As a step toward an understanding of the mechanism of super cloud cluster (SCC), the effect of surface friction on the instability of Kelvin wave is discussed, together with the condition of the vertical profile of parameterized convective heating. In this paper (Part I), linear theory is developed with an equatorial beta-plane model in which the amount of convective heating is assumed to be proportional to the vertical velocity at the top of the boundary layer. Results from the eigenvalue problem suggest that the effect of surface friction is, at least under such parameterization, indispensable for essential understanding of SCC; this study is along the same line as Wang (1988) with respect to the importance of surface friction. Issue is focused on three unstable modes. One is a stationary mode excited with much heating at lower levels, and identified with conditional instability of the first kind (CIFK). The other two are Kelvin modes that contrast with each other; one is destabilized by enough heating at upper levels, even without surface friction (referred to as NFK: Non-Frictional Kelvin mode), and the other is destabilized by moderate heating at lower levels under the control of surface friction (FK: Frictional Kelvin mode). NFK has phase velocities of more than 10ms-1, and its growth rates are on the order of 10-3s-1 at small wavelengths. On the contrary, FK is characterized by smaller phase velocities of less than 10ms-1, and by only slight dependence of its growth rates on wavelength. In addition, the convergence in the boundary layer exhibits a phase shift slightly eastward relative to the convergence aloft; this is the key feature that induces effective feedback between convection and the wave. In view of the large phase velocity and the small preferred scale, NFK may be interpreted as a Kelvin mode of the type that has been treated in many previous studies on SCC. Difficulties in explaining the observed phase velocity and preferred scale of SCC are inherent in NFK. By contrast, FK can give a realistic phase velocity and preferred scale, and clear up major difficulties in the previous studies. Inclusion of surface friction is expected not only to serve as the missing link of the past linear theories of SCC, but also as a basis of future non-linear experiments with explicit moisture processes.
The summer rain shower which develop in central Japan under otherwise fair-weather conditions during the summer of 1985 were investigated, making use of routine observational data from weather stations. According to the statistical analysis, the diurnal cycle of precipitation exhibited a distinct peak around 1800 LST, while little rainfall occurred from 0000 to 1200 LST. This distinct peak was associated with the summer rain shower developed over the heated land surface during the afternoon. The rain shower activity increased as the atmospheric static stability for dry (or moist) convection decreased, under conditions that the atmospheric precipitable water exceeded 40mm. The rain shower was concentrated in the mountainous regions which consist of several mountain ranges having the horizontal scale of about 100km. The areas where the rain shower was concentrated largely did not move with time. The degree of spatial concentration of the rain showers however was weaker under higher activity conditions of the rain showers. A thermally induced local circulation developed over central Japan during the daytime under fair weather and weak synoptic wind conditions from the spring to summer seasons. This circulation was strongly dependent on the topography, and converged in the mountainous regions. According to the previous study, the daytime thermally induced circulation contributes to an increase in the water vapor content over the mountainous regions through the moist air advection from the plain and basin regions, and the greatest amounts of water vapor are accumulated over the mountainous regions in the late afternoon when the horizontal scale of topography is close to 100km. Specific humidity measured in the mountainous area displayed an afternoon maximum, exceeding that at the basin bottom in the afternoon. The increase in water vapor over the mountainous regions is expected to contribute to the development of cumulus clouds, which was confirmed by decreased sunshine duration during the late afternoon over the mountainous regions. These results suggest that summer rain showers are triggered over the mountainous regions by the thermally-induced local circulation.
The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere has been well simulated in the first version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (Takahashi, 1996). The model horizontal resolution is T21 with 60 layers of vertical resolution of about 500m in the upper troposphere and lower stratosphere, with a moist convective adjustment scheme. The period of the simulated oscillation is 1.5 years, which is slightly shorter than that observed. The equatorial wave behavior in the model is examined using the spectral analysis method. The westerly acceleration phase of the QBO-like oscillation in the model is due to Kelvin waves and eastward-propagating gravity waves. The easterly acceleration phase of the oscillation is due to westward-propagating n=1 equatorial gravity waves, random westward-propagating gravity waves and Rossby waves propagating from the mid-latitudes in the northern winter hemisphere to the equatorial region. The Rossby-gravity wave has a role in the easterly acceleration of the QBO-like oscillation in the present general circulation model.
Balloon, lidar and satellite observations indicate that the concentration of stratospheric background sulfate aerosol is gradually and globally increasing due to anthropogenic and/or natural causes. By considering the above fact, this study investigates the extent to which the background aerosol increase can change the stratospheric temperature through the perturbation in radiative heating alone. The temperature change is evaluated with a seasonally-marched, fixed dynamical heating model, which prescribes tropospheric conditions and stratospheric dynamical heating. Radiative perturbation and hence temperature perturbation respond linearly to doubled and tripled background levels, the stratospheric loads of which are 0.0087 and 0.0174, respectively, at 0.55μm. The doubled background level yields radiative forcings of about -0.18, 0.03 and -0.15Wm-2 for solar, terrestrial and net radiation, respectively. For the tripled background level, the middle and lower stratosphere in low latitudes warms by about 0.15K due primarily to the perturbation of terrestrial radiation with very weak annual oscillation. In high latitudes, on the other hand, the middle and lower stratosphere cools by about 0.15K due primarily to the perturbation of solar radiation with pronounced semiannual oscillation (maxima around solstice and minima around equinox) of about 0.1K. The causes of the latitudinal difference and the interhemispheric difference are described in relation to the background temperature and its seasonal march.
Basic dynamics on temporal variations of the atmospheric predictability is investigated both with conceptual models of one- and two-dimensional dynamical systems and with a simplified atmospheric circulation model introduced by Legras and Ghil (1985). As a measure of the predictability, we use the Lorenz index α that gives an ensemble average of the error growth rate for a prescribed time interval (Lorenz, 1965). We try to find the relation between the predictability variation and quasi-stationary (QS) states, which occur when the trajectory of the solution passes near a local minimum point (MP), which is either an unstable stationary point (US) or a non-stationary local minimum point (MIN), in phase space. At a MIN the speed of the trajectory has a local minimum value in phase space (Mukougawa, 1988). In any one-dimensional dynamical system there is a unique relation that α increases monotonically during QS states. In multi-dimensional dynamical systems, on the other hand, there is not such a relation between α and the QS states. During QS states related to a US, it is possible that α varies in more than one manner; α increases monotonically, decreases monotonically, has a maximum, or has a minimum, depending on the trajectory. During QS states related to a MIN, on the assumption that the trajectory exists close enough to the MIN, α shows one of the four relations mentioned above depending on the property of each MIN. If we consider trajectories only on the attractor, every QS state related to MP has its own tendency in the variation of α, which is one of the four relations. The same relation as in one-dimensional dynamical systems is found in some chaotic solutions in the Legras and Ghil model, although it seems to be just one of the four possibilities.
We carried out a three-week observation campaign using the MU radar and meteorological (C-, X-, Ku-band) radars during the Baiu season in 1991 (17 June-8 July). A complete data set of three-dimensional atmospheric motions with high reliability and resolution was produced after removing raindrop echoes. The Baiu front was located to the south of the radar site during 17-24 June, and to the north during 25-28 June. After 29 June, surface medium-scale (meso-α-scale) cyclone centers passed near the MU observatory, and the tropopause jet stream became strong (one or two days) after the low-level jet stream appeared near the surface cyclone center. We have investigated meso-β-scale vertical velocity fluctuations and precipitating cloud clusters with temporal scales of several hours for various locations relative to the surface cyclone centers: (i) one case in the northern side of a cyclone center on the Baiu front, (ii) four cases near developing surface cyclone centers and (iii) one case in the southern side of the Baiu front far from a cyclone center. For (i) and (ii), the vertical distributions of upward-velocity regions were strongly dependent on the level of lowest stratiform turbulence near the tropopause level (LSTT) and the frontal surface. The upward velocities in the cases (ii) were associated with the developing cloud cluster on the northern side of the surface warm front, while they were dominant over the region of meso-β scale but did not always have precipitation on the northern side of the surface cold front. For (iii) some upward-velocity regions without precipitation penetrated LSTT. Each meso-β-scale fluctuation for all the cases (i)-(iii) included a number of upward-velocity peaks corresponding to meso-γ-scale disturbances. Some of them were coincident with surface rainfall (and lower-tropospheric precipitation echo) peaks. Based on all the observational evidence mentioned above, we propose a schematic picture of the ‘hierarchical structure’ of the vertical velocity fluctuations near the Baiu front. This is partly the same as the well-known multi-scale structure composed of meso-α-scale cyclones, meso-β-scale cloud clusters and meso-γ-scale precipitating clouds, but covers more broad clear regions which cannot be observed by foregoing studies based on only meteorological radars and satellites.
A physical initialization method for the economical prognostic Arakawa-Schubert scheme (EPAS) is developed to incorporate observed precipitation data into a numerical weather prediction model. The method adopted is a variational approach which minimizes the difference between the first guess and the initialized model variables, subject to strong constraints on precipitation areas and precipitation rates. The physical initialization method is divided into 2 parts: (1) Adjustment of the initial thermodynamic variables such that the model precipitation areas diagnosed from the adjusted variables become consistent with the observed precipitation areas at the initial time. (2) Adjustment of the initial cloud-base mass flux (Mb) in such a way that the model precipitation rates are equal to the observed precipitation rates. In order to examine the impact of the physical initialization method, forecast experiments for a case study of Typhoon WALT (T9407) were performed. The results of the forecast experiments indicate that the physical initialization method eliminated the spin-up error of precipitation forecast for the first one hour, and reduced the positional error of precipitation forecasts for the first few hours. The results suggest that the adjustment of the initial cloud base mass flux was essential for eliminating the spin-up error. It is also found that the adjustment of the initial cloud base mass flux contributed to reduce the positional error by strengthening the model convective precipitation around the observed heavy-rain area, and that the adjustment of the initial thermodynamic variables reduced the positional error by removing extreme moist-convective instability in the observed rain-free areas.
A meso-β-scale convective system (MCS) passed over the Tokai District of Japan on 8 Sep., 1994. Only one multicell thunderstorm (Storm C) exceeded the tropopause level in the MCS. This paper presents the evolution of the kinematic and radar echo structures of Storm C observed by the dual-Doppler radar system of Nagoya University. Storm C lasted more than 2 hours and its elongated axis was normal to the direction of mean cell motion. The early developing stage of Storm C was characterized by a downdraft on the left flank (left flank downdraft or LFD) and the development of cells on the right flank above the outflow of the LFD. A mid-level rear inflow descending within the storm and a new downdraft on the rear flank (rear flank downdraft or RFD) were observed in the explosively developing stage. The mid-level rear inflow was accelerated and localized in the vicinity of an anticyclonic vortex developed in a nearby decaying storm. Cells developed strongly above the merged outflows of the LFD, the RFD and the descending mid-level rear inflow. In the decaying stage, the intensified RFD and mid-level rear flow divided the main updraft region on the right flank into two and their outflows prevented the outflow of the LFD from spreading to the right flank. On the basis of the observational results, the effect of the vortex on the behavior of the rear inflow as well as the influence of the rear inflow on the evolution of Storm C were discussed.