Modeling ice particle aspect ratio evolution during riming
Open Access
- Author:
- Jensen, Anders Alstrup
- Graduate Program:
- Meteorology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 19, 2015
- Committee Members:
- Jerry Y Harrington, Dissertation Advisor/Co-Advisor
Johannes Verlinde, Committee Member
Eugene Edmund Clothiaux, Committee Member
Miriam Arak Freedman, Committee Member
Hugh Morrison, Special Member - Keywords:
- ice
microphysics
modeling
rime
riming
snowflake - Abstract:
- The first part of this dissertation describes and tests a single-particle ice growth model that evolves both ice crystal mass and shape as a result of vapor growth and riming. Columnar collision efficiencies in the model are calculated using a new theoretical method derived from spherical collision efficiencies. The model is able to evolve mass, shape, and fall speed of growing ice across a range of temperatures, and it compares well with wind tunnel data. The onset time of riming and the effects of riming on mass and fall speed between −3◦C to −16◦C are modeled, as compared with wind tunnel data for a liquid water content of 0.4 g m −3 . Under these conditions, riming is constrained to the more isometric habits near −10◦C and −4◦C. It is shown that the mass and fall speed of riming dendrites depend on the liquid drop distribution properties, leading to a range of mass-size and fall speed-size relationships. Riming at low liquid water contents is shown to be sensitive to ice crystal habit and liquid drop size. Moreover, very light riming can affect the shape of ice crystals enough to reduce vapor growth and suppress overall mass growth, as compared with those same ice crystals if they were unrimed. Part two details a novel bulk microphysics model that simultaneously evolves mass, shape, and density due to both riming and vapor growth. A direct result is that conversion rates between, for example, snow and graupel, are not needed. The method is unique because ice crystal shape is predicted and two measures of size are included as prognostic variables. Ice is nucleated as either planar or columnar, determined by temperature. The model is tested in comparison to a traditional bulk scheme in a kine- matic framework, where feedbacks between microphysics and dynamics are neglected. Separating planar and columnar ice, predicting ice crystal shape, and allowing shape to evolve naturally during vapor growth and riming results in ice crystals being sorted in physical space because fall speed depends on shape. Simulations show that predicting ice crystal shape leads to a different spatial precipitation rate distribution compared to the traditional method of separating and converting between ice categories. It is also shown that when system dynamics evolve in time, both spatial precipitation rate distribution and domain total precipitation rate depend on snow and graupel conversion thresholds in traditional models. Finally, in certain environments columnar ice rimes while planar ice does not showing that riming is habit dependent. The impact of ice crystal habit on precipitation rate can be just as prominent as an order of magnitude change in cloud number concentration. In part three the microphysics parameterization is tested in a WRF squall line case. The results show that the modeling approach to ice microphysics successfully cap- tures features of the squall line including the transition zone and enhanced stratiform precipitation region. Traditional bulk microphysics schemes have difficulty modeling these features consistently. Comparing the results with microphysical retrievals and other modeling studies, it is shown that rimed ice advected out of convective updrafts can be the sole source of enhanced stratiform precipitation without additional mass growth in the mesoscale updraft. It is also shown that the transition zone is sensitive to microphysical properties including ice distribution shape. Vapor growth and aggre- gation without riming can also cause enhanced stratiform precipitation, but in this case significant vapor growth occurs in the mesoscale updraft. Finally, aggregation can in- crease reflectivity in the transition zone, thus eliminating the reflectivity trough there. Aggregation is also shown to extend the enhanced stratiform precipitation region.