No 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.

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.

One of the subprograms of the Polar-Experiment North (POLEX-North), "Observation of. Wintertime Clouds and Precipitation" was carried out at Inuvik (68*22'N, 133*42'W), Northwest Territories, Arctic Canada, from December 1, 1979 to January 5, 1980. When the warm air from the Pacific Ocean advected over the observation station, dendritic crystals were observed as rimed and snowflakes, and at times, graupel particles. Corresponding to these snow crystals, a PPI radar echo showed convective type. On the other hand, when the polar air masses covered over the observation station, as expected from the temperature condition, the prevailing shapes . of snow crystals were combination of bullets, columns and crossed plates. Corresponding to these air masses, a PPI radar echo was a stratiform type. The maximum precipitation intensities in a successive snowfall during the observation period were in the order of 10-3 to 100mm*hr-1.Calculating the mass using the number flux and the maximum precipitation intensity, they fell into the range between 1*10-3mg and 1*10-2 mg. The equivalent diameters corresponding to these masses were from 0.12mm to 0.27mm.The Z-R relation, Z=13R1.2 and Z=9R1.1 were obtained for crossed plates and column type snow crystals, respectively. These relations were nearly equal to Z=10R1.0 for ice crystals introduced by Sato et al. (1981) for the light precipitation intensity during the summer season at the South Pole. About the relation between the maximum size and number flux, the increase in number flux increased the maximum size when the cloud thickness was thick and the increase in number flux decreased the maximum size when the cloud thickness was very thin. Based on the relation above described, a correlation between cloud thickness, maximum and minimum temperatures in clouds, weather conditions of synoptic scale and the shapes of snow crystals was summarized in Fig. 15.

Abstract. 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 closeup 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 -20øC 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. IntroductionArctic WANTS (water, aerosol, nuclei transportation, and snow) experiments were carried out at Inuvik, Northwest Territories, Canada, from 14 December 14, 1995 to January 15, 1996. The purpose of this field experiment was to better understand the weather systems, water cycle, behavior of precipitation, aerosols, and ice crystal processes in the western Canadian Arctic region during midwinter, focusing primarily on the interaction between larger-scale (synoptic scale and mesoscale) properties and cloud physical (microscale) properties.