Characteristics of baroclinic waves in a differentially heated rotating fluid annulus are studied experimentally over a wide range of angular speed. The annulus geometry is properly selected to produce unmodulated waves so that tilted-trough vacillation with a definite period and high rotational symmetry comparable to steady wave and amplitude vacillation is obtained. Its characteristics (vacillation period, drift period and frequency spectra) are examined in detail. Time variations of the top-surface velocity fields and the horizontal temperature fields in an upper layer during a cycle of tilted-trough vacillation are described ; the temperature fields were constructed indirectly from data obtained with a small number of probes to reduce probe effects. The dependence of the rates of radial heat transport on the angular speed and the dominant wavenumber is also presented.
To gain some information for the initialization process in maintaining the proper tropical circulations, the kinetic energy of tropical gravity waves among vertical modes, meridional indices, and zonal wave numbers is investigated for the NCAR Community Climate Model. Kinetic energy is decomposed into divergent and nondivergent parts. In particular, the divergent kinetic energy is split into zonal and meridional components which measure the strengths of the Walker and Hadley circulations, respectively. The gravity waves associated with the third internal mode corresponding to wave number 3 and meridional index 1 play the most important role in the Walker circulation. However, the gravity waves related to the third internal mode, the longest zonal wave, and the first meridional index contribute the most to the Hadley circulation. To retain proper tropical circulations in the forecast, we suggest that the gravity waves associated with the internal modes 2-6, meridional indices 1-6, and zonal wave numbers 1-11 not be adjusted. Adjustment of gravity waves in the initialization using only a cutoff frequency or period is not recommended. The results based only on the normalized tropical data are qualitatively similar to those for the global region but normalized by the global gravitational energy in Ko et al., 1989b. This implies that the normalized values are more appropriate than the absolute values in the energetics analysis.
The Baiu/Meiyu frontal zone (BFZ), the South Pacific convergence zone (SPCZ), and the South Atlantic convergence zone (SACZ) are significant convergence zones in the sub-tropics. These zones (hereafter Sub-Tropical Convergence Zones ; STCZs) are significant sub-tropical frontal zones characterized with strong moisture convergence, frontogenesis in equivalent potential temperature fields, and generation of convective instability. To clarify the reasons why the STCZs form in the specific areas, we investigated large-scale circulations in the sub-tropics globally compared with those around the STCZs. Moreover, the large-scale circulations around the STCZs were compared between active and inactive periods of the STCZs. The STCZs appeared where the following two conditions were quasi-stationarily satisfied ; (1) sub-tropical jets flow in the sub-tropical latitudes (30°-35°) and (2) low-level poleward flows prevail along the western peripheries of the sub-tropical highs. Over the areas where one or two of the conditions were not satisfied, no, or only weak, precipitation areas appear in the sub-tropics. The STCZs were intensified (weakened) when the two conditions were satisfied (not satisfied). Moreover, significant precipitation zones similar to the STCZs also temporarily appeared elsewhere in the sub-tropics, if the two conditions were satisfied there even for a short period. The poleward flows were indispensable for the strong moisture convergence, the frontogenesis in equivalent potential temperature fields, and the generation of convective instability, in the sub-tropics, which are the characteristics of the sub-tropical frontal zones. The poleward flows were formed geostrophically in the eastward pressure gradient between the sub-tropical highs and heat lows developed in the high-pressure-zones in the sub-tropics. Monsoon convections and land-surface heating over the continents were important as heat sources to form the heat lows.
In this study, the rainfall distribution and synoptic conditions concerning behavior of the frontal zones over East Asia during the Baiu season have been investigated in terms of the length of consecutive heavy rainfall duration in the southwestern part (between 130°E and 140°E) of the Japanese Islands. In the cases of consecutive rainfall which lasts several days in the southwestern part of Japan, the Subtropical Frontal Zone (STFZ), which corresponds to a convective cloud zone extending from the inland region of China, causes rainfall in Japan as the Baiu Frontal Zone (BFZ). The Eurasian Polar Frontal Zone (EPFZ) is located along the 50°N latitude line as a maximum zone of meridional temperature gradient. On the other hand, when rainfall lasts a day or two, the EPFZ is located between 35°N and 40°N to the east of 120°E, and rainfall in the southwestern part of Japan is caused by a sub-synoptic-scale cyclone moving along the EPFZ. In this case, the STFZ is inactive and it comes very close to or merges with the EPFZ. The structure of the BFZ of this case is considered to be different from that of the STFZ but comparable with that of the EPFZ. Moisture content and moisture flux around Japan during the heavy rainfall period are larger in the former case than in the latter case. Moreover, preceding the start of the rainfall of longer duration in the southwestern part of Japan, an area of active convective clouds appears in the inland region of China.
Density currents in jet shear flows are considered analytically in order to investigate the interaction between density currents and environmental shear. This study is an extension of Benjamin (1968)and Xu (1992), in which the environmental flows are uniform and of uniform shear, respectively. The environmental flows in this note have the same magnitude at both the upper and lower rigid boundaries, and an eastward maximum in the middle level. The result shows that, in spite of the eastwards negative shear in the upper layer, the depth and propagation speed of the density current increase as the shear increases.