Spatial and temporal variation of rainfall over New Zealand

Abstract

A hidden semi-Markov model (HSMM) was applied to high temporal resolution rain rate data, and, after determining a transition structure for all of New Zealand, the model's parameters from fits at about 90 sites were mapped. The addition of bogus sites was required before a verification gave an acceptable result for some annual statistics over a 0.1° by 0.1° (i.e., about 10 km) spaced grid. Synthetic rainfall series were generated at each of the grid points, and various aspects of rainfall climatology were mapped. The rainfall process can be viewed as a succession of precipitation events which individually break down into a succession of episodes of either rain type precipitation (i.e., large spatial scale and persistent) or shower type precipitation (i.e., small-scale and short lived). The synthetic data sets showed that most rainfall events consist of a single shower and the rest rarely consist of more than two interchanges between rain and showers. These interchanges take place between periods of rain and showers centered at the higher end of the range of rain rates, and the transition from rain to showers is about three times more likely than that from showers to rain. The area with the highest probabilities for these transitions is the Southern Alps in the west of the South Island of New Zealand. Copyright 2008 by the American Geophysical Union.