Rainfall redistribution over low hills due to flow perturbation S.G. Bradley a , *, W.R. Gray b , L.D. Pigott a , A.W. Seed a , C.D. Stow a , G.L. Austin a a Department of Physics, University of Auckland, 38 Princes Street, Auckland 1, New Zealand b National Institute for Water and Atmospheric Research, P.O. Box 14 901, Wellington, New Zealand Received 20 October 1994; revised 12 March 1997; accepted 16 March 1997 Abstract A simple potential flow model is developed for airflow over low hills. The model readily allows incorporation of simple rain microphysics so that drop trajectories can be traced in the region of a hill complex. Rainfall intensity variations are found in the horizontal and in the vertical, and a straight- forward modification to the model allows for time dependence to be studied. The model is shown to serve as a useful diagnostic tool to describe rain drop redistribution due to perturbed airflow. In particular, increased rainfall is predicted in the lee of hill peaks and depleted rainfall is predicted on ridges. The redistribution effects dominate at low levels, in contrast to predictions based on seeder–feeder enhancement. Comparison with field measurements shows good qualitative agreement over low hills and, in some cases, close quantitative agreement. Redistribution of rain appears to be a significant process that should be considered in rainfall interpolation and prediction schemes involving hilly terrain.  1997 Elsevier Science B.V. Keywords: Enhancement; Mesoscale; Modelling; Rainfall; Wind flow 1. Introduction Rainfall rates over low hills are sometimes considerably higher than over surrounding level terrain. A mechanism to explain this phenomenon was originally proposed by Bergeron (1965). He suggested that rain from a widespread ‘‘seeder’’ cloud sweeps out droplets from low-level ‘‘feeder’’ clouds which have grown over the hills through forced uplift of moist air. Bader and Roach (1977) used a simple air-flow trajectory model and vertically falling drops to calculate orographic rain enhancement over the hills of South 0022-1694/97/$17.00  1997 Elsevier Science B.V. All rights reserved PII S0022-1694(97)00054-1 Journal of Hydrology 202 (1997) 33–47 * Corresponding author. Fax: +64 9 3737445; e-mail: s.bradley@auckland.ac.nz