Relationships between air mass properties and mesoscale rainfall in New Zealand's Southern Alps

• Published in: Atmospheric research Vol. 52; no. 4; pp. 261 - 282
• Main Authors: Wratt, D.S, Revell, M.J, Sinclair, M.R, Gray, W.R, Henderson, R.D, Chater, A.M
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 2000; Elsevier
• Subjects: Air mass; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; New Zealand, Southern Alps; Orographic rainfall; Rain gauge transects; Rainfall intensity; Rainfall spillover; Tasman Sea; Water in the atmosphere (humidity, clouds, evaporation, precipitation); New Zealand; South Pacific; Pacific Ocean; Tasman Sea; Oceania; Southern Alps, New Zealand; Air mass; Rainfall spillover; Rain gauge transects; Orographic rainfall; Rainfall intensity; Orography; Rain; Mesoscale; Intensity
• ISSN: 0169-8095; 1873-2895
• DOI: 10.1016/S0169-8095(99)00038-1

This paper identifies relationships between air mass properties and mesoscale rainfall when moist air blows over New Zealand's Southern Alps from the Tasman Sea. Around 50% of the variance in six-hourly rain volumes summed across three separate cross-mountain raingauge transects and in six-hourly rain volume spilling across the alpine divide are statistically explained by the following properties of the approaching air mass: relative humidity, wind velocity normal to the mountains, air mass stability and synoptically induced upward motion. These factors also explain about 25% ( r≈0.5) of the variance in the downwind distance reached by the spillover rainfall. For the highest 10% of six-hourly rainfalls, spillover distance and magnitude are negatively correlated with the 700 or 500 hPa temperature. Multiple linear regression equations suitable for predicting rainfall intensity and spillover are developed. A progression is described in the magnitude and depth of vertical motion and resulting condensation rates over the mountains as the properties of the incoming air mass evolve through a storm. These changes, together with greater downwind advection of ice particles compared to raindrops, explain the observed statistical relationships between the air mass properties and mountain rainfall.