Analysis of Meteorological Precursors to Ordinary and Explosive Cyclogenesis in the Western North Pacific

John R. Gyakum Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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Richard E. Danielson Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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Abstract

Thirty-five cases of cyclogenesis that occurred during the cold seasons from 1975 to 1995 in the western North Pacific Ocean are studied to determine common and disparate dynamic and thermodynamic structures in both the ordinary and rapid developments. An analysis of 1000-hPa height and 1000–500-hPa thickness anomalies with respect to the 20-yr climatology reveals the following results. Though each sample of cyclogenesis is characterized by a favorable-appearing thickness trough–ridge structure, important differences are found. Both the upstream surface anticyclone and the downstream precedent cyclone are preferentially stronger at the beginning of the most rapid cyclogenesis in the strong sample. Because of the consequently stronger equatorward flow, the 1000–500-hPa thickness anomaly in the strong sample is colder by approximately 40 m (∼2°C) in the region of incipient cyclogenesis and eastward by 1500 km.

A harmonic time series analysis of NCEP gridded fields partitions the geopotential height fields into high- (corresponding to synoptic-scale waves) and low-frequency wave components. This analysis shows the 500-hPa synoptic-scale disturbances that trigger both ordinary and rapid cyclogenesis are easily tracked as early as 72 h prior to the event. These triggering disturbances, 72 h prior to the most rapid cyclogenesis, are found most typically in central Siberia. Additionally, the synoptic-scale trough–ridge couplet is stronger at the onset of development for the explosive sample, suggesting a stronger large-scale forcing for cyclogenesis.

To gain insight into possible physical mechanisms associated with these structural differences, the SST anomalies (with respect to a 30-yr climate) in the rapid developments are compared with those of the weaker systems. Though there is no statistically significant difference in SST anomalies, the preferentially colder tropospheric air mass in the strong sample suggests this sample to be characterized by stronger surface fluxes. Indeed, the NCEP reanalyses reveal both the sensible and latent heat fluxes to be 50–75 W m−2 greater in the rapid development cases in the region along their subsequent cyclone tracks. These statistically significant differences are also reflected in moisture budget analyses, which reveal surface evaporation to be larger in the explosive cases. This evaporation component contributes importantly to the computed precipitation in each class of cyclogenesis.

Corresponding author address: Dr. John R. Gyakum, Dept. of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal, QC H3A 2K6, Canada.

Email: gyakum@zephyr.meteo.mcgill.ca

Abstract

Thirty-five cases of cyclogenesis that occurred during the cold seasons from 1975 to 1995 in the western North Pacific Ocean are studied to determine common and disparate dynamic and thermodynamic structures in both the ordinary and rapid developments. An analysis of 1000-hPa height and 1000–500-hPa thickness anomalies with respect to the 20-yr climatology reveals the following results. Though each sample of cyclogenesis is characterized by a favorable-appearing thickness trough–ridge structure, important differences are found. Both the upstream surface anticyclone and the downstream precedent cyclone are preferentially stronger at the beginning of the most rapid cyclogenesis in the strong sample. Because of the consequently stronger equatorward flow, the 1000–500-hPa thickness anomaly in the strong sample is colder by approximately 40 m (∼2°C) in the region of incipient cyclogenesis and eastward by 1500 km.

A harmonic time series analysis of NCEP gridded fields partitions the geopotential height fields into high- (corresponding to synoptic-scale waves) and low-frequency wave components. This analysis shows the 500-hPa synoptic-scale disturbances that trigger both ordinary and rapid cyclogenesis are easily tracked as early as 72 h prior to the event. These triggering disturbances, 72 h prior to the most rapid cyclogenesis, are found most typically in central Siberia. Additionally, the synoptic-scale trough–ridge couplet is stronger at the onset of development for the explosive sample, suggesting a stronger large-scale forcing for cyclogenesis.

To gain insight into possible physical mechanisms associated with these structural differences, the SST anomalies (with respect to a 30-yr climate) in the rapid developments are compared with those of the weaker systems. Though there is no statistically significant difference in SST anomalies, the preferentially colder tropospheric air mass in the strong sample suggests this sample to be characterized by stronger surface fluxes. Indeed, the NCEP reanalyses reveal both the sensible and latent heat fluxes to be 50–75 W m−2 greater in the rapid development cases in the region along their subsequent cyclone tracks. These statistically significant differences are also reflected in moisture budget analyses, which reveal surface evaporation to be larger in the explosive cases. This evaporation component contributes importantly to the computed precipitation in each class of cyclogenesis.

Corresponding author address: Dr. John R. Gyakum, Dept. of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal, QC H3A 2K6, Canada.

Email: gyakum@zephyr.meteo.mcgill.ca

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