The two major difficulties, i. e., the excessive speed and the smallest preferred scale, encountered by wave-CISK as an explanation for the 40-50 day tropical intraseasonal oscillation (or the Madden-Julian oscillation, MJO), as demonstrated in many studies, are examined. In addition, wave-CISK is contrasted with a more promising framework for interpreting MJO recently proposed by Chao and Lin. The central cause for the two difficulties can be attributed to the convective heating formulation used. Also, depending on how one defines it, the wave-CISK concept as the foundation for interpreting MJO is at best inadequate and at worst erroneous.
Response of a stably stratified two-layer atmosphere to low-level heating is investigated by obtaining and analyzing analytic solution for the two-dimensional, steady-state, linear perturbation. The ambient wind is assumed to be constant and the Brunt-Vaisala frequency to be piecewise constant in each layer. The diabatic heating is specified from the surface to a certain height in the lower layer. In this study, discussion is made on only the case that the stability in the lower layer is larger than that in the upper layer. A steady-state solution is possible only when the upper layer is not neutrally stratified. If the upper layer is neutrally stratified, the incident wave is totally reflected from the layer interface and the wave resonance in the lower layer can result in a wave breaking eventually in the absence of dissipation. The lower layer depth to produce the maximum magnitude of the vertical velocity in the lower layer is presented in terms of the vertical wavelength of dominant gravity wave and the stability ratio between two layers. The magnitude of the maximum vertical velocity for this lower layer depth is larger than that for the uniform stability case and it increases as the stability ratio between two layer decreases. The vertical velocity in the upper layer is also amplified by the stability ratio between two layers. The reflection coefficient of waves at the layer interface and the transmission coefficient through it are obtained in terms of the stability ratio. It is shown that the transmission coefficient is larger than the reflection coefficient. The lower layer depth to produce the maximum magnitude of the horizontal velocity perturbation is the same as that for the ducting condition by Lindzen and Tung. In the upper layer, the magnitude of the horizontal velocity perturbation is the same as that for the uniform stability case regardless of the stability ratio. Position of the maximum positive horizontal velocity perturbation at the surface is shifted toward the heating center from the downstream side as the stability ratio decreases. The momentum flux for the two-layer case is much larger than that for the uniform stability case because both the horizontal and vertical velocity perturbations in the lower layer are amplified by the wave reflection from the layer interface.
We carried out statistical analyses to investigate the characteristics of snowfall distributions over the Ishikari Plain of the island of Hokkaido, Japan. Using AMeDAS's hourly precipitation data between 1982 and 1991, we determined that the snowfall distribution could be classified into two regions and using the rotated empirical orthogonal function : one is a mountain-centered distribution, and the other is a plain-centered distribution. The characteristics of the temporal variations of the two spatial patterns showed that the plain-type snowfalls apt to occur when it is colder, i. e., in the morning or in the latter part of winter. Next, we investigated the relationship between the plain-type snowfall and synoptic-scale and mesoscale environments. We were able to establish that the plain-type snowfalls are accompanied by offshore cold winds that blow in the planetary boundary layer against the winter northwesterly monsoon. When the snowfall continues for more than one day, the offshore wind originates in the large scale katabatic winds that spread around the whole river basin behind the coast. This finding suggests that the source of the offshore wind is the pooled cold air on the plain produced by the confluence of the katabatic flow from the surrounding mountains. We proposed that continuous drainage from the pool enables the coastal snowfall to continue. We also investigated the large-scale atmospheric conditions, using the data for the objective analysis. When a plain-type snowfall occurs, the westerly wind speed and the temperature at the upper troposphere are respectively weaker and lower than these factors during snowfall with no offshore wind. These synoptic-scale environments encourage the katabatic flow, and eventually prolong the coastal snowfall. Moreover, these synoptic-scale environments are similar to those that operate during snowfalls in other regions facing the Japan Sea.
During the intensive observation period of the Baiu Front Heavy Rainfall Experiment in July 1988, a Baiu frontal rainband passed over the Kyushu District, and its kinematic structure was observed by dual Doppler radar. The purposes of this paper are to investigate the thermodynamic structure of the rainband using a retrieval method and to clarify its maintenance mechanism. The rainband had many similarities to those for tropical and mid-latitude squall lines. The dry rear inflow from the northeast merged with the convective downdrafts and resulted in low-level divergent out-flows. The warm and moist southwesterly converged with the outflow directed toward the leading edge and caused the rearward sloping convective updrafts. The retrieved perturbation temperature field showed a positive anomaly in the convective updrafts above a height of 4 km, and a negative anomaly due to evaporation of raindrops in the dry rear inflow behind the gust front. The perturbation pressure pattern was consistent with the temperature and flow patterns and showed a meso-high behind the gust front and a meso-low beneath the warm anomaly of the convective updraft. In the low-level of the stratiform region, a meso-low was generated owing to the subsidence warming of the descending rear inflow. The distributions of temperature and pressure perturbations suggest that the convective updraft was maintained by the upward pressure gradient force below a height of 4 km, while it was maintained by a positive buoyancy above this level. The horizontal pressure gradient force associated with the mid-level meso-low in the convective region accelerated the lifted air to the rear, and the convective updraft tilted to the rear of the system. The raindrops were transferred to the rear of the system and fell into the convective downdraft region, which did not prevent the convective updraft from developing and the intense rainband was maintained.
An investigation was conducted into the effect of the interference of the migrating diurnal tide and a quasi-stationary wave of zonal wavenumber 1 to produce a zonally symmetric diurnal tide, making use of a general circulation model. It was found that the new zonally symmetric tide is created at the upper stratosphere in the winter hemisphere when the quasi-stationary wave amplifies. The amplitude of the new tide was found to be comparable to or larger than that of the tide propagating from the surface. This creation is expected to occur when the forced Rossby waves of zonal wavenumber 1 have large amplitude and the polar night jet-shift or warming event occurs.
The equations of motion describing two-dimensional internal gravity waves were analyzed to derive an expression for the horizontal buoyancy flux-i.e., the correlation between buoyancy (or temperature) and horizontal velocity-in various cases involving vertical shear. It is shown that, although this correlation vanishes at zeroth order for a single wave, its first order contribution is nonzero due to shear, even for steady, conservative, incompressible waves. Departures from steady, conservative motion or quasi-compressibility also cause a nonzero correlation. Several cases were analyzed and some numerical results obtained for waves approaching a critical layer of reduced intrinsic phase speed. With weak shear, the buoyancy flux is small relative to vertical momentum flux, as expected from the perturbation theory. Strong vertical shear enhances the buoyancy flux within the shear zone and causes partial reflection beneath, producing a nonzero correlation (at zeroth order) in this region. These effects may explain recent observations of zonal wind and temperature cospectra in the equatorial lower stratosphere.
The static relationship between anomalies of sea surface temperatures (SSTs) and air-sea heat fluxes in the North Pacific is investigated using the datasets of SSTs and heat fluxes through the sea surface computed from COADS for 40 years (1951 to 1990). In the low-latitude ocean, through a whole year, the downward heat fluxes (oceanic heat gain) negatively correlate with SST anomalies : when SST anomalies are negative (positive), then the ocean gains more (less) heat. On the contrary, in winter and spring in the mid-latitude ocean, the downward heat fluxes positively correlate with SST anomalies : when SST anomalies are positive (negative), then the ocean gains more (less) heat. The static relationship between SST anomalies and the upward heat fluxes (oceanic heat loss) is also examined. It is found that the situation is almost completely opposite with those between SST anomalies and the downward heat fluxes.