A Diagnostic Study of the Early Phases of Sixteen Western North-Pacific Cyclones

Abstract

We study the phenomenon of rapid cyclogenesis by examining the time evolution of the synopticscale environment for the 36-h period prior to most rapid intensification. We perform this study with the construction of composites for eight explosively-developing cyclones whose maximum intensification commences within a 5 latitude-longitude region in the vicinity of the Kuroshio Current. A comparison is made with corresponding composites of eight weakly-developing lows within the same region. The set of 16 cases is a comprehensive sample of events for nine cold seasons. Synoptic-scale features for the strong cases, distinct from those of the weaker cases, are detected throughout the 36-h dynamical conditioning period.The stronger cases form to the south and west of Japan with antecedent propagation predominately over the maritime environment. The weaker systems, in contrast, either form in situ in our study region, or travel eastward from the Sea of Japan over the Island of Honshu. The unique thermodynamic precursors of the stronger systems include: a warm 1000-500hPa thickness anomaly that travels from the East China Sea northeastward into the Kuroshio region, and a cold thickness anomaly and thermal jet east of our study area. The dynamical consequence of the warm anomaly includes the development of an anomalous southwesterly thermal wind to the northwest where incipient surface development occurs.We examine a representative strong case in which initial surface development occurs to the northwest of the warm anomaly in a zone of quasi-geostrophic forcing for ascent. The system subsequently propagates into a favorable location for further development downstream of a second 500hPa trough within the northwesterly polar jet. At the onset of explosive deepening, the cyclone is propagating towards a second region of anomalously cold air and strong baroclinity.

A multiscale analysis of a case of slow growth/rapid cyclogenesis during CASP II

“…At this time, the system is located downstream of a strong southwesterly thermal jet and upstream of anomalously cold air and a thermal jet in the wake of a predecessor cyclone to the east of Greenland. Such a structure is similar to that found by Kelly et al (1994) for a sample of rapidly-intensifying cyclones at lower latitudes in the western North Pacific. In that study, it was hypothesized that either or both strong downstream dynamical forcing for ascent/low level convergence and anomalously strong sensible heat fluxes in the cold air could be factors in the subsequent development.…”

“…Analysis of a Case of Slow Growth I Rapid Cyclogenesis /27 0000 UTC 28 FEB 1992 I200UTC 28FEB1992 30/ John R. Gyakum and Ronald E. Stewart low to the northeast of the incipient cyclone in the North Pacific sample studied by Kelly et al (1994), and our study shows a similarly-located (with respect to the cyclogenesis region) precedent deep low near 960 hPa southeast of Greenland (Fig. 2c).…”

“…The most rapid 24-h pressure faîl (35 hPa) begins at this time; therefore, the structure of the thickness anomaly field may be compared directly with the results of Kelly et al (1994) in which a sample of rapidly-intensifying systems is studied. Our Fig.…”