SIMULATION OF EXTREME NEW-ZEALAND PRECIPITATION EVENTS .1. SENSITIVITY TO OROGRAPHY AND RESOLUTION

Three extreme precipitation events with peak observed rainfall of grea ter than 700 mm over the South Island of New Zealand were simulated us ing the DAR hydrostatic mesoscale model nested within the ECMWF analys es. The ECMWF analyses for two of the events showed a low-level jet wi th mixing ratios greater than 12 g kg(-1) crossing tbe South Island of New Zealand during the heavy precipitation near a cold front. The thi rd case, which had smaller mixing ratios, occurred as a tow-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 how in the model grid. Orography had a strong influence on the amount of precipitation: the peak preci pitation was related to orographic slope while the area-averaged preci pitation was related to the maximum orographic elevation. The model su ccessfully simulated nearly half the peak observed precipitation and o ver half the area-averaged precipitation (determined by hydrological m eans) in two of the cases and much less in the third case. Refining th e horizontal resolution from 30 to 15 km also increased the peak preci pitation amounts. However, the area-averaged precipitation in the 15-k m 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 wav e consistent with linear theory, in spite of the nonsteady flow, irreg ular orography, and the large amount of diabatic heating present. The amplitude of the mountain wave increased with mountain height and reso lution. The absence of a mountain wave in the run with the artificial barrier orography indicates unrealistic flow for that configuration.