A simple moist model useful to understand some basic properties relevant to the origin of the intraseasonal disturbances in the tropics is constructed by use of the linear shallow-water equations on a sphere. The dynamical equations are supplemented by the moisture equation as proposed by Gill (1982). The mutual interaction of equatorial dynamics and convective activity is demonstrated by releasing localized, initial anomalies for either temperature or velocity. In particular, it is found that there are two intrinsic ways of large-scale moist adjustment processes in the tropics. Firstly, it is shown that the active heating region associated with the convergence of low-level winds propagates eastward spontaneously with expanding the zonal scale of the velocity field. The phase speed of this organized pattern is much smaller than that of the corresponding free Kelvin wave because the buoyancy effects are much reduced by the convergence-dependent heating. These results are consistent with the recent observational studies of the 30-50 day oscillation in the tropics. Secondly, the present model also demonstrates that the burst of westerly winds may excite a westward propagating cross-equatorial cyclone pair more easily than the burst of easterly winds. This vortex pair is mainly composed of the gravest Rossby waves with moist processes and is quite similar to the long-lived synoptic cyclone pair often observed over the warm SST region in the tropics.
Asian summer monsoon simulated by a 5-layer tropospheric general circulation model (MRI•GCM-I) is described. Gross features of the July circulations are well simulated. Although the detailed rainfall distribution in the monsoon region, e.g. a maximum over northeastern India, is not well reproduced, the time evolution of the total diabatic heating distribution over the entire monsoon region was reasonably simulated. One month period from mid-April is a transition period in the model from the northern winter regime to the summer regime. A reverse transition occurs between September and October. It is pointed out that both the transitions do not occur smoothly, but rather rapidly, and that circulation changes associated with the monsoon onset are not confined to the monsoon region but extend globally. The present simplified treatment of the land surface processes made the model overestimate the latent heat flux over the Tibetan Plateau compared to the observed one (e.g., Luo and Yanai, 1985). It is found out that reduction of the field capacity of the model ground water over the Tibetan Plateau tends to improve surface energy balances there. Quasi-periodic oscillations are found in the model monsoon system. Time evolution of them is quite similar to the one described by Krishnamurti and Bhalme (1976), although the simulated oscillation period is not quasi-biweekly but 8-9 days. The oscillation period seems to be influenced by dominant quasi-10 day period of Kelvin waves found in the model tropics. As a result, the timing of the monsoon onset coincides with one of the favorable timings for the intensification of the upper level easterlies over the equatorial Indian Ocean provided by the Kelvin wave. Preceding the monsoon onset, a long-lasting westward moving disturbance was formed in the model lower troposphere in the south of the equator over the Indian Ocean. Further studies are needed to confirm the reality of this disturbance and the role of it on the monsoon onset.
An Observing System Experiment was carried out using the JMA global forecast/analysis system and the FGGE Main Level II-b data sets. Parallel sets of analyses and 5-day forecasts were produced for the three observing system configurations and the impact of the space based observing system was statistically and synoptically evaluated. Space based data are indispensable for the analyses in low latitude and the southern hemisphere. Even in the northern hemisphere, satellite data improve the analysis over the Pacific region and thereby improve the forecast over North America downstream of that region. The impact on the analysis at a certain time level appears as the sum of the impact of data existing at that time and the impact of data from previous time levels assimilated through the analysis and forecast cycles. On the other hand, satellite data have little impact on the forecast around East Asia because sonde data basically determine the quality of the analysis upstream of that region.
Disturgances of intermediate time-scale (10-30 days) were studied by use of twice daily data of geopotential heights for 11 years from 1969 to 1979. The disturbances with an amplitude of height of more than 150m were analyzed. High and low pressure systems with durations of longer than 7 days and sizes of larger than about 1, 000km in diameter were selected for the analysis. The number of them was fifty. Most of them occur in the two regions of the North Pacific and the North Atlantic, and migrate eastward, southward and westward. Their three dimensional structures show that baroclinic energy conversion from potential to kinetic energy plays a significant role in their life span.
An observational study is made on the relationship between the development of the subtropical high around Japan and the convective activity in the northwestern tropical Pacific, and their association with wave propagations over the North Pacific during August 1984. It was found that several occasions of intensified convection around the north of the Philippines were immediately accompanied with the development of the barotropic high around Japan, and both of them were followed by eastward propagations of intensified geopotential height anomalies, at a speed of 10-12m/s, from the north of the Philippines (negative height anomaly) through over Japan (positive height anomaly) extending to the west coast of North America. These anomalies were associated with troughs and ridges of zonal wavenumber 6-7 over the North Pacific on the montly mean 500mb height field of August 1984. A comparison was made between the .monthly mean 500mb height anomaly field and propagation of wave packet simulated by a linear barotropic model with a forcing around the north of the Philippines. Both of them showed quite similar patterns, suggesting that the development of the barotropic high around Japan was associated with energy propagations of barotropic Rossby wave excited by intensified convections around the north of the Philippines.
We study the looping motion of tropical cyclone by a numerical simulation for the case of no steering current in order to investigate the relation between the asymmetry and the motion of model vortex. The dynamically balanced vortex is used initially to avoid an initial unrealistic motion of the vortex. The mean winds in each pressure level rotate counterclockwise in the developing stage in both f-and β-plane cases. As the mean wind is produced by the asymmetric distribution of winds, the rotation of the mean wind is considered as an inertial wave propagating on the eyewall. We apply the linear wave theory to a modified Rankine vortex which has an internal boundary in no gravity field. The angular velocity of this inertial wave is estimated approximately at a half of the vorticity in the outer region of the eyewall. If the distribution of azimuthal wind is in proportion to 1/r approximately, the looping motion will be absent as the vorticity becomes zero in the outer area of the eyewall. The looping motion is commonly cyclonic, because the vorticity near the eyewall is positive due to the surface friction and the lateral mixing of momentum.
The annual deposition of Sr-90, Cs-137, Pu-239, 240, and Pu-238 were observed from 1959 to 1984 at the Meteorological Research Institute (MRI). In order to interpret the serial trends of the annual radioactive deposition at the MRI, a semi-empirical box model of atmospheric transport was developed. The model divides the atmosphere of the Northern Hemisphere into four compartments: the atmosphere above 21km, stratosphere below 21km, active mixing and exchange (AME) layer near the tropopause, and the troposphere. The transfer between the compartments follows the firstorder kinetics. The half residence times for transfer between upper and lower stratospheric compartments, between the lower and AME layer compartments, and between the AME layer and troposphere are 0.5, 0.7 and 0.3yrs, respectively. It is revealed that as a long-term monitoring of the annual deposition of radioactive debris in the mid-latitude area, the model quantitatively permits the calculation of stratospheric inventories and trends of annual deposition of debris injected into the stratosphere which are characterized by apparent residence times of 0.5 to 1.7yrs. This simple model is useful to predict the annual deposition amount of radioactive debris from the thermonuclear explosion for practical purposes.
The concentration of atmospheric CO2 was measured continuously, 300m offshore of the main island of Okinawa, Japan for the period from February, 1983 to February, 1985. A diurnal variation of CO2 concentration was found during the warm months of March through October, with an enhanced magnitude in the summer season, strongly depending on the atmospheric stability. The CO2 concentration, obtained by applying a statistical data selection scheme, showed that the seasonal CO2 variation reaches a maximum in late April and a minimum in mid-September with the peak-to-peak amplitude of 7.2ppmv. During these measurements, the rate of secular CO2 increase was about 1.6 ppmv/year. These results are consistent with those of our aircraft measurements in the lower troposphere over Okinawa for the same period.
A model based on single particle diffusion is introduced to evaluate the magnitude of "meandering" in plume dispersion where the instantaneous plume meanders around its mean axis. It is assumed that the plume can be regarded as a series of puffs, each of which will move as a whole along a trajectory of its own circular arc in the mean flow. Then a dispersion equation of puff centers is obtained to evaluate the magnitude of the variance of puff center positions; the variance is regarded as a measure of plume meandering. The dispersion equation was successfully tested against the observation performed under the condition that the mean wind velocity was 8.8m/s and the sampling duration was 3 minutes. The smoke puffs were observed over about 500m downwind distances from a 200m high chimney and the mean arc radius of their trajectories was 55m and the mean angular velocity was 0.033rad/s.