Analysis of meteorological precursors to ordinary and explosive cyclogenesis in the Western North Pacific

Thirty-five cases of cyclogenesis that occurred during the cold seasons fro m 1975 to 1995 in the western North Pacific Ocean are studied to determine common and disparate dynamic and thermodynamic structures in both the ordin ary and rapid developments. An analysis of 1000-hPa height and 1000-500-hPa thickness anomalies with respect to the 20-yr climatology reveals the foll owing results. Though each sample of cyclogenesis is characterized by a fav orable-appearing thickness trough-ridge structure, important differences ar e found. Both the upstream surface anticyclone and the downstream precedent cyclone are preferentially stronger at the beginning of the most rapid cyc logenesis in the strong sample. Because of the consequently stronger equato rward now, the 1000-500-hPa thickness anomaly in the strong sample is colde r by approximately 40 m (similar to 2 degrees C) in the region of incipient cyclogenesis and eastward by 1500 km. A harmonic time series analysis of NCEP gridded fields partitions the geopo tential height fields into high (corresponding to synoptic-scale waves) and low-frequency wave components. This analysis shows the 500-hPa synoptic-sc ale disturbances that trigger both ordinary and rapid cyclogenesis are easi ly tracked as early as 72 h prior to the event. These triggering disturbanc es, 72 h prior to the most rapid cyclogenesis, are found most typically in central Siberia. Additionally, the synoptic-scale trough-ridge couplet is s tronger 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 str uctural differences, the SST anomalies (with respect to a 30-yr climate) in the rapid developments are compared with those of the weaker systems. Thou gh there is no statistically significant difference in SST anomalies, the p referentially colder tropospheric air moss in the strong sample suggests th is 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 subseq uent cyclone tracks. These statistically significant differences are also r eflected in moisture budget analyses, which reveal surface evaporation to b e larger in the explosive cases. This evaporation component contributes imp ortantly to the computed precipitation in each class of cyclogenesis.