An Alpine Rainstorm: Sensitivity to the Mesoscale Upper-Level Structure

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Abstract
Rainstorms that occur on the south side of the European Alps during spring and fall are usually accompanied at the tropopause level by a single elongated filament of intruded stratospheric air. This study examines the dependency of one such storm (the Piedmont event of 5–6 November 1994) upon the filament’s structure. The focus is on quantitative precipitation forecasting and particular attention is given to the timing, distribution, and amplitude of the storm’s rainfall. In a first step the storm’s synoptic evolution and the upper-level filament’s mesoscale structure are examined cursorily using data from a range of sources (ECMWF operational fields, radar estimate of accumulated precipitation, Meteosat water vapor imagery, and two limited-area pseudoforecasts of different duration to 0600 UTC 6 November). This examination helps identify distinct elements of the filament’s potential vorticity (PV) distribution. In the second step a set of atmospheric states are generated for 1800 UTC 4 November that differ only in their representation of the aforementioned PV elements, and this set forms the initial state for a series of 30-h simulations to 0600 UTC 6 November. The simulated developments exhibit the same general synoptic behavior, but there are marked differences in the precipitation fields. Diagnostic analysis links the discrepancies to a chain of influences involving a different evolution of the filament aloft that impacts upon the underlying prefrontal low-level jet and, thereby, upon the strength and location of the storm’s rainfall. This single case study serves to demonstrate that the simulated rainfall of the event is sensitive to the finescale features of the tropopause-level flow. One inference is that an adequate initial specification of these features might be a necessary prerequisite for accurate QPF of Alpine rainstorms.

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