Predicting rainfall in hilly terrain: the role of mesoscale processes and cloud microphysics

The nowcasting of precipitation by a combination of radar and satellite image extrapolation and mesoscale models is reasonably well established. However, in the event of moist air being forced over significant topography heavy rainfall can result which has proved quite difficult to predict quantitatively. Topography leads to a diverse range of modifications to the large scale wind patterns. These modifications, in the form of topographically-induced mesoscale circulations, can modify cloud and precipitation-producing microscale processes. The net effect of the topography is to redistribute and modify the amount of precipitation that would have occurred in the absence of topography and, in some cases, to cause rainfall which would otherwise not have occurred at all. Radars are about the only way to correctly differentiate distinct dominant rainfall production processes which occur at different times in a given location. These different processes lead to different geographical distributions and amounts of rainfall. Field experimental results are presented which illustrate this situation in New Zealand where heavy, flood producing orographic rainfall associated with strong north west airflows over the Southern Alps and onshore winds associated with Sub Tropical Cyclones lead to several different outcomes. Suggestions are made as to how to proceed including the more detailed use of satellite data to help determine the dominant