On the Structure of the &amp;ldquo;Baiu Front&amp;rdquo; and the Associated Intermediate-scale Disturbances in the Lower Atmosphere

Evolution process and fine structure of a medium-scale (*1000km) cloud cluster developed over the western portion of the Baiu front (Continental China region) and propagated to the eastern portion (the Japan Islands and the Pacific) are investigated.In this paper (Part I), special attention is put on variations in cloud/rainfall features, associated disturbances and environmental situations throughout evolution process of the cloud cluster.This cloud cluster was generated at the eastern foot of the Tibetan Plateau 12 July 1979, and propagated to the east toward the Japan Islands, along *30N latitude circle with a phase speed of 1000km/day. Two days after its generation, the cloud cluster caused heavy rainfalls (*100 mm in one hour) over the southwest Japan. After this, the cluster developed into a depression as it propagated over the Pacific along the subtropical baroclinic zone (the Baiu frontal zone). The following features are found: 1). This medium-scale cloud cluster had a long life time (a few days). Its shape seen in GMS images changed evidently during the analysis period (12GMT, 12 July*12GMT, 15 July). The evolution process is subdivided into four stages, i.e., formation, developing, transitional and cyclone-formation stages.2). Over the Continent (formation and developing stages), the cloud cluster has an isolated oval-shape and developed associating with a weak positive vorticity core which was embedded in the southern branch of the mid-tropospheric jet stream (southern frontal zone). This positive vorticity core is thought to be formed by lee-cyclogenesis due to the Tibetan Plateau. Over Japan and the Pacific (cyclone-formation stage) the cloud cluster was elongated in shape and developed into a depression in the baroclinic zone (Baiu front), coupled with a short wave trough in the upper troposphere, which was propagated southeastward from the northern frontal zone over the Continent.3). Cloud/rainfall features of the cloud cluster over the Continent were quite different from those observed over the Japan Islands. That is, the cloud cluster developed in the unstable tropical air over the Continent consisted of only convective clouds. In contrast to that, the cluster in the subtropical baroclinic zone consists of both convective and stratiform clouds, or convective and uniform echoes as recognized in radar. 4). Rainfall in the cloud cluster reached a maximum intensity at the end of the developing stage.After this, the cluster shrinked in extension, as the posititive vorticity core in the middle layer became less distinct.In the cyclone-formation stage, rainfalls reached another maximum, which was followed by the transformation of the cluster into a large cloud system of a frontal depression. 5). The cluster was composed of a few mesoscale (100km) cells. The minimum of TBB in the mesoscale cells was *-70* (cloud top height of *16km). 6). The cloud cluster showed periodic variation with a period of several hours throughout the four stages.This variation is attributed to alternate developments of...

Section: 2supporting

confidence: 67%

A Medium-Scale Cloud Cluster in a Baiu Front

Akiyama¹

1984

“…We define the former as the Baiu frontal subsynoptic-scale depression, and the latter as the synoptic-scale cyclone. In some papers, similar subsynoptic-scale depression is defined as the ''medium-scale depression'' (e.g., Matsumoto et al 1970) or ''subsynoptic-scale depression'' (e.g., Ninomiya and Shibagaki 2003).…”

Section: The West-east Elongating Baiu Frontmentioning

confidence: 99%

Synoptic- and Meso-.ALPHA.-Scale Variations of the Baiu Front Simulated in an AGCM

Ninomiya

Enomoto

Nishimura

et al. 2003

The present paper studies features of the Baiu front and associated precipitation systems simulated in the climatological SST run by an AGCM T106L52 (primitive equation spectral model, the maximum wave-number of 106, with 52 vertical levels) in comparison with the features found in observational studies.The Baiu front is properly simulated in 1-20 June Y07 (the 7th year after the spin-up integration). In this ''Baiu phase'', the large-scale circulation systems, such as the upper cold lows and blocking ridge in the northern latitudes, westward extending Pacific subtropical anticyclone, monsoon westerly, and subtropical jet stream, are simultaneously maintained. Under this condition, the synoptic-and meso-ascale variations of the Baiu front are simulated. The southward shift of the front occurs when a synopticscale cyclone develops in association with a westerly trough. When westerly troughs are inactive, weak subsynoptic-scale depression is formed in the frontal zone. A few meso-a-scale precipitation systems are generated in the trailing portion of the preceding depression, and form a ''Baiu precipitation system family'' with a length of@2000 km. This indicates that the simulation of the realistic Baiu front depends on the maintenance of proper large-and synoptic-scale circulation systems.However, the Baiu precipitation zone disappears in the early July, as the west-east elongating barotropic anticyclone, which is usually seen over Japan in August in the real atmosphere, is situated over @37 N.

“…The author (Matsumoto, Fujita and Asai,1962) showed previously that a remarkable southerly low-level jet was observed at the time of the severe rainfalls in June 1961 and it was super-geostrophic. The super-geostrophic low-level jet comes out even in the averaged wind field over several days (Matsumoto, Yoshizumi and Takeuchi, 1970: Matsumoto, Ninomiya and Yoshizumi, 1971). The fact that also the geostrophic wind assumes maximum value at about the same level as the low-level jet is linked with the characteristic temperature distribution mentioned above through thermal wind relationship.…”

Section: Solenoidal Circulation Around the Unbalanced Low-level Jetmentioning

confidence: 99%

“…The author has pointed out the importance of the second type of the low-level jet in his recent studies on severe rainstorms (Matsumoto, Yoshizumi andTakeuchi, 1970: Matsumoto, Ninomiya andYoshizumi,1971). The low-level jet is undoubtedly a smaller scale phenomenon and little is known about its structure and mechanism simply because of the lack of adequate observations.…”

Section: Introductionmentioning

confidence: 99%

Unbalanced Low-Level Jet and Solenoidal Circulation Associated with

Matsumoto¹

1972

A southerly low-level jet appeared over Kyushu on July 7, 1970 when a large amount of precipitation was observed to the west of jet axis. The mesoscale thermal structure around the jet core shows an indirect indication of adjustment with the wind field but it is found to be highly unbalanced.The warmer area appearing below the jet core to the east is characterized by dryness, which is possibly generated by an indirect solenoidal circulation.Similar relationship among wind, temperature and relative humidity field is found in the lower troposphere even for the monthly mean values of June 1970.