Hugh Morrison

	Hugh Morrison

Hugh Morrison (MMM), works on cloud modeling and the development of cloud microphysics parameterizations. The general approach is to develop a bulk microphysics parameterization that assumes an underlying functional form for the hydrometeor size distributions, as opposed to bin microphysics parameterizations that explicitly resolve the size distributions (and are therefore computationally very expensive). The new bulk scheme predicts both the mixing ratios and number concentrations of several cloud and precipitation species. The prediction of both mixing ratios and number concentrations increases the degrees of freedom and allows for more a more realistic treatment of cloud-aerosol interactions and indirect aerosol effects than using simpler schemes predicting mixing ratios only.

The new scheme has been tested using a kinematic framework with a prescribed flow field in collaboration with W. Grabowski (MMM), using a bin microphysics model as the benchmark. A key finding is that the predictions of both cloud and precipitation (rain) number concentrations are needed to reproduce the bin model results. The bulk microphysics scheme has also been applied to a dynamical cloud model with a focus on improving the numerical treatment of supersaturation and including the effects of entrainment and mixing. A new treatment of the ice phase will also be developed in conjunction with Grabowski. Morrison is also working in collaboration with A. Gettelman (CGD) and P. Rasch (CGD) to develop a new stratiform cloud microphysics package for the NCAR Community Climate System Model (CCSM). A number of modifications and simplifications to the bulk microphysics scheme were needed for implementation into a large-scale climate model. These include diagnostic rather than prognostic treatment of precipitation (for both mixing ratio and number concentration), consideration of sub-grid distributions of cloud water and dynamics, and coupling with a fractional cloudiness scheme. The goals are to improve representation of cloud-aerosol interactions and indirect aerosol effects, improve current climate biases, and maintain a high degree of computational efficiency. As co-chair of the GEWEX Cloud System Study Polar Cloud Working Group (with J. Pinto - RAL), Morrison has helped to foster relationships with the international community. This has included the organization of the Mixed-Phase Arctic Cloud Experiment (MPACE) model intercomparison project in collaboration with S. Klein (Lawrence Livermore National Laboratory) and the US DOE Atmospheric Radiation Measurement (ARM) Cloud Parameterization and Modeling Working Group. The goal is to investigate how models of varying complexity simulate the stratiform mixed-phase clouds that are endemic to the Arctic. These clouds have a significant radiative impact on the atmosphere and surface and are therefore critical to simulations of Arctic weather and climate. Work is underway to develop additional case studies for a future model intercomparison, which is likely to also involve Arctic stratiform mixed-phase clouds, but in the context of much different environmental conditions compared to those of MPACE (e.g., sea ice versus open ocean).

Funding Sources

This research is supported by the National Science Foundation.