Yoshio Asuma

Structures and Environment of Explosively Developing Extratropical Cyclones in the Northwestern Pacific Region

Abstract 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 un...

Numerical Study of Explosively Developing Extratropical Cyclones in the Northwestern Pacific Region

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: a...

Precipitation features observed by Doppler radar at Tuktoyaktuk, Northwest Territories, Canada, during the Beaufort and Arctic Storms Experiment

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.

Wintertime precipitation behavior in the western Canadian Arctic region

To investigate the precipitation formation process in the two major types of weather systems (“Pacific origin” and “storm track” types) in the western Canadian Arctic region, an X-band vertical pointing Doppler radar, microwave radiometer were installed at Inuvik, Northwest Territories, Canada, during the midwinter of 1995/1996. Precise precipitation observations specially for ice crystals with replicator, microscope, and close-up photos were also conducted at the same place. Precipitation formation mechanisms were quite different between these two major weather systems. For the Pacific-origin-type disturbance, warm and moist air was advected from the Pacific Ocean between 1.5 and 3.5 km msl. Convective echo patterns appeared by radar, and a large amount of precipitable water vapor and liquid water path were estimated by a microwave radiometer. As the air temperature was below the freezing point, the liquid water was identified as being in the supercooled state. Densely rimed dendrites and graupel particles were observed predominantly on the ground. Collision and coalescence processes of supercooled cloud droplets were dominant. On the contrary, for the storm track disturbance the moisture came from the Arctic Ocean, and strong winds were observed on the ground. The air mass was colder than −20°C throughout the layer. Stratiform echo patterns were observed by radar, a smaller amount of precipitable water vapor, and only a small liquid water path were observed by a microwave radiometer. It was suggested that the condensation growth was predominant. Snow crystal shapes of plates, columns, and bullet rosettes were observed predominantly on the ground.

Supercooled drizzle formed by condensation-coalescence in the mid-winter season of the Canadian Arctic

Abstract Supercooled drizzle (freezing drizzle) was observed at Inuvik, N.W.T., Canada (68°22′N, 133°42′W) on December 20, 21 and 27, 1995. Meteorological conditions in which the supercooled drizzle could form under low temperatures (colder than −20°C) in the mid-winter season of the Canadian Arctic were examined from the sounding data and data measured by a passive microwave radiometer at ground level. The following results were obtained. (1) Supercooled drizzle fell to the ground with ice pellets and frozen drops on snow crystals. (2) The maximum size of supercooled drizzle particles increased as the depth of cloud layer saturated with respect to water increased. (3) Because a layer of air temperature higher than 0°C was not detected from the sounding data at Inuvik, melting of snow particles was impossible. It was concluded, therefore, that supercooled drizzle was formed by the condensation–coalescence process below freezing temperature.

Aggregation of needle snow crystals

Abstract For the purpose of studying the early process of snowflake growth, characteristics of the aggregation (collision–adhesion process) of needle snow crystals were investigated by examining photomicrographs of snowflakes composed of two crystals. The adhesion type of the snowflakes was classified into crossed adhesion and point adhesion types. When the sizes of the two crystals were similar, which is indicative of nearly equal fall velocity, there was an increased number of the crossed adhesion type, whereas the point adhesion type was predominant when there was a large difference in size, thus fall velocity. In the case of the crossed adhesion type, the angle between the c -axes of the two crystals was usually greater than 70°, whereas the angle in the case of the point adhesion type showed no such distinct feature. Our findings suggest that the crossed adhesion type snowflakes are formed mainly through the interaction of the flow fields around two crystals (the collision due to relative horizontal motions) and the point adhesion snowflakes were formed through the inertial collision between smaller crystals and larger ones (the collision due to the velocity difference).

Organizations and the interior characteristics of winter monsoon clouds by dual-polarization Doppler radar observation in Hokuriku, Japan

Abstract To understand the characteristics of winter monsoon clouds, dual-polarization Doppler radar observations were carried out at Mihama, Hokuriku, Japan. A series of organized disturbances associated with the passage of a cyclonic circulation was analyzed. The following scenario was proposed. Environmental circumstances, for example, temperature and wind profiles, surface heat and moisture fluxes, etc., determine the organizational style of mesoscale convections. This organization regulates the convection interior flow pattern including the location and intensity of updrafts. Subsequently, it influences interior microphysical properties, for example, the types of precipitation particles, their growth and distributions, their electrification, etc. In this paper, an observational example following this scenario is presented.

Radar observations of tropical clouds on the Manus Island, Papua New Guinea

During the Intensive Observation Period (IOP) of TOGA-COARE (the Tropical Ocean-Global Atmosphere program, a Coupled Ocean-Atmosphere Response Experiment), Doppler radar observation was carried out on Manus Island, Papua New Guinea. In this study, as a preliminary analysis of tropical cloud in the western part of Pacific Ocean, two cases of convective clouds are analyzed, one is the isolated convective cloud which appeared at the center of the island, the other is linear convective rainband associated with gust front. The common finding of these analyses is the temporal separation of warm rain processes and cold rain processes. This feature causes the timing of the anvil appearance and of the wind gust occurrence.<<ETX>>