Simulation of Extreme New Zealand Precipitation Events. Part II: Mechanisms of Precipitation Development

Abstract

The extreme precipitation event that occurred on 27 December 1989 over the South Island of New Zealand was simulated using the DAR hydrostatic mesoscale model nested within the ECMWF analyses. The model simulated nearly half of the peak observed rainfall for this storm (greater than 700 mm) and captured the location and timing of the intense precipitation. The heavy precipitation developed while a deep layer of moist subtropical air along a cold front ascended the high terrain of the South Island. The intense orographic ascent was associated with a low-level jet core with wind speeds of over 20 m s?1 ahead of the cold front. An upper-level trough and jet streak entrance region were also present upstream of the South Island during the event, aiding the ascent over the mountains and deepening the layer of moist air. The air crossing the mountain was nearly saturated throughout the troposphere and had only weak moist vertical stability near the cold front. Almost all of the simulated precipitation formed in the low troposphere through forced ascent, with only minimal convection behind the cold front. Two sensitivity experiments were conducted to investigate the effects of orography and latent heating on the development of precipitation in the simulations. Weak upstream blocking by the orography was present, enhancing the ascent upstream and causing a slight moistening of the midtroposphere. The latent heat, maximized near the surface on the upwind side of the mountain, caused increased upward motion and precipitation over the orography and decreased ascent upstream, tending to dry and stabilize the air there. The latent heat release weakened the blocking effect of the orography and altered the mountain wave through reduced effective dry static stability.