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 produ ction of extraordinary rainfall. To uncover the precise mechanism of orogra phic rainfall occurring in full-physics MM5 simulations of the 1994 Piedmon t flood, the authors carried out simulations with the same real-data initia l and boundary conditions, but with the real orography replaced by an ideal ized one. With excellent agreement between real and idealized orography on the rainfall rate versus time in the Piedmont area, analysis of the idealiz ed-orography simulation provides a clear picture of the model's mechanism o f orographically induced rainfall. As noted in previous studies of the 1994 Piedmont case, a moist saturated airflow has a reduced effective static st ability and tends to flow over the mountains, while an unsaturated airstrea m is stable and tries to flow around (toward the left in the Northern Hemis phere). In the 1994 Piedmont case, there was a strong horizontal gradient o f moisture; thus the western moist part of the airstream flows over, while the eastern drier part is deflected westward around the obstacle, and so a convergence is produced between the airstreams. This effect is explored usi ng a simple version of MM5 wherein the flow, moisture distribution, and ide alized orography are varied within the observed range. Quantitative as well as qualitative rainfall rates and flow features of the full-physics MM5 si mulations are captured with the simple model.