Vertical Wind Motions Important in Cirrus Ice Concentration
High altitude ice clouds (cirrus) affect Earth’s outgoing long wave radiation and help regulate upper tropospheric humidity. The sensitivity of climate predicted by global models is affected by the representation of cirrus microphysical properties. U.S. Department of Energy-funded researchers used numerical simulations and airborne measurements to evaluate the impact of physical processes on ice concentrations in midlatitude cirrus clouds. A computationally efficient modeling approach that incorporates key cirrus physical processes was used to simulate thousands of cloud cases. The model results were compared to aircraft observations taken during two field campaigns, including the ARM Small Particles in Cirrus (SPARTICUS) campaign. The researchers found that in simulations with only homogenous nucleation processes, the ice concentration statistics were sensitive to the magnitude of vertical wind perturbations. When vertical wind perturbations were adjusted to agree with aircraft measurements in simulations, there was good agreement between the simulated and observed ice concentration frequency distributions. Additional simulations showed that at higher ice nuclei concentrations or warmer temperatures, heterogeneous ice nucleation dominated the total simulated ice concentration. The study also found that including the impact of ice crystal sedimentation on the evolution of the ice crystal concentration is an important factor when comparing to aircraft observations, as aggregation of ice crystals can significantly reduce ice concentrations. These results provide important guidance for the parameterization of ice nucleation in global models, which currently do not resolve cloud-scale vertical motions.
Jensen, E.J., R. P. Lawson, J. W. Bergman, L. Pfister, T. P. Bui, and C. G. Schmitt. 2013. “Physical Processes Controlling Ice Concentrations in Synoptically Forced, Midlatitude Cirrus,” Journal of Geophysical Research Atmospheres 118, 5348–60. DOI: 10.1002/Jgrd.50421.