The characteristics of explosively developing extratropical cyclones in the northwestern Pacific region are analyzed using the global objectively Analyzed dataset (GANAL) provided by the Japan Meteorological Agency (JMA). In the present paper, these cyclones are classified into three types, depending on positions of formation and of rapid development: OJ cyclones originate over the eastern Asian continent and develop over the Sea of Japan or the Sea of Okhotsk; PO-L cyclones are also formed over the Asian continent and develop over the northwestern Pacific Ocean; and PO-O cyclones are formed and develop over the northwestern Pacific Ocean. Statistical analyses suggest that OJ cyclones frequently appeared in late fall and had the smallest deepening rates of the three types; PO-L cyclones had medium deepening rates and frequently occurred in early and late winter; and PO-O cyclones mainly occurred in midwinter and had the largest deepening rates. Two kinds of composite analyses were conducted to understand the structures and the mechanisms of development. The first composite analysis used geographically fixed coordinates. The results suggest that the favorable atmospheric conditions for the development of each type of cyclone are closely connected to the presence and extension of the cold air mass over the Asian continent. In addition, these conditions are closely related to seasonal variations across the area. The other analysis of cyclone mesoscale structure, using cyclone- relative coordinates at the maximum deepening rate, suggests that OJ cyclones had a short-wave, upper-level jet streak and a strong baroclinic zone in the lower level. PO-L cyclones, associated with a zonally stretched jet stream, had a remarkable midlevel baroclinic zone. PO-O cyclones with a strong jet streak also had a distinct baroclinic zone in the midlevel, and a large water vapor budget (precipitation minus evaporation) appeared around the cyclone center. These cyclone structures reflected vorticity, temperature, and moisture advection, that is, larger-scale atmospheric conditions that affected cyclone development.
Abstract Numerical simulations of six explosively developing extratropical cyclones in the northwestern Pacific Ocean region are conducted using a regional mesoscale numerical model [the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)]. Cyclones are categorized according to the locations where they form and develop: Okhotsk–Japan Sea (OJ) cyclones originate over the eastern Asian continent and develop over the Sea of Japan or the Sea of Okhotsk, Pacific Ocean–land (PO–L) cyclones also form over the Asian continent and develop over the northwestern Pacific Ocean, and Pacific Ocean–ocean (PO–O) cyclones form and develop over the northwestern Pacific Ocean. Two cases (the most extreme and normal deepening rate cases for each cyclone type) are selected and simulated. Simulations show that the extreme cyclone of each type is characterized by a different mesoscale structure and evolutionary path, which strongly reflect the larger-scale environment: an OJ cyclone has the smallest deepening rates, associated with a distinct upper-level shortwave trough, a clear lower-level cold front, and a precipitation area that is far from the cyclone center; a PO–L cyclone has moderate deepening rates with high propagation speeds under zonally stretched upper-level jets; and a PO–O cyclone has the strongest deepening rates associated with large amounts of precipitation near its center. Sensitivity experiments involving the latent heat release associated with water vapor condensation show that PO–O cyclones rarely develop without a release of latent heat and their structures are drastically different from the control runs, while OJ cyclones exhibit almost the same developments and have similar structures to the control runs. These tendencies can be seen in both extreme and normal deepening rate cases. These results reveal that the importance of latent heat release to explosive cyclone development varies among the cyclone types, as is reflected by the cyclone origin, frontal structure, moisture distribution, and jet stream configuration.
The characteristics of radar echoes of the precipitating snow clouds over the Ishikari Bay, Hokkaido, Japan were investigated statistically during four months, February and March of 1983 and 1984. In this analysis, the precipitating snow clouds were divided into three types: typical monsoon, final period of monsoon type and low pressure type based on the daily weather maps. Examined elements in this analysis were as follows; Size distributions of radar echo population, percentage occupied by the strong echo areas as against total precipitating echo areas, time changes of echo areas, locations of the center of individual large echo areas, life time of the echoes, and so on. Highly interesting characteristics were obtained; the log-normal distribution was obtained for the size ranges of radar echoes of all snowfall types; the percentage occupied by the echoes stronger than level 2 accounted for less than 40% of all echo areas; the total echo area showed sharp fluctuations; the locations at which the radar echo areas when the snow clouds invading the plain reached their maximal sizes were along the coast line; the average echo area having a life time of 1 hour was approximately 25km2 etc.
In the fall of 1994, the Beaufort and Arctic Storms Experiment (BASE) was held to collect information on the structure and evolution of mesoscale weather systems over the southern Beaufort Sea and the Mackenzie River delta of the western Canadian Arctic. As part of the experiment, X-band Doppler radar observations were carried out at Tuktoyaktuk, a village on the shore of the Beaufort Sea. In this paper, the precipitation features, structure, and moisture transport associated with two distinctly different weather systems that were observed during BASE are described with a variety of datasets. Climatologies of storm activity in the area suggest these two types of different weather systems, the so-called Pacific origin and storm track disturbances, are the most frequently observed in this region during the fall months. The characteristic feature of a Pacific origin weather system is a pronounced layering of the air masses. In the upper layer, the air mass is of Pacific origin and is associated with a deep low in the Gulf of Alaska. As a result it is moist and is capable of producing precipitation. In contrast, the lower layer is initially of continental origin and is associated with a secondary lee cyclogenesis event in the Mackenzie River basin. As the secondary disturbance moves to the east, there is a shift in the wind direction that advects air from the Beaufort Sea into the lower layer. This results in a moistening of the lower layer that allows precipitation from the upper layer that had previously evaporated in the lower layer to be enhanced and reach the surface. The detailed structure of this type of storm is strongly affected by the topography of the region and the presence of open water in the southern Beaufort Sea. The storm track weather system is markedly different and is associated with the passage of a mesoscale low over the southern Beaufort Sea. In this sort of system, there is a well-defined frontal structure of a type previously identified in the midlatitudes. Two different precipitation regimes are identified that are associated with the passage of the warm and cold front. In this sort of system, the sources of moisture are the Bering Sea and the open water in the southern Beaufort Sea.
