- A two-way interactive nested-grid system is developed for the PSU/NCAR three-dimensional mesoscale model and tested using real data and raw terrain. A mesh structure which minimizes numerical noise is designed, and a procedure to obtain compatible coarse grid mesh (CGM) and fine grid mesh (FGM) terrain conditions is presented.
An 18-hour simulation of the Johnstown flood of July 1977 is conducted using a modified version of the Fritsch-Chappell convective parameterization scheme and a high-resolution PBL package. Compared to observations, the model reproduced the size, propagation rate and orientation of the mesoscale convective complex and squall line that were instrumental in producing the flooding rains. The evolution of the planetary boundary layer, cool outflow boundaries and surface pressure perturbations, such as meso-(beta) scale lows, highs, ridges and troughs, were also reproduced well. Other mesoscale features, conform well to the observed feature. Of particular significance is that the model-forecasted rainfall amounts and distribution are similar to the observed.
In this simulation, the model produced and maintained a meso-(beta) scale warm-core vortex with a space scale of 100-200 km in diameter and a time scale of more than 18 hours. Associated with the vortex, an intense vertical circulation developed. This circulation produced a significant amount of non-convective precipitation (30-40% of the total rainfall). It is found that equivalent potential temperature and horizontal momentum are approximately conserved following air-parcel motion in the warm core. At upper levels, the vortex behaves like an "obstacle," forcing the horizontal wind flow around it. In addition to the meso-vortex, the simulation clearly shows the generation of internal gravity waves associated with the squall line.
A number of sensitivity experiments indicated that the evolution of the mesoscale convective systems responds strongly to changes in the following parameters: (i) initial data; (ii) inclusion and strength of parameterized moist downdrafts; (iii) diabatic heating (convective, non-convective latent heating and planetary boundary layer processes); (iv) virtual temperature; (v) mesoscale short wave; and (vi) high-resolution terrain forcing.
The results imply that it may be possible to forecast the meso-(beta) scale structure, evolution and, in particular, precipitation of mesoscale convective weather systems. (Abstract shortened with permission of author.).
- Dissertation Note:
- Ph.D. The Pennsylvania State University 1985.
- Source: Dissertation Abstracts International, Volume: 46-11, Section: B, page: 3763.
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