To make progress on understanding the precise mechanism of orographic rainfall occurring in our full-physics MM5 simulations of the 1994 Piedmont flood, we carried out simulations with the same real-data initial and boundary conditions, but with the real topography replaced by an idealized one. With excellent agreement between real- and idealized-topography on the rainfall rate vs. time in the Piedmont area, analysis of the idealized-topography simulation provides a clear picture of the models mechanism of orographically induced rainfall. As noted in previous studies of the 1994 Piedmont, a moist saturated airflow has a reduced effective static stability and tends to flow over the mountains, while an unsaturated airstream is stable and tries to flow around (toward the left in the Northern Hemisphere). In the 1994 Piedmont case and others, there was a strong horizontal gradient of moisture; thus the moist part of the airstream flows over while the dry part is deflected to the left of the obstacle, and so, a convergence is produced between the airstreams. We explore this effect using a simple version of MM5 wherein the flow, moisture distribution, and idealized topography are varied within the observed range. Quantitative as well as qualitative rainfall rates and flow features of the full-physics MM5 simulations can be captured with the simple model.
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