Rainfall and snowfall are critical inputs to the New Zealand economy and as weather and climate change over the coming decades strategic planning must incorporate forecast changes in precipitation patterns and amounts. By varying the input parameters of a simple parametric model of orographic rain we are able to produce estimates of the changes in the orographic component of individual precipitation events under various global climate change scenarios.

The technique has been applied to an area of New Zealand using climate scenarios developed for the year 2100. Results suggest that, for the area chosen, a 2ºC change in temperature would lead to a 6 to 7 % increase in both maximum and catchment averaged precipitation rates for the storm case modelled. Similar increases are seen from a 10% increase in wind speed. Increasing both temperature and wind speed together leads to a 16% increase in precipitation. These changes describe only the orographic rain rate component and only the part of that due to increases in wind speed and temperature, and do not address other factors such as the changes in frequency or duration of storms. Nonetheless, the increases are significant and sufficient to indicate that regional climate change research needs to incorporate orographic precipitation processes to improve the reliability of climate change forecasts in regions such as New Zealand.