Rain gauge, radar, and atmospheric observations during a prolonged northwesterly storm in November 1994 have been used to study factors influencing the distribution of precipitation across the Southern Alps. Despite the persistent northwesterly flow, the location and intensity of precipitation varied markedly during this storm, providing an excellent dataset for these investigations. Data from 36 recording gauges in the northern half of the Alps were supplemented by data from 57 daily gauges, which were partitioned into 6-h values. These data were grouped according to distance from the alpine divide, and best-fit transect curves, normalized for rainfall intensity, were established every 6 h. The fraction of the total transect precipitation falling in leeside catchments varied between 0.11 and 0.70, while a ''spillover distance'' index varied between 6 and 29 km. Comparison with atmospheric profiles of temperature and wind from Hokitika on the west coast of New Zealand and with European Centre for Medium-Range Weather Forecasts analyses revealed that precipitation was confined upwind of the divide during a period of blocked flow near the start of the storm, and only extended into leeside catchments with the onset of stronger flow and reduced static stability. Regression equations involving these factors explained up to 93% of the spillover variations. It is suggested that ascent and precipitation maxima are shifted upstream during blocked flow, while spillover is enhanced during stronger and/or unstable flow as the upstream influence lessens and snow and ice particles drift farther downwind before falling below the freezing level. Further case and modeling studies are needed to demonstrate the wider applicability of these findings.
Abstract. The Southern Alps field experiment was designed to identify the dominant rainfall processes in intense orographic events in the South Island of New Zealand and included the deployment of a rain gauge network and meteorological radar. Multiscaling statistics, used to characterize the rainfall from a single extreme event, revealed both orographic and temporal changes in the rainfall nature, with significantly more incessant rainfall observed in the higher-altitude regions. Central to this work was physical interpretation of the statistical parameters, which contributes toward forming links between multiscaling analysis and meteorological processes necessary for practical applications of multiscaling statistics. A further step was taken by combining the statistical results with other meteorological data to infer details of the physical processes, hence providing an example of the utility of multiscaling characterization of rainfall for improving our understanding of physical rainfall processes. Evidence is presented of lateral broadening of precipitating elements as the alpine divide is approached and is used, in conjunction with the wind profile, to explain the quasiincessant rainfall observed near the divide.
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