Physical processes controlling the evolution of ice concentration in cirrus clouds

 

Authors

Eric Jensen — NASA - Ames Research Center
Paul Lawson — SPEC, Inc.

Category

Modeling

Description

Several past studies have compared measured cirrus ice concentrations with calculations based on nucleation theory. However, such calculations only indicate the peak ice concentrations occurring just after nucleation events. Various cloud processes (e.g., differential sedimentation, entrainment, dispersion, and aggregation) conspire to reduce mean ice concentrations as the cloud evolves. Here, we use both a one-dimensional cloud model and a three-dimensional cloud-resolving model to evaluate the impact of these processes on the evolution of ice concentration through the life cycle of cirrus clouds. Results are compared statistically with airborne measurements of midlatitude cirrus ice concentration during the DOE SPARTICUS and NASA MACPEX campaigns. We will show that mean ice concentrations are reduced substantially by processes occurring after nucleation events, and this issue should be taken into consideration when comparing with observations that necessarily represent a range of cloud ages. We find that radiatively driven small-scale convection results in considerable variability in cirrus microphysical properties. In particular, frequency distributions of ice concentration are broader when three-dimensional effects are included than in equivalent one-dimensional simulations. Under some conditions, convective motions can considerably enhance ice concentrations in the lower parts of cirrus.