Using a nonlinear primitive equation model of the tropical atmosphere, we have studied the large scale dynamics associated with the organization of super-cloud clusters (SCC) and the effect of nonlinear mean flow interaction and lateral forcing on the evolution of SCC and the Madden and Julian Oscillation (MJO) over the tropical western Pacific. It is found that the nonlinear interaction between convection and the large scale circulation associated with the organization of SCC can excite a variety of symmetric divergent motions along the equator and rotational motions away from the equator. The former can be identified as eastward propagating moist Kelvin waves and westward propagating inertia-gravity waves and the latter as Rossby waves. The interaction between the SCC and a basic flow induced by a northern wintertime heat source distribution gives rise to quasi-stationary modes over the western Pacific which may be identified with mixed Rossby-gravity wave. Westerly vertical wind shear over the western Pacific modifies the vertical tilt of the MJO and favors its growth. During the northern winter, cold surges from the East Asian continent exert strong control on the development of SCC in the equatorial regions by inducing pressure differential across the maritime continent and the western Pacific, leading to enhancement of surface wind convergence. When the influence of the subsidence motion associated with a pre-existing SCC/MJO is weakened, the cold surge-induced pressure anomaly can lead to the development of new SCC with intermediate time scales. The time interval between intermediate SCCs is about 8-10 days which is approximately the transit time of the MJO across the span of the warm pool of the western Pacific. During the northern summer, the SCCs in the equatorial region are much less organized with mixed eastward and westward propagating signals. In the northern subtropics (15°N-20°N), westward propagating synoptic scale waves are found in regions with strong westerly shear. These waves grow by latent heating as well as by extracting wave energy from the mean westerly vertical shear in the lower troposphere. Overall, the results suggest that the organization and location of SCC and the associated multi-scale motions are strongly dependent on the evolving seasonal mean flow.
In the framework of the linear theory by using the Eady model, properties of optimal baroclinic perturbations which attain the maximum growth over a prescribed time interval (τ) are examined in connection with the skill forecast problem in numerical weather prediction (NWP). The development of the optimal perturbation depends not only on the linear stability of the basic flow, but also on the time interval τ to assess its growth. In the unstable region, the optimal perturbation grows almost exponentially in time as the unstable normal mode. Its structure gradually coincides with the unstable normal mode as τ becomes large. The corresponding optimal initial disturbance becomes the adjoint of the unstable mode. For smaller τ (τ≤3 days), however, the phase line of the optimal perturbation has a larger inclination than the unstable normal mode, which plays a negligible role in this early development. In the neutral region, the maximum amplitude of the optimal perturbation grows algebraically in time as Cτ2+1 (C is a constant). The optimal initial disturbance has a plane wave structure leaning against the shear. The phase line becomes more horizontal and the vertical scale decreases inversely with τ. On the other hand, the excited optimal perturbation has an almost barotropic structure, and is composed mainly of the two neutral non-singular normal modes with different phase speeds. The growth of the optimal perturbation is quantitatively well understood by the Orr mechanism for larger τ (τ≥2 days). For shorter τ, however, the interference between the two neutral non-singular normal modes has a major contribution to the development. The structure of the excited optimal perturbation is also explained by considering the projectability for each mode. This study suggests that a high resolution model which is comparable to the operational full NWP model is necessary to predict accurately the forecast skill of the NWP model.
We study the phenomenon of rapid cyclogenesis by examining the time evolution of the synoptic-scale environment for the 36-h period prior to most rapid intensification. We perform this study with the construction of composites for eight explosively-developing cyclones whose maximum intensification commences within a 5° latitude-longitude region in the vicinity of the Kuroshio Current. A comparison is made with corresponding composites of eight weakly-developing lows within the same region. The set of 16 cases is a comprehensive sample of events for nine cold seasons. Synoptic-scale features for the strong cases, distinct from those of the weaker cases, are detected throughout the 36-h dynamical conditioning period. The stronger cases form to the south and west of Japan with antecedent propagation predominately over the maritime environment. The weaker systems, in contrast, either form in situ in our study region, or travel eastward from the Sea of Japan over the Island of Honshu. The unique thermodynamic precursors of the stronger systems include: a warm 1000-500hPa thickness anomaly that travels from the East China Sea northeastward into the Kuroshio region, and a cold thickness anomaly and thermal jet east of our study area. The dynamical consequence of the warm anomaly includes the development of an anomalous southwesterly thermal wind to the northwest where incipient surface development occurs. We examine a representative strong case in which initial surface development occurs to the northwest of the warm anomaly in a zone of quasi-geostrophic forcing for ascent. The system subsequently propagates into a favorable location for further development downstream of a second 500hPa trough within the northwesterly polar jet. At the onset of explosive deepening, the cyclone is propagating towards a second region of anomalously cold air and strong baroclinity.
The trajectories of passive tracers in the global atmosphere were statistically analyzed with emphasis on the exchange paths crossing the boundary between the Northern and Southern Hemispheres. Twice-daily three-dimensional wind data for January and July 1989 from the European Centre for Medium-Range Weather Forecasts were used for the trajectory calculation. The statistics of the latitudes of tracers indicated that the partial barrier between the Northern and the Southern Hemispheric air masses was at 15°S and 5°S in January and at 10°N and 30°N in July. Tracers moving from the northern side of the boundary to the southern side and those moving in the opposite direction were selected with a criterion for the trajectory. The trajectories were classified into three types: one passing through only the lower part of the troposphere; one passing through only the upper part of the troposphere; and one going into the tropics from the lower troposphere, lifted to the upper troposphere in the ITCZ, and entering the other hemisphere. The trajectory from the summer to the winter hemisphere consists of an upper-only path and a lifted path, whereas the trajectory from the winter to the summer hemisphere consists of a upper-only path and a lower-only path. Although individual trajectories were sensitive to the methods of computation, a set of trajectories was insensitive to them. The regional characteristics of the trajectories were discussed by making a comparison among the three trajectories obtained from instantaneous wind fields, and from twice-daily, and monthly-averaged wind fields.
An investigation was conducted into the interference among the tidal wave, the 5-day, and 10-day normal-mode Rossby waves in the summer upper stratosphere, making use of a general circulation model. It was found that the EP flux due to the interference has a period of slightly longer than one day, and that two kinds of periodic structures of the EP flux also exist, having about 10-day periods. One structure is an amplitude modulation of the quasi-diurnal variation while the other is a long-term variation in the low-pass filtered data. It was also found that the acceleration of the zonal-mean zonal wind is mainly composed of the zonally symmetric diurnal tide. The EP flux due to the diurnal tide for zonal wavenumber 1 and the normal-mode Rossby waves of zonal wavenumber 1 plays a minor role in the acceleration of the zonal-mean zonal wind. When the Rossby waves simultaneously amplify, however, the EP flux brings about a noticeable long-term forcing of the zonal-mean zonal wind. The interference of the tide and normal-mode Rossby waves appearing in the NMC analysis data were also examined. The diurnal tide was analyzed as a quasi-stationary wave due to the once-daily sampling. It was found that the observed 5-day and 10-day variations in the EP flux are superficial and result from aliasing phenomena due to the once-daily sampling.
Lagrangian motion in a steady, baroclinic annulus wave is investigated numerically by following a tracer particle trajectory for a long period. Even for the regular Eulerian flow field of steady waves, the trajectory shows a chaotic nature, which is an example of “Lagrangian turbulence”. However, the chaotic trajectory has some organized structures depending on its position in the annulus. Based on the structure, the annulus of fluid is divided into the following regions: the upper-level and lower-level jets, the cyclonically and anticyclonically trapped regions, and the inner, outer and lower boundary layers. Some isolated regions in which the marked particle has never stayed for that period are also found inside the cyclonically trapped region and around the anticyclonically trapped region. Statistics over the long period show a preferred cyclic route of the region transitions: the outer boundary layer→the upper-level jet→the inner boundary layer→the lower-level jet or the lower boundary layer→the outer boundary layer. The number of dwell periods of the particle in the trapped regions is much smaller than that in the cyclic route, but the average time of one stay in the trapped regions is longer. A Lagrangian view of the heat transport in the steady annulus wave is obtained: The fluid particle absorbs a large amount of heat in the outer boundary layer and releases it in the inner boundary layer, while it nearly conserves its temperature in the interior regions. Inward heat transport is small in every one cycle of the meander of the jets.
Development of a meso-scale low family over the northeastern Japan Sea on 3-4 January 1987 is studied. The meso-scale low family formed in the northwestern part of a parent polar low. Local low-level frontogenesis occurred as a result of the horizontal differential thermal advection between polar air streams from the continent to the southwestern side of the parent polar low and warm air streams from the Pacific to the northwestern side of the polar low. A low-level shear line of WNW-ESE orientation formed in the frontogenesis zone. While upper-air observations showed a thermal gradient of ∼1.5K/100km and a lateral wind shear of ∼15(m/sec)/100km across the shear line, dense surface observations indicated a strong lateral shear of 5-10(m/sec)/10km within the shear zone of 20-30km width. Two meso-scale lows developed along the shear line with an interval of ∼200km. Each meso-scale low showed the warm core structure and brought strong gusts of ∼25m/sec with intense precipitation. While the generation of these lows in the strong shear zone suggested the barotropic process, the meso-scale cyclogenesis in association with the local frontogensis indicated the baroclinic generation process. It was examined whether the observed meteorological parameters would satisfy the criterion of barotropic shear or Eady's type baroclinic instability in the shallow layer obtained from linear instability analyses. It was shown that the maximum growth rates of these instabilities appeared around a wavelength of 200km, although the maximum growth rate of the baroclinic instability calculated from sparse aerological data was considerably smaller than that of the barotropic instability calculated from dense surface data.
Several ozone retrievals from DIAL (DIfferential Absorption Lidar) measurements have been proposed. Each retrieval method has its own definition of the range resolutions and therefore one cannot easily compare the ozone concentration errors resulting from the use of these different methods. In this paper, assuming the atmospheric model quoted from the U. S. Standard Atmosphere 1976 and the system constants of our lidar system, we numerically compared the differences between some representative DIAL methods by means of estimating the error caused by smoothing. Initially we defined a common range resolution as a scale. Using the common scale, we focused on the distortions arising from smoothing and the uncertainty of statistical signal errors. These estimates revealed that some algorithms reduce statistical error but make substantial distortions at higher altitude. It was possible to distinguish for each algorithm both the statistical errors and the systematic errors.
In this note, the analytical model of Smith, which describes steady 2-dimensional downslope windstorms in uniform environmental flows with constant density, is extended into a sheared environmental flow having density decrease with height. Both the potential temperature and a vorticity-like quantity, which are conserved along the streamlines, are assumed to be linear functions of the stream function. By this assumption, the model becomes analytically tractable in spite of both the shear and density decrease of the environmental flow. The result shows that the vertical profiles of environmental horizontal velocity and potential temperature have a great influence on the downslope windstorms as well as do their total differences between the upper and lower boundaries.
Amplification of disturbances superimposed on a uniform basic straining flow is analytically investigated in a linearized barotropic dissipative system on an f-plane. The dissipation is assumed to be of a diffusive type. The temporal evolution of disturbance energy, which initially is imparted isotropically at every wave number, is examined. The results obtained are the following. (1) For a pure rotation, the straining flow does not affect the disturbance energy. (2) In the near future, the disturbance energy is amplified by the deformation, and the rotation does not partake it. (3) In the distant future, amplification of the disturbance energy by the straining flow also occurs, which depends on the rotation as well as the deformation. The behaviour crucially depends on which component, deformation or rotation, dominates.