New Model Formulation for Entrainment-Mixing Processes in Clouds
As clouds evolve, outside dry air is mixed in (or entrained) by turbulent motions in the clouds. This dry air causes cloud droplets to evaporate. As also observed in the field, cloud modelers assume either “homogeneous” mixing (all cloud droplets evaporate the same amount) or “heterogeneous” mixing (some droplets evaporate more than others). These different mixing processes give rise to distinct cloud properties. Having an accurate representation of these processes is critical for improving large scale models. Unfortunately, there has been no single parameterization that spans the full spectrum of observed entrainment-mixing processes. Now, U.S. Department of Energy scientists at Brookhaven National Laboratory (BNL) have filled this gap and developed a new model formulation, based on in situ aircraft measurements collected at the Atmospheric Radiation Measurement program’s Southern Great Plains site and numerical simulations with the Explicit Mixing Parcel Model (EMPM). They introduced a new microphysical measure, the homogeneous mixing degree, and explored the potential of using this measure to quantify a continuum of entrainment-mixing mechanisms and relate it to the entrainment-mixing dynamics. The parameterization may now be used in models that have both droplet mass and number information (“two-moment microphysics schemes”). BNL scientists are implementing the scheme into a cloud-resolving model and investigating its influence on these cloud model results. The long-term goal is to develop a mixing scheme for use in global climate models.
Lu, C., Y. Liu, S. Niu, S. Krueger, and T. Wagner. 2013. “Exploring Parameterization for Turbulent
Entrainment-Mixing Processes in Clouds,” Journal of Geophysical Research-Atmospheres 118,185–194. DOI: 10.1029/2012JD018464.