A DIAGNOSTIC STUDY OF THE EARLY PHASES OF 16 WESTERN NORTH-PACIFIC CYCLONES

We study the phenomenon of rapid cyclogenesis by examining the time ev olution of the synoptic-scale environment for the 36-h period prior to most rapid intensification. We perform this study with the constructi on of composites for eight explosively-developing cyclones whose maxim um intensification commences within a 5-degrees latitude-longitude reg ion in the vicinity of the Kuroshio Current. A comparison is made with corresponding composites of eight weakly-developing lows within the s ame region. The set of 16 cases is a comprehensive sample of events fo r nine cold seasons. Synoptic-scale features for the strong cases, dis tinct from those of the weaker cases, are detected throughout the 36-h dynamical conditioning period. The stronger cases form to the south a nd west of Japan with antecedent propagation predominately over the ma ritime environment. The weaker systems, in contrast, either form in si tu in our study region, or travel eastward from the Sea of Japan over the Island of Honshu. The unique thermodynamic precursors of the stron ger systems include: a warm 1000-500 hPa thickness anomaly that travel s from the East China Sea northeastward into the Kuroshio region, and a cold thickness anomaly and thermal jet east of our study area. The d ynamical consequence of the warm anomaly includes the development of a n anomalous southwesterly thermal wind to the northwest where incipien t surface development occurs. We examine a representative strong case in which initial surface development occurs to the northwest of the wa rm anomaly in a zone of quasi-geostrophic forcing for ascent. The syst em subsequently propagates into a favorable location for further devel opment downstream of a second 500 hPa trough within the northwesterly polar jet. At the onset of explosive deepening, the cyclone is propaga ting towards a second region of anomalously cold air and strong barocl inity.