Weather Mod Critical Issues Report

• Project Type: General OWC
• Project Name: Weather Modification
• Title: Critical Issues in Weather Modification Research
• Date: 1/1/2004
• Water Conservation - Doc Type: Final Report

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3.4 Cloud modeling of cloud seeding effects

This has been a continuing effort conducted by a few cloud modeling groups over
the past thirty years. Simulations of many types of cloud seeding experiments have been
accomplished. Much of the work depended on simplifications of the microphysics and of
the dynamics, but even so basic effects were evident that will likely stand the test of more
sophisticated treatments suggested in the NRC report, Some of the findings are listed
below,

The NRC report failed to critically review the development of cloud models over
the past 20 to 30 years, with respect to cloud seeding simulations, and with respect to
natural cloud precipitation simulations. No NRC committee member was particularly
active in the modeling field, except in the dynamics of clouds. The report concentrated
on the future use of complex microphysical and three-dimensional, time-dependent
research cloud models that in general are of little use in operations now. They failed to
evaluate what has been developed and what could be applied with current computer
power and model capabilities on operational projects.

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Bulk-water microphysical techniques were used in most of the cloud models in
the early days and are currently being used in large-scale weather prediction models.
This process, assumes zero terminal velocity for the cloud water and cloud ice, relatively
small terminal velocities for snow content, modest values for rain, and the largest vertical
velocities for graupel and hail. The velocities vary with the quantity of precipitation
content at a grid point. Such a framework allows for the production of rain from cloud
water, the formation of cloud ice at appropriate observed temperatures, the production of
snow from supercooled water and cloud ice or the depositional growth of cloud ice, and
the production of graupel/hail from frozen rain (via probabilistic freezing) or interactions
between the liquid and ice contents. If rain does not form from cloud liquid (as is the
case in many higher latitude clouds) then it forms later in the lifetime of the cloud
through melting of ice particles, The growth ofthe graupel/hail considers both wet and
dry growth processes, Nearly thirty interactive processes among the various water
processes (such as accretion, collection, aggregation, etc.) are simulated. The paper by
Lin et al. (1983) describes the early development that is the basis for many of the models.

It has become more common in recent years for bulk microphysics schemes to
predict two moments such as hydro meteor mixing ratio and concentration (Ferrier et aI.,
1995; Meyers et aI., 1997; Reisner et aI., 1998). A somewhat different paradigm is to
emulate an explicit bin model by prescribing basis functions for the drop size
distributions such as gamma or log-normal distributions (Clark, 1976; Clark and Hall,
1983) and explicitly predict the evolution ofthose basis functions by vapor
deposition/evaporation, stochastic coalescence, and sedimentation. Tzivion et al. (1994)
predict three parameters that fully define the basis functions: mixing ratio, number
concentration, and a third moment. Milbrandt and Yau (2004) have implemented such a
model for application to hailstorm simulations, This model does a much better job of
representing hail processes than the earlier bulk-water microphysical methods without the

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