Combining measurements by satellite and aircraft with climate modeling to evaluate the efficacy of cirrus cloud thinning

Abstract:

Cirrus cloud thinning or CCT is a climate intervention method for primarily cooling the mid-to-high latitudes, based on increasing the flux of outgoing longwave radiation (OLR) by making cirrus clouds more transparent to OLR. This transparency is achieved by altering the formation mechanism of cirrus cloud ice crystals from homo- to heterogeneous ice nucleation (henceforth hom and het), and that is achieved by enriching the concentration of ice nucleating particles in the upper troposphere. Thus, CCT is viable only when hom makes a significant contribution to the net cloud radiative effect (CRE). There have been at least 12 modeling studies attempting to ascertain the cooling potential of CCT, but due to many uncertainties relating to the prediction of hom, these studies predict a global annual mean change in CRE ranging from ~ 0 to -2.2 W m-2. To obtain a more reliable radiative cooling estimate, measurements are needed to constrain the climate models.

This study constrains the Whole Atmosphere Community Climate Model version 6 (WACCM6) with CALIPSO satellite retrievals of cirrus cloud effective diameter De to estimate the CCT radiative cooling effect. Although these De retrievals have been validated against De from several field campaigns, they are validated here against a climatology of cirrus cloud properties obtained from 24 field campaigns consisting of 150 flights. Moreover, ice particle number concentration N is calculated from retrieved De and the in situ climatological ice water content, and shown to be consistent with N retrievals based on a CloudSat-CALIPSO lidar-radar method.

The cirrus cloud ice particle size distribution in WACCM6 was constrained to conform with these De retrievals that depend on temperature, latitude, season and land fraction (land vs. ocean). The treatment of ice particle fall speeds was also revised. Two WACCM6 simulations were differenced to obtain the radiative contribution of hom; one based on the observed De and one based on De corresponding to het conditions (where retrieved N was minimal). The experimental design assumes hom-affected cirrus occur only outside the ± 30 °latitude zone. Outside this zone, the maximum CCT CRE difference was -2.4 and -2.5 W m-2 in the northern and southern hemispheres, respectively, indicating that CCT may be a viable climate intervention method.