Estimating long-term annual precipitation in a mountainous region from a diagnostic model

Abstract

Much of New Zealand's precipitation is modulated by windflow over its orography. Rain-gauge networks are normally sparse, and because measurement programmes are relatively expensive, there are considerable benefits in modelling and mapping spatial patterns of precipitation in order to provide information at sites where no data exists. In this paper a simple, numerically robust diagnostic model (VDEL) for estimating orographic precipitation is used to estimate annual precipitation normals for a mountainous region of the southern North Island, New Zealand. The VDEL model, developed initially to analyse storm precipitation patterns, is based on the estimation of orographically forced vertical motion as Vs·∇Zs where ∇Zs is the height of the terrain and Vs the low-level horizontal windflow upstream of the mountain range. Precipitation patterns were diagnosed by VDEL for four main windflow directions and calibrated against annual precipitation normals by means of a multivariate regression mapping function. Areal average precipitation, estimated from catchment runoffs, was reproduced with errors in the 5 to 10 per cent range. © 1997 the Royal Meteorological Society.