Simulation of Extreme New Zealand Precipitation Events. Part I: Sensitivity to Orography and Resolution

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Abstract
The extreme precipitation events with peak observed rainfall of greater than 700 mm over the South Island of New Zealand were simulated using the DAR hydrostatic mesoscale model nested within the ECMWF analyses. The ECMWF analyses for two of the events showed a low-level jet with mixing ratios greater than 12 g kg−1 crossing the South Island of New Zealand during the heavy precipitation near a cold front. The third case, which had smaller mixing ratios, occurred as a low-level jet and crossed the South Island while a low redeveloped downstream. Three different orographies were used with the 30-km horizontal resolution model runs, with progressively increased terrain heights. The highest orography was created by artificially inserting the effective barrier of the Southern Alps to northwesterly flow in the model grid. Orography had a strong influence on the amount of precipitation: the peak precipitation was related to orographic slope while the area-averaged precipitation was related to the maximum orographic elevation. The model successfully simulated nearly half the peak observed precipitation and over half the area-averaged precipitation (determined by hydrological means) in two of the cases and much less in the third case. Refining the horizontal resolution from 30 to 15 km also increased the peak precipitation amounts. However, the area-averaged precipitation in the 15-km runs was not significantly larger than in the 30-km runs, suggesting more concentrated precipitation over a smaller area. All simulations, except the artificial barrier orography case, produced a mountain wave consistent with linear theory, in spite of the nonsteady flow, irregular orography, and the large amount of diabatic heating present. The amplitude of the mountain wave increased with mountain height and resolution. The absence of a mountain wave in the run with the artificial barrier orography indicates unrealistic flow for that configuration.

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