SIMULATION OF EXTREME NEW-ZEALAND PRECIPITATION EVENTS .2. MECHANISMSOF PRECIPITATION DEVELOPMENT

The extreme precipitation event that occurred on 27 December 1989 over the South Island of New Zealand was simulated using the DAR hydrostat ic mesoscale model nested within the ECMWF analyses. The model simulat ed nearly half of the peak observed rainfall for this storm (greater t han 700 mm) and captured the location and timing of the intense precip itation. The heavy precipitation developed while a deep layer of moist subtropical air along a cold front ascended the high terrain of the S outh island. The intense orographic ascent was associated with a low-l evel jet core with wind speeds of over 20 m s(-1) ahead of the cold fr ont An upper-level trough and jet streak entrance region were also pre sent upstream of the South Island during the event, aiding the ascent over the mountains and deepening the layer of moist air. The air cross ing the mountain was nearly saturated throughout the troposphere and h ad only weak moist vertical stability near the sold front. Almost all, of the simulated precipitation formed in the low troposphere through forced ascent, with only minimal convection behind the fold front. Two sensitivity experiments were conducted to investigate the effects of orography and latent heating on the development of precipitation in th e simulations. Weak upstream blocking by the orography was present, en hancing the ascent upstream and causing a slight moistening of the mid troposphere. The latent heat, maximized near the surface on the upwind side of the mountain, caused increased upward motion and precipitatio n over the orography and decreased ascent upstream, tending to dry and stabilize the air there. The latent heat release weakened the blockin g effect of the orography and altered the mountain wave through reduce d effective dry static stability.