ALBERT

Description: Over the last 1.5 years a CALIPSO satellite retrieval for cirrus clouds has been developed for number concentration N〈sub〉i〈/sub〉, effective diameter D〈sub〉e〈/sub〉 and ice water content IWC. Using four years of CALIPSO data, these retrieved properties agree favorably with corresponding aircraft measurements from Krämer et al. (2020, ACP), but also show evidence that cirrus clouds form in two ways; (1) through heterogeneous ice nucleation (het) only (henceforth het cirrus) and (2) through het and homogeneous ice nucleation (hom) combined (henceforth hom cirrus). Hom cirrus have higher N〈sub〉i〈/sub〉 and IWC and smaller D〈sub〉e〈/sub〉 relative to het cirrus. The visible extinction coefficient α〈sub〉ext〈/sub〉, defined as 3 IWC/(ρ〈sub〉i〈/sub〉 D〈sub〉e〈/sub〉), is thus sensitive to changes in hom. Plots of log N〈sub〉i〈/sub〉 vs. log α〈sub〉ext〈/sub〉 for various temperature intervals reveal different slopes for het and hom cirrus, with hom cirrus prevailing when α〈sub〉ext〈/sub〉 〉 0.3 km〈sup〉-1〈/sup〉 between 235-215 K with median N〈sub〉i〈/sub〉 ranging from ~ 100 to ~ 300 L〈sup〉-1〈/sup〉, respectively. In relation to increasing α〈sub〉ext〈/sub〉, the het regime over land is characterized by quasi-constant N〈sub〉i〈/sub〉 with increasing D〈sub〉e〈/sub〉 while the hom regime is characterized by increasing N〈sub〉i〈/sub〉 with decreasing D〈sub〉e〈/sub〉. This was verified using a simple model based on hom theory, where predicted N〈sub〉i〈/sub〉, D〈sub〉e〈/sub〉 and IWC associated with hom matched corresponding CALIPSO retrievals (as indicated by relatively high N〈sub〉i〈/sub〉). Over oceans, while hom cirrus represent 17% to 45% of all sampled cirrus (depending on latitude) they are responsible for 55% to 81% of the estimated net cloud radiative effect for all cirrus clouds.

Description: Over the last 1.5 years a CALIPSO satellite retrieval for cirrus clouds has been developed for number concentration Ni, effective diameter De and ice water content IWC. In general, these retrieved properties agree favorably with corresponding aircraft measurements from Krämer et al. (2020, ACP), but also show evidence that cirrus clouds form in two ways; (1) primarily through heterogeneous ice nucleation (het) only (henceforth het cirrus) and (2) through het and homogeneous ice nucleation (hom) combined (henceforth hom cirrus). Het cirrus represent 55% to 80% of all sampled cirrus (depending on 30° latitude band and appear to be most sampled during field campaigns) and have visible extinction coefficients αext 〈 0.3 km-1. Hom cirrus have αext 〉 0.3 km-1, and outside the tropics, are responsible for 72% to 81% of the estimated net cloud radiative effect for all cirrus clouds (depending on latitude band). They are often associated with warm fronts and orographic gravity waves. Using a simple model based on hom theory, the model-predicted Ni, De, and IWC are found to agree well with CALIPSO retrievals of these cloud properties that, based on retrieved Ni, correspond to hom, thus corroborating the role of hom. These results indicate that the climate intervention method known as cirrus cloud thinning (CCT) could be effective during Polar Night (i.e., winter, when Arctic Amplification is strongest). Moreover, hom cirrus in climate models may be underestimated due to the uniform treatment of pre-existing ice over prescribed atmospheric layers (i.e., ice nucleation is near cloud top).

Description: Research on the climate intervention method known as cirrus cloud thinning or CCT has produced mixed results regarding its efficacy, largely due to uncertainty over the role of homogeneous ice nucleation (i.e., hom) in cirrus cloud formation. Over the last 1.5 years we have developed a new CALIPSO satellite remote sensing method for cirrus clouds that reveals cirrus clouds form in two ways; (1) through heterogeneous ice nucleation (het) only (henceforth het cirrus) and (2) through het and hom combined (henceforth hom cirrus). Het cirrus have the greatest frequency of occurrence and appear to be most sampled in field campaigns, while hom cirrus have extinction coefficients 〉 0.3 km-1 and are responsible for ~ 80% of the estimated net cloud radiative effect for all cirrus clouds outside the tropics. Using a simple model based on hom theory, the model-predicted ice particle number concentrations N, effective diameters De, and ice water contents IWC are found to agree well with CALIPSO retrievals of these cloud properties that correspond to hom (based on retrieved N). Moreover, hom cirrus in climate models may be underestimated due to the uniform treatment of pre-existing ice over prescribed atmospheric layers (i.e., does not account for ice nucleation occurring primarily near cloud top). Finally, radiation transfer model calculations based on our CALIPSO cirrus cloud profiles suggest that a transition from het to hom cirrus resulting from CCT could lead to a -5 W m-2 cooling of the Arctic during winter when Arctic Amplification is strongest.