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A Diagnostic Study of the Extratropical Precipitation Resulting from Tropical Cyclone Bola

Mark R. SinclairNational Institute of Water and Atmospheric Research, Wellington, New Zealand

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Abstract

This is the second of two papers on tropical storms entering middle latitudes in the southwest Pacific. This study focuses on the heavy rainfall associated with Tropical Cyclone Bola during 6–8 March 1988 following its passage from the tropics to a position new northern New Zealand. The heaviest precipitation fell on the upwind side of the Gisborne Ranges, where 5-day rainfall totals exceeding 800 mm were recorded.

Precipitation diagnoses from the 2.51 × 2.5° ECMWF dataset seriously underestimated the intensity of observed rainfall over the mountainous terrain. This failure resulted primarily from an inability to resolve the large orographic component of this rainfall. A simple scheme, based on ascent caused by flow over topography, was able to replicate the huge rainfall amounts on the upwind side of the mountain only when details of the local topography to approximately 10 km were included.

A quasigeostrophic ω diagnosis from the ECMWF data showed that large-scale ascent occurred where upper-level vorticity advection and increasing low-level thermal advection acted in phase. Strongest ascent occurred in the region between the difluent exit region of a westerly jet to the north of Bola and the confluent entrance to a second jet southeast of the South Island of New Zealand. A cross section revealed an associated two-cell vertical circulation pattern similar to that observed in other parts of the world. The heavy rain occurred just downstream from a low-level confluence where tropical air from Bola's eastern flank met cooler air from the southeast. Frontogenesis and moisture convergence associated with this confluence possibly helped to focus ascent near the Gisborne Ranges, although this could not be confirmed, because of the lack of observations.

Abstract

This is the second of two papers on tropical storms entering middle latitudes in the southwest Pacific. This study focuses on the heavy rainfall associated with Tropical Cyclone Bola during 6–8 March 1988 following its passage from the tropics to a position new northern New Zealand. The heaviest precipitation fell on the upwind side of the Gisborne Ranges, where 5-day rainfall totals exceeding 800 mm were recorded.

Precipitation diagnoses from the 2.51 × 2.5° ECMWF dataset seriously underestimated the intensity of observed rainfall over the mountainous terrain. This failure resulted primarily from an inability to resolve the large orographic component of this rainfall. A simple scheme, based on ascent caused by flow over topography, was able to replicate the huge rainfall amounts on the upwind side of the mountain only when details of the local topography to approximately 10 km were included.

A quasigeostrophic ω diagnosis from the ECMWF data showed that large-scale ascent occurred where upper-level vorticity advection and increasing low-level thermal advection acted in phase. Strongest ascent occurred in the region between the difluent exit region of a westerly jet to the north of Bola and the confluent entrance to a second jet southeast of the South Island of New Zealand. A cross section revealed an associated two-cell vertical circulation pattern similar to that observed in other parts of the world. The heavy rain occurred just downstream from a low-level confluence where tropical air from Bola's eastern flank met cooler air from the southeast. Frontogenesis and moisture convergence associated with this confluence possibly helped to focus ascent near the Gisborne Ranges, although this could not be confirmed, because of the lack of observations.

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