Factors Affecting the Distribution and Spillover of Precipitation in the Southern Alps of New Zealand—A Case Study

Abstract

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.

Spatial analysis of precipitation in a high-mountain region: exploring methods with multi-scale topographic predictors and circulation types

“…Even with idealized topographic settings and flow configurations (e.g., isolated hill or ridge, constant flow), situations can be distinguished where precipitation maxima occur over the windward slope, the crest, or the downwind slope of a topographic obstacle (e.g., Sinclair et al, 1997;Smith, 1979). Distributions depend on the height and scale of the obstacle, and the strength, static stability and moisture profile of the impinging flow.…”

“…Distributions depend on the height and scale of the obstacle, and the strength, static stability and moisture profile of the impinging flow. More complex topographic shapes, transient weather systems, convection, and the drift of hydrometeors quickly complicate the picture (e.g., Cosma et al, 2002;Fuhrer and Schär, 2005;Houze et al, 2001;Roe, 2005;Sinclair et al, 1997;Steiner et al, 2003). Therefore, the distribution of long-term mean precipitation is, in many regions, a superposition of several distinct responses to topography, which act at different space scales, involve several characteristics of the topography (not just height), and pertain to different flow situations.…”

“…Moreover, the distribution of rain gauges in complex terrain is often biased, with a majority of measurements taken at valley floors, while steep slopes and high elevations are underrepresented (e.g., Sevruk, 1997). The sampling bias entails a risk of systematic errors in spatial interpolation that can impinge upon estimates on a larger scale, such as for averages over river catchments (e.g., Daly et al, 1994;Sinclair et al, 1997).…”

A case study of orographic rainfall processes incorporating multiscaling characterization techniques

“…2. Rapid generation of widespread rainfall, without the addition, the alpine rainfall process needed to explain why, delay required for standard autoconversion, was attributed to even in the statically unstable postfrontal environment lateral spreading of hydrometeors generated upwind of the [Sinclair et al, 1997], intense cells observed in the lowland foothills throughout the air mass. These hydrometeors could and foothills do not propagate to the alpine divide.…”

Effects of Unsaturated Moist Froude Number and Orographic Aspect Ratio on a Conditionally Unstable Flow over a Mesoscale Mountain

“…For broader mountain ranges, the maximum rainfall tends to be positioned on the upstream side of the mountain (Rauber 1992), while the maximum in precipitation tends to be positioned more toward the mountain peak or lee slope for narrow mountains (Sinclair et al 1997). Colle (2004) studied the relationship between orographic precipitation distribution and environmental conditions and terrain geometry for a stable airstream impinging on a two-dimensional, idealized mountain.…”