The effect of turbulence on mixed phase clouds: LES experiments

Theoretical work by Korolev and Field (2008) (KF08) demonstrates that if a vertical velocity exceeds a threshold velocity and the associated vertical ascent reaches above a certain threshold altitude, mixed phase clouds can be produced. KF08 employed many simplifying assumptions concerning microphysics and mixing.

In this work we present a series of 2-D/3-D LES experiments that relax some of these assumptions, in order to investigate the role of turbulence and microphysics in the generation of mixed phase clouds. All experiments investigate mixed phase cloud generation within a stable profile by using varying shear strengths to trigger turbulence. The shear is applied at different temperatures with varying initial shear layer thickness and three different initial background ice mixing ratios. We present results for the following microphysical set-ups

i) condensation only (no ice or liquid sedimentation)

ii) condensation and sublimation (no ice or liquid sedimentation), similar to KF08

iii) condensation, sublimation, riming (no sedimentation)

iv) same as iii but with sedimentation.

Using this suite of experiments we will demonstrate an analogous set of conditions to KF08 are required for a turbulent environment, i.e. for the simplest case (i) TKE must exceed a threshold TKE for the activation of water and the maintenance of a mixed phase cloud. Further, we demonstrate how increasing microphysical complexity impacts this threshold and the relationship of TKE to water generation.