MMM
SEMINAR NCAR
The Simulation of a
Hailstorm using a
Multi-Moment Bulk
Microphysics Scheme
Jason A. Milbrandt1,2 and M. K. Yau1
2Meterological
Research Branch (Environment
Bulk
microphysics schemes are becoming increasingly important in both atmospheric
research and numerical weather prediction models. With sufficient computer power it is possible
to use schemes that predict more than one parameter of the hydrometeor size
distributions. A number of schemes
predict two quantities, the mass content and total number concentration, for
each hydrometeor category. The relative
dispersion of each size spectrum is normally held constant.
An
analysis of the role of the spectral dispersion in bulk schemes indicates that
this parameter plays an important role in the overall prediction of the
hydrometeor fields by affecting the computation of sedimentation and
microphysical growth rates. In view of the
importance of this parameter, two alternatives to the fixed-value approach are
presented. The first is a two-moment
method, where the relative dispersion is diagnosed from the predicted moments;
the second is a three-moment approach, where the dispersion parameter is
independently prognosed. Using these proposed approaches, a new
multi-moment bulk scheme has been designed.
The scheme was interfaced with the Canadian MC2 mesoscale model and used to conduct high-resolution (1-km) simulations of a supercell hailstorm. Comparisons are made between the full triple-moment control simulation and sensitivity runs which used various one-moment and two-moment versions of the scheme. We examine how the different approaches affected the simulated storm structure, hail sizes, and quantity and phase of the precipitation at the surface. It is evident from this study that there is a fundamental improvement in overall simulation skill for two-moment over one-moment schemes. Further, the variable-dispersion approaches are able to control the problems associated with excessive size-sorting due to differential sedimentation that are inherent in standard fixed-dispersion two-moment schemes
Thursday, 15 September 2005,
3:30 PM
Refreshments
NCAR-Foothills Laboratory
Bldg 2 Auditorium (Rm1022)