Investigating Cirrus Cloud Formation Processes from Combined CALIPSO Lidar and IIR Observations

Over the last two years, an improved CALIPSO satellite retrieval for cirrus clouds has been developed for number concentration N_i, effective diameter D_e and ice water content IWC derived from D_e and visible extinction coefficient, α_ext, as IWC = α_ext ρ_i D_e /3. Based on four years of CALIPSO data, these retrieved properties agree favorably with the cirrus cloud climatology of Krämer et al. (2020, ACP) that is based on numerous field campaigns throughout the world. The cirrus IWC climatology of Schiller et al. (2008, JGR) gives higher IWCs than the former and agrees well with these CALIPSO retrievals when cirrus optical depth > 0.3.

When related to the visible extinction coefficient α_ext, D_e exhibits a maximum. Moreover, T_r – T_top related to α_ext (T_r = radiative temperature; T_top = temperature at cloud top), also exhibits a maximum at or near this D_e maximum. Note that T_r – T_top is a proxy for cirrus geometrical thickness because T_r is typically near the cloud center. This maximum D_e is interpreted as a transition region in terms of extinction which separates cirrus clouds formed through heterogeneous ice nucleation only (henceforth het and het cirrus) and through het and homogeneous ice nucleation (hom) combined (henceforth hom cirrus). Hom cirrus have higher N_i and IWC relative to het cirrus.

Thus, α_ext is sensitive to changes resulting from hom that separates the het and hom cirrus regimes. This led to a conceptual model of cirrus cloud formation that integrates these two types of cirrus cloud, which is partially confirmed by a simple model based on hom theory and is also supported by other published studies on cirrus clouds.

The above analysis allowed us to globally map the fraction of hom cirrus by season. While in the tropics (± 30° latitude) this fraction is typically < 15%, outside the tropics this fraction varies by ~ 20% to 35% during the winter. Estimating the cloud radiative effect (CRE) as the product of cloud frequency of occurrence and cloud optical depth, the cirrus CRE over ocean outside the tropics during winter is dominated by the hom cirrus fraction.

Based on theoretical expectations, the balance of het cirrus and hom cirrus may partly depend on the concentration of mineral dust at cirrus levels, with a lower hom fraction when dust levels are relatively high. The fraction of hom cirrus has a strong seasonal dependence far downwind of the main Asian dust sources (Taklimakan and Gobi deserts) and throughout the Southern Ocean; higher in winter relative to summer. This inversely coincides with dust loading and dust aerosol optical depth in these regions at cirrus levels, suggesting that seasonal changes in mineral dust concentration may alter the hom cirrus fraction. This could have a significant impact on the cirrus CRE as shown by numerous studies. In addition, there is clear evidence of a Twomey effect for het cirrus clouds, assuming there are more ice nuclei over land relative to oceans.