Mechanisms of intense alpine rainfall

Numerical studies by the authors and others of the 1994 Piedmont flood show that the orographically modified flow was a critical element for the production of extraordinary rainfall. It was shown in those studies that the orographically modified flow is very sensitive to the moist thermodynamic properties of the airsteam impinging on the Alps, since they determine the effective static stability of the flow. The complexity is compounded by the fact that the particular size, height and shape of the Alps puts even the dry airflow into a regime (for typical air speeds and static stabilities) where there is little guidance from analytical theory. To help unravel the tangle of interdependent flow features produced by moist-thermodynamic effects and complex topography we have conducted a series of MM5 simulations of decreasing complexity.

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.