Factors influencing ice formation and growth in simulations of a mixed‐phase wave cloud

Abstract

In this paper, numerical simulations of an orographically induced wave cloud sampled in‐situ during the ICE‐L (Ice in Clouds Experiment ‐ Layer clouds) field campaign are performed and compared directly against the available observations along various straight and level flight paths. The simulations are based on a detailed mixed‐phase bin microphysics model embedded within a 1‐D column framework with the latest parameterizations for heterogeneous ice nucleation and an adaptive treatment of ice crystal growth based on the evolution of crystal habit. The study focuses on the second of two clouds sampled on 16th November 2007, the in‐situ data from which exhibits some interesting and more complex microphysics than other flights from the campaign. The model is used to demonstrate the importance of both heterogeneous and homogeneous nucleation in explaining the in‐situ observations of ice crystal concentration and habit, and how the ability to isolate the influence of both nucleation mechanisms helps when quantifying active IN concentrations. The aspect ratio and density of the simulated ice crystals is shown to evolve in a manner consistent with the in‐situ observations along the flight track, particularly during the transition from the mixed‐phase region of the cloud to the ice tail dominated by homogeneous nucleation. Some additional model runs are also performed to explore how changes in IN concentration and the value of the deposition coefficient for ice affect the competition between heterogeneous and homogeneous ice formation in the wave cloud, where the Factorial Method is used to isolate and quantify the effect of such non‐linear interactions. The findings from this analysis show that the effect on homogeneous freezing rates is small, suggesting that any competition between the microphysical variables is largely overshadowed by the strong dynamical forcing of the cloud in the early stages of ice formation.