Predicting orographic rainfall: the role of small mobile radars

The heavy and flood producing rainfall in New Zealand is always associated with hilly terrain and thus the micrphysical processes going on in the bottom few hundred meters of the atmosphere is of vital importance in predicting the severity of the impact of a sub tropical cyclone or rapidly developing low. The measurement of orographic rainfall by radar is severely limited by the presence of the hills or mountains producing the orographic effects and the difficulty that a distant radar has in observing the precipitation processes near the ground. Conventional visible and IR satellite data is often difficult to use because of extensive cirrus cloud cover.

We have used small mobile X band weather radars ( with spatial resolution of 100m and a temporal resolution of 6 seconds) to rapidly deploy to mountainous regions in at risk areas. Results are presented for the land fall of a Sub Tropical cyclone near Gisborne, NZ, where strong effects due to local small scale topography as well as the effects of the mountain range as a whole are shown