3.4
A new approach for predicting the evolution of snow size spectra
David L. Mitchell, DRI, Reno, NV; and V. Grubisic and A. W. Huggins
A new microphysical model is under development that is very unique in terms of its assumptions and formulation. It is like the Mitchell snow growth model for growth by diffusion and aggregation, except it is driven by supersaturation and not changes in ice water content (IWC) as before. Supersaturation determines particle growth and nucleation rates with a dependence on ice particle shape for both processes. A new treatment for ice particle fallspeeds is used, which is coupled with the microphysical processes, including aggregation. Developed from the 0th and 2nd moment conservation equations with respect to mass, particle growth was formulated in terms of size distribution parameters. This analytical basis provides for physical insight and computational efficiency.
The model is currently operating in a steady state height dependent mode, which is well suited for radar work. The NWS WSR-88D radar employed throughout the U.S. is often sited on mountain peaks in the intermountain west, the beam tilting slightly upwards in order to prevent occultation by topography. In the Lake Tahoe area, the lowest radar echo is typically about 1.0-1.5 km above the Sierra Nevada mountain barrier, and snowfall rates at this level are often much less than those reported at ground level. An equation was derived for estimating the IWC from the radar reflectivity (dBZ), given an estimate of the corresponding mean ice particle size, D. Using this equation and a D - temperature relation, the model was initialized at the lowest radar echo and used to predict snowfall rate at ground level. Observed and predicted snowfall rates will be compared, as well as observed and predicted dBZ values when the model is initialized near cloud top.
Some microphysical findings include (1) the relatively constant mean ice particle size and number concentration typically observed in the lower regions of natural frontal clouds were predicted for the first time, largely due to a shut down of the aggregation process, resulting from negligible dispersion in fall speeds for large aggregates; (2) an aggregation efficiency Ea around 0.1 appears to produce spectral evolution similar to observations, consistent with laboratory measurements of Ea; (3) the dependence of mass growth rate on ice particle and size distribution shape appears stronger than previously realized; (4) the aggregation process often does not have a strong impact on snowfall rates.
Session 3, Cold Cloud Microphysics
Tuesday, 4 June 2002, 8:30 AM-9:59 AM
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