The optimization of an algorithm is presented, for the estimation of rainfall over the oceans. The empirical algorithm previously developed was based on a thorough statistical analysis of SMMR and SSM/I data, as well as on radar data obtained during the GATE experiment. Since some results seemed overestimated, we performed a critical revision of the derivation of the algorithm, and calculated new coefficients. This modified algorithm gave satisfactory results in the comparison between the SSM/I derived rainfall and the corresponding values obtained by radar's in the framework of the GPCP program.
Distinct medium-scale, eastward-travelling waves in the middle and upper troposphere have been detected by using both the MU radar wind data with fine time resolution and the data provided by the Japan Spectral Model (JSM) with fine horizontal resolution. The medium-scale waves appear to exist ubiquitously in a latitudinal band of 30-40°N during the spring season, when travelling long waves with a period of 3-4 days are observed. Through a detailed analysis with spectral and correlation methods, some of the wave characteristics are studied. The maximum amplitude of 4 ms-1 for the geopotential-height wave component is observed at 250 hPa. The meridional and vertical phase tilts are small. The phase differences of the geopotential height with the horizontal wind and temperature are geostrophic and hydrostatic, respectively. The zonal wavelength and wave period are about 2100 km and 26 h, respectively. The zonal phase velocity of 22 ms-1 is about twice as large as that of the long waves, indicating that the medium-scale waves are not due to the higher harmonics of the long waves travelling together but due to a unique mechanism of generation.
A modified version of the typhoon bogus developed at the Japan Meteorological Agency has been installed in the Australian Bureau of Meteorology's tropical prediction system. The construction of the bogus vortex is described and the impact on forecasts during several tropical cyclone events is illustrated. Improvements in prediction are obtained in nearly all cases. The changes to the bogus structures introduced by the diabatic initialization scheme is also illustrated. It is shown that the method of initialization can have a large effect on the imposed structure and can affect the quality of prediction of both the tropical cyclone itself and the large scale flows quite remote from the storm center. It is suggested that without careful initialization occasional erratic behaviour can result from mass-wind imbalance, inconsistency between the imposed vortex structure and the grid resolution, and rejection of the vortex by the forecast model. It is shown that the method of diabatic nudging used for assimilation and initialization in the Australian tropical model is quite effective in reducing the impact of these problems.
A meso-β-scale kinetic energy (KE) budget of a midlatitude mesoscale convective system (MCS) and its effects on larger-scale flow are investigated using a 21-h high-resolution real-data simulation of an intense squall system that occurred during 10-11 June 1985 PRE-STORM. It is found that the ageostrophic cross-contour generation in the front-to-rear (FTR) ascending flow provides a major source of KE to the squall system, whereas the horizontal flux divergence is a primary sink. The vertical flux divergence is a KE sink (source) in the FTR ascending flow below (above) an upper-level jet, but it is always a source in the rear-to-front (RTF) descending flow. Due to the downward KE transport in the RTF flow, the presence of the jet stream containing most of its KE along the line appears to affect the intensity of the squall system and surface gust fronts several hundreds kilometers away to the front and to the southwest of the system. The effects of moist convection on atmospheric motion of all resolvable scales are examined in a wavenumber space. With deep convection incorporated, the model produces little changes in the KE spectrum for smaller wavelengths but significant alterations in both the spectral magnitude and slope for longer wavelengths. The convectively generated alterations appear to be a consequence of the intermittent development of the MCS, rather than reverse KE cascade.
Some of the rotating annulus experiments with radial differential heating show stepwise transitions of flow regimes from steady axi-symmetric flow to vacillation via a steady wave regime as the rotation rate increases. The stepwise regime transitions are investigated numerically with a semi-spectral model of a three-dimensional Boussinesq fluid, and the results are interpreted with bifurcation theories. The transition between axi-symmetric flow and a steady wave regime is characterized by hysteresis ; the criterion for the disappearance of an established steady wave differs from the criterion for the onset of the steady wave. The branch of the steady wave solution does not bifurcate from that of the axi-symmetric flow at the point where the axi-symmetric flow becomes unstable. Instead, the steady wave branch has another type of critical point (interpreted as a "limit point" ) at which it disappears. The transition from steady wave to tilted-trough vacillation is interpreted as a Hopf bifurcation ; a periodically fluctuating solution bifurcates from the steady wave branch when the steady wave solution loses its stability.
To understand various development mechanisms of disturbances in the Eady problem, the quasigeostrophic equation system is extended to include the horizontal boundary conditions. First, we distinguish continuous-spectrum waves from the modal waves by using potential vorticity. Next, we derive the "wave momentum" and clarify its relationship with the Eliassen-Palm flux. Finally, the necessary condition for the development of waves in the baroclinic flow is expressed by evaluating the EP-flux divergence. In Farrell (1984) or Thorncroft and Hoskins (1990), shallow vortices develop algebraically, as the result of wave-wave interaction of a boundary modal wave and internal continuous-spectrum waves. These interactions are clearly indicated by the EP-flux convergence around the critical level.
Aircraft observation of aerosol particles was carried out at an altitude of approximately 4.5 km from 0°-34°N during 7-10 March 1989. Marked spatial variation in the number concentrations of aerosol particles with radii between 0. 15 and 5 μm was observed in the middle troposphere, with concentrations ranging from 0. 1 to 10cm-3. Sulfate-containing particles (both H2SO4 and (NH4)2SO4) were present in the observation regions as an important background aerosol. However, high concentrations of particles were observed intermittently in the tropical atmosphere in association with the vertical transport of sea-salt particles through convective clouds. Asian dust-storm particles also contributed to high concentrations of particles with radii r &ge ; 1 μm at mid-latitudes. The present study suggests that the transport of sea-salt particles and mineral particles contribute strongly to increases in the number concentration of aerosol particles in the middle troposphere. Modification of sea-salt particles collected over the tropical region is also discussed in this paper.
In order to investigate effects of physical processes of H2O on the general circulation of the atmosphere, a general circulation model which excluded all physical processes of H2O was constructed. A great difference of the general circulation between without all physical processes of H2O and with them is obtained. For the case that the surface albedo is assumed to be 0.1 everywhere, the global mean surface temperature without them is 279K, while the temperature with them is 289K. The pole-to-equator temperature difference near the ground surface without them is about l00K, while the difference with them is about 40K. And the vertical and meridional temperature profiles without them are also quite different from the profiles with them. The maximum zonally averaged zonal wind without them is located on 60-70°latitude, while the wind with them is located on 30-50°latitude. The horizontal width of the direct meridional circulation without them, which is well known as the Hadley circulation, is almost the same as that with them. However, the intensity of the direct circulation without them is much weaker than that with them. The activity of eddies, and the intensity and the horizontal width of the indirect meridional circulation without them are also quite different from those with them. The differences of heat balances between without them and with them at the ground surface and in the atmosphere are investigated and discussed in detail. Effects of the radiation process by H2O, and effects of evaporation, precipitation and the advection of water vapor on heat balances are discussed.
We have carried out the numerical weather prediction experiments with JMA's operational global spectral model for the Australian Summer Monsoon onset during AMEX phase II period. With the moisture data derived from GMS TBB data for the initial condition, the onset forecast is remarkably improved.