Ice clouds: from ice crystals to their response in a warming climate

Abstract

Atmospheric ice crystals form from a variety of different sources and at different temperatures. Between 0 °C and -38 °C, liquid water and ice crystals can coexist. Cloud ice initiated by freezing of cloud droplets at these temperatures needs to be catalyzed by an ice nucleating particle (INP). Further growth then often involves collisions of ice crystals and cloud droplets or depositional growth of the ice crystals at the expense of the cloud droplets due to the lower water vapor pressure over the ice crystal than over the liquid droplet surface. At temperatures colder than -38 °C, ice can not only originate from freezing of cloud droplets, but also by freezing of deliquesced aerosols and direct deposition of water vapor onto an INP. The complexity of the ice formation processes is reflected in the spread of simulated cloud ice contents in the current generation of global climate models (GCM). This work describes the implementation and first results of a new cloud microphysics scheme in the ECHAM6-HAM2 GCM aimed to reduce the number of weakly constrained parameters involved in the representation of cloud ice formation and evolution. It does no longer rely on heuristic conversion rates between in-cloud ice crystals and precipitating snow but uses only one single, prognostic ice category which better represents the spectrum of ice crystals in clouds. Because precipitating snow is no longer diagnosed, the trajectory of ice crystals must be fully prognostic. Numerical stability of vertical advection is achieved by an adaptive time step in the microphysics routine which leads to an increase in computation time of roughly 25%. The new scheme significantly reduces the conceptual complexity of the model. Tuning parameters for the ice crystal fall speeds and the conversion to snow are no longer needed. With the introduction of a new cloud cover parameterization the high bias of high cloud cover in the base model version ECHAM6.3-HAM2.3 could be reduced. Overall, the new model is in reasonable agreement with observations in key variables while some deficiencies remain. New model diagnostics are introduced to disentangle the relative importance of ice formation pathways to provide a sound cause-and-effect relation between the simulated cloud fields and the process parameterizations. This analysis revealed that immersion and contact freezing in supercooled liquid clouds only dominate ice formation in roughly 5% of the simulated clouds, a small fraction compared to roughly 64% of the clouds governed by freezing in the cirrus temperature regime below -38 °C. Furthermore, we could demonstrate that even in the mixed-phase temperature regime between -38 °C and 0 °C, the dominant source of ice is the sedimentation of ice crystals that originated in the cirrus regime. The new scheme is used to assess changes in the cloud fields in response to a warming climate. The equilibrium response of the global mean surface temperature to an instantaneous doubling of atmospheric carbon dioxide concentrations is found to be 3.8 °C which is within the spread of the current generation of GCMs but substantially larger than the base model version ECHAM6.3-HAM2.3 with a value of 2.5 °C. This difference could be narrowed down to different cloud optical depth feedbacks and needs further investigation. Even though clouds are predominantly glaciated already below temperatures of roughly -5 °C, the cloud phase feedback is suppressed. Since most cloud ice is formed in clouds with a large vertical extent and high optical thickness, phase transitions do not significantly increase the optical depth of the cloud.