Background
Ice cloud formation is very sensitive to the updraft velocity.
The characteristic updraft velocity for ice nucleation (w*) in ACME/CAM5 is parameterized a function of TKE and has an artificial upper bound of 0.2 m/s.
Based on parcel model simulations and in-situ aircraft measurements, we proposed a new treatment of w*, assuming the subgrid variation of vertical velocities follows the Gaussian distribution.
It has been implemented and tested in ACME, along with other improvements for ice cloud parameterizations.
Method
A new sub-grid parameterization of the characteristic updraft velocity (w*) for ice nucleation was proposed by Zhang et al. (in preparation). It is calculated by averaging the vertical velocities in the updraft portion of an assumed sub-grid distribution. In the present scheme, the subgrid variation of vertical velocities is assumed to follow the Gaussian distribution.
Results
Comparison between the parameterized w* and the inverted “true” w* derived from parcel model simulations and aircraft measurements.
It has a large impact on instantaneous ice nucleation events, but the change to the global annual mean cloud forcing is rather small.