Exploring sources of ice crystals in cirrus clouds: comparative analysis of two ice nucleation schemes in CAM6

Abstract. Ice crystal formation in cirrus clouds is poorly understood, and its representation remains a challenge in global climate models. To enhance the understanding, a novel ice nucleation parameterization based on the Kärcher (2022) (K22) scheme is introduced into the NCAR Community Atmosphere Model version 6 (CAM6). To investigate ice formation in cirrus clouds, sensitivity tests are conducted to analyze three ice sources from orographic gravity wave (OGWs), convective detrainment, and turbulence. These tests employ both the K22 scheme and the default Liu and Penner (2005) (LP05) scheme. Model evaluation includes 6-year climatology and nudged simulations representing the Small Particles in Cirrus (SPARTICUS) and O2 / N2 Ratio and CO2 Airborne Southern Ocean Study (ORCAS) campaigns. Both schemes simulate that convection detrained and turbulence-induced ice crystals are concentrated in low- to mid-latitudes, whereas OGW-induced ice crystals are concentrated in mid- to high latitudes. Compared to the LP05 scheme, the K22 scheme generates a higher number of ice crystals. The simulated cloud microphysical properties using the K22 scheme align well with observations for orographic cirrus during the SPARTICUS campaign. In orographic cirrus over high terrains at mid- to high latitudes, both schemes identify OGW-induced ice crystals as the dominant ice source. Due to its distinct competition parameterizations, the K22 scheme exhibits less contribution from minor ice sources (convection detrained and turbulence-induced). This underscores the significance of competition mechanisms within ice nucleation schemes and helps clarify regional and dynamical controls on ice sources in cirrus clouds. The application of two distinct nucleation schemes provides valuable insights into the dominant ice sources in cirrus clouds.