Space-time variability of heavy orographic rainfall

Heavy rainfall is the result of slow-moving or quasi-stationary storm systems, or storm cells repeatedly tracking over the same area. High rain rates, produced by amply moisture supply, combined with a significant residence time, are the sure recipe for extreme rainfall accumulations. Topographic barriers often act to generate heavy rainfall and focus it locally. The study of the detailed mechanisms of this orographic forcing, and how it affects the dynamics and microphysics of heavy precipitation-producing storm systems, was the focus of the Mesoscale Alpine Program (MAP), which entailed a Special Observing Period (SOP) on the south side of the European Alps that lasted from September through November 1999.

Several heavy rainfall events were observed during the MAP SOP. The hydrologic response at the land surface to the intense rainfall was significant, resulting in local flooding, debris flows, and at least one fatality. The events earlier in the SOP, especially the Intensive Observing Periods (IOPs) 2, 3, and 5 were characterized by moist, potentially unstable air being lifted at the Alpine barrier, triggering convection that resulted in short-term rainfall accumulations with local maxima in excess of 200 mm.

The heavy rainfall event of 19-21 September 1999 (IOP 2b) is used to highlight the great spatial and temporal variability of rainfall, particularly over the western side of the Lago Maggiore region. Data from a high-density rain gauge network demonstrate a significant temporal variability in rainfall intensity and extreme spatial gradients in rain accumulation (100 mm/km). Objective storm cell tracking analyses are used to show how this heavy rainfall event was composed of storm cells that were either quasi-stationary or fast-moving, with the largest point-rainfall accumulations produced by the former. Analyses of lightning and multiple polarization radar data reveal differences in storm microphysics, with the fast-moving storm cells generating significantly more lightning flashes as a result of more active mixed-phase processes, involving higher-density ice particles and supercooled liquid water.