Space-time analysis of rainfall in relation to topography for heavy precipitation events observed during MAP

Several heavy rainfall events were observed on the Mediterranean side of the Alps during the Special Observing Period (SOP) of the Mesoscale Alpine Program (MAP). 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 Observation Periods (IOPs) 2, (September 17-21), 3 (September 24-27), and 5 (October 2-5), 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. Smaller-scale topographic features appear to have played an important role in the generation (or intensification) of convective cells, as revealed by inspection of radar data.

Particular attention is given to the spatial and temporal distribution of rainfall relative to the topography. To explain the observed spatial rainfall distribution, objective tracking analyses are performed based on Monte Lema and/or S-Pol radar data to document formation, evolution, and decay of radar-echo cells and their motion relative to the larger precipitation system and the terrain. Time-series analyses of cell characteristics (including maximum intensity and height of echo centroid), together with storm-microphysical information based on S-Pol multiple polarization radar and lightning data, shed light on the importance of warm-rain processes for heavy rainfall events. Rainfall amounts will be estimated based on radar and rain gauge information.