT can only be effective if hom cirrus contributes a significant portion of the total cirrus cloud net radiative effect (CRE). This goal (of characterizing the fraction of hom cirrus globally and seasonally) was accomplished through the development of a CALIPSO satellite remote sensing method for cirrus clouds over the last few years, which retrieves the cloud ice particle number concentration N, effective diameter D_e, and ice water content IWC. These retrievals agree favorably with the cirrus cloud climatology of Krämer et al. (2020, ACP) that is based on numerous field campaigns throughout the world.

Hom cirrus were identified by their relatively high N and found to also exhibit relatively high IWC, whereas only hom cirrus over land exhibited relatively small D_e. This revealed that the extinction coefficient α_ext was well suited for evaluating the transition from het to hom cirrus since α_ext = 3 IWC/(ρ_i D_e). When D_e was plotted against α_ext for cloud radiative temperature T_r intervals of 4 K, a D_e maximum occurs revealing the transition between het-dominated cirrus and hom cirrus. This maximum is believed to occur due to the corresponding high N produced by hom, and the resulting conditions near ice saturation keeping ice particle growth rates (and sizes) low. Below some T_r threshold, a D_e maximum was no longer found, indicating that hom was not having a significant impact on D_e at these lower temperatures. In this way, a hom cirrus region in T_r-α_ext space could be defined, bounded by D_e,max for α_ext and the threshold T_r for temperature. In addition, a simple model predicting N, D_e and IWC for pure hom conditions exhibited excellent agreement with the pure-hom regions sampled by CALIPSO.

The above analysis made it possible to globally map the fraction of hom cirrus by season. While in the tropics (± 30° latitude) this fraction is typically < 15%, outside the tropics zonal means of this fraction vary by ~ 20% to 35% during the winter. Over the Southern Ocean the hom fraction is often ~ 40% during winter, exhibiting a large seasonal change. Similar large seasonal changes also occur east of the Asian desert regions, suggesting that the hom fraction is largely determined by the concentration of ice nuclei such as mineral dust. Estimating the cloud radiative effect (CRE) as the product of cloud frequency of occurrence and cloud optical depth (OD), the cirrus CRE over oceans outside the tropics during winter is dominated (> 50%) by the OD-weighted hom cirrus fraction. These results suggest that Mother Nature is already conducting CCT and that CCT may exhibit a significant cooling effect partly due to the larger optical depth of the hom cirrus fraction.