We study the basic features of linear stationary response of a global atmosphere to tropical heating in connection with effects of planetary rotation. For this purpose, with the linearized shallow water equations on a sphere, we calculate the horizontal structure of circulations for a wide range of the parameter ε(0_??_ε_??_10000), the square of the ratio of the rotational velocity of the planet to the velcoity of the gravity wave. First, we examine responses to tropical heating which is concentrated in an area of finite extent and symmetric about the equator. When ε is small, the induced circulation is simply a direct circulation. An increase of ε makes the circulation confined to the equatorial region and intensifies the westerly inflow in the west of the heating region. The origin of the east-west asymmetry in the circulation is discussed in the light of vorticity balance. For ε_??_100, the circulation is mainly composed of Kelvin and Rossby waves. Gill's solution (1980) obtained in the simplified model is similar to ours for ε=100 or 1000. Second, the case that the tropical heating is antisymmetric about the equator is examined. For small ε a direct flow crossing the equator predominates. When ε becomes large, the flow meanders in the west of the heating region. For ε=10000, the flow crossing the equator becomes negligible and the two closed circulations are formed separately in each hemisphere. For ε=10, we calculate circulations for some values of Rayleigh friction and Newtonian cooling rate; the results are sensitive to this value. We discuss the effects of dissipation processes on steady balance in the light of equations of motion and thermodynamics. We examine also the response to heating which is spread over the sphere.
The propagation and structure of the tropical intraseasonal oscillation are studied by the use of a three-dimensional linear response model and compared with those obtained from the FGGE data. It is assumed that the imposed thermal forcing oscillates with a 40 day period and propagates eastward. Although the amplitude of the forcing is assumed to be large over a certain longitude band and small elsewhere, the responding zonal wind oscillation has significant components that propagate eastward around the earth as observed. This oscillation is also associated with an observed longitudinal node in the region of the maximum forcing. When the imposed forcing is strictly confined over some longitudes, the zonal wind oscillation propagates eastward and westward away from the forced region as in the case of a two-dimensional model. The eastward moving wavenumber 1 component is associated with the observed wave pattern of the combined Kelvin-Rossby mode in the upper troposphere, while it is dominated by that of the Rossby mode in the lower troposphere. This component also takes the observed structure of the Walker cell modified by a frictional meridional convergence in the boundary layer. The dominance of the Rossby mode in the lower troposphere is due to the effect of surface friction.
In order to investigate the behavior of an ensemble of cloud clusters over a tropical ocean with the zonally uniform SST surrounded by the dry continent, numerical experiments have been conducted using a global spectral model. The long time-integrations were performed under the perpetual April SST conditions corresponding to the Northern Hemisphere spring to maintain the north-south symmetry of SST. The following results have been obtained; (1) On the equator cloud clusters tend to be located at the eastern end of moist region over the ocean (3000km westward from the eastern boundary). Thus, the location of cloud clusters on the equator is determined by the mutal interaction between cloud clusters and moisture fields. At the eastern boundary of the ocean, evaporation from the ocean is balanced with the westward advection of dry air. At the western boundary, meridional moisture transport also contributes to the moisture budget. (2) In the subtropics cloud clusters tend to be located in the western part of the ocean. Time-average maximum of the precipitation exists 3000km-6000km off the western coast of the ocean. This is due to the fact that the moisture flux convergence is enhanced around the border between the ocean and the dry continent with no moisture supply. It implies that the dry continent influences the activity of cloud clusters over the ocean.
A prediction experiment is performed with a very-fine-mesh (42km at 60°N) primitive equation model for a heavy snowfall event in the Hokuriku District associated with a convergent cloud band over the Japan Sea. Using the result of the experiment, together with observed data including special soundings, we elucidate the structure of the convergent cloud band with a line of active convection on its southwestern edge. The atmosphere around the cloud band has the following characteristic features: - a low-level convergence, middle-level divergence zone accompanied by intense positive vorticity along the line of active convection, - a warm and weak-wind zone along the line of active convection embedded in cold airmass - a weak stable layer on the northeastern side of the line of active convection as a boundary between two layers: a lower layer with northerly cold air flow toward the line of active convection and an upper layer with southwesterly warm air flow which has been heated in the line of active convection, - a west-northwesterly air flow on the southwestern side of the line of active convection with weak vertical wind shear and nearly neutral stratification up to the top of the cold airmass, - a strong-wind zone at the divergent level (700-6OOmb) 300-500km northeast of the line of active convection. On the basis of cross-section and trajectory analyses, we can make a clear picture of the atmosphere around the cloud band. Heat and water vapor budget analysis indicates that over the southern Japan Sea the mesoscale thermal structure around the cloud band is largely maintained by localized release of latent heat in large-scale cold air advection field.
Forecast experiments on a long-lived meso-α-scale convective system (MaCS) in the Baiu frontal zone were made by using a 13-level 64km-mesh primitive equation model. This MaCS passed over the East China Sea and Kyushu (western part of Japan) on 14 July 1979 and developed into a Baiu frontal depression when it moved over the Pacific coast of Japan on 15 July 1979. Three 24-hour forecast experiments (Exp. CNT: control experiment, Exp. SMM: experiment from the initial condition of small specific humidity in the lower troposphere, and Exp. HET: experiment with the prescribed condensation heating in the first 1-hour of the time integration) were made from the initial state at 09LST 14 July 1979. The result of Exp. CNT showed slow spin-up of the MαCS in the early few hours. The spin-up of the MαCS in Exp. SMM was significantly slower than that in Exp. CNT. Exp. HET indicated improvement of the MαCS's spin-up. The forced condensation heating in the first 1-hour had strong influence throughout the whole integration period. The timely spin-up of the MαCS caused by the forced heating in the model resulted in the timely change of circulations around the MαCS, and this change contributed to the further development of the MαCS and the associated Baiu frontal depression.
The linear theory of land and sea breeze circulation (LSBC) shows that, in the absence of the Coriolis force and under the hydrostatic approximation, there exists a similarity solution. In this solution, the horizontal coordinate is scaled by Nκ1/ω*-3/2, the vertical coordinate by κ1/2ω*-l/2 the horizontal velocity by gαΔT/N, the vertical velocity by gαΔT•ω*/N2 and the pressure by gαΔTκ1/2ω*-1/2, respectively, where ω* and ΔT are the frequency and amplitude of the temperature variation at the ground, respectively, N the Brunt-Vaisali frequency corresponding to the basic density stratification, κ the eddy thermal diffusivity, g the gravity acceleration and α the thermal expansion coefficient. The eddy Prandtl number is assumed to be unity. In the immediate neighborhood of the coastline, a small region in which non-hydrostatic effects are significant and the similarity solution is invalid is present. The horizontal and vertical dimensions of the non-hydrostatic region are of the order of (κ/N)1/2 and the vertical velocity becomes of the same order of the horizontal one in this region. Outside of the region, however, the similarity solution remains always valid. When the Coriolis force is present, the solution outside of the non-hydrostatic region depends only on the non-dimensional Coriolis parameter f defined by f*/ω*. If the horizontal dimension λ* of LSBC is defined by the distance from the coastline at which the non-dimensional velocity of the onshore wind becomes equal to 0.03, λ* is given by λ*=Nκ1/2ω*-3/2F(f), where F(f) is a universal function off. F remains almost constant (about 2.1) for f<1(latitude less than 30°). When f becomes larger than 1, however, F starts to decrease rapidly and becomes equal to 0.9 for f=2.0 (at the Arctic or the Antarctic). Effects of the eddy Prandtl number and the non-linear process on the flow characteristics are also discussed.
Weekday-weekend differences of air temperature and other meteorological parameters in the central part of Tokyo are described on the basis of data for 25 years. Temperature is lower on Sundays than on weekdays by about 0.2°C in the daytime and about 0.1°C in the nighttime on the 25year average. The magnitude of the weekday-weekend temperature difference depends on seasons and meteorological conditions, and has increased during the 25 years. In recent years temperature is lower also on Saturday evening. In the daytime of Sundays pressure is higher by about 0.05mb than on weekdays. This fact indicates that the weekday-Sunday difference of temperature occurs over a depth of several hundred meters above the surface. In the nighttime no pressure difference is found between weekdays and Sundays, hence the temperature difference is likely to be confined in a layer much shallower than in the daytime. Weekday-weekend differences are also found for wind speed, relative humidity, and solar radiation.
A new method of estimating vertically-integrated liquid-water amount and identifying precipitating clouds by Nimbus 7 SMMR data was presented on the basis of calculations of microwave radiative transfer. Comparison with other methods showed that the new method gives reasonable estimates for both precipitating and non-precipitating clouds. The method was applied to clouds in the western North-Pacific region in February of 1980, and some characteristics of integrated liquidwater amount, the relationship between averaged integrated liquid-water amount and cloud amount in each latitudinal zone, etc. were studied.
A method to detect cloud cover in the Antarctic using only the infrared channels of AVHRR is discussed. From the data of NOAA-7 received at Syowa Station, the difference in the brightness temperature of each channel appeared to be useful for the identification of clouds. The brightness temperature of the channels 3 (3.7μm) and 4 (11μm) shows the positive difference when the thickness of clouds are in some particular range, and then tends to show negative difference for the thick cloud. Thin clouds have the difference in the brightness temperature between channels 4 and 5 (12 μm). These tendencies are explained by the radiative properties of model clouds theoretically calculated. On the graph of these temperature difference against the channel 4 brightness temperature, pixels of the same cloud distribute on the particular arch starting from the clear pixel. From the arch, clouds can be distinguished from the ground surface. The particle size, temperature and thickness of the cloud can also be inferred. At the low temperature over the inland snow surface, many troubles arise. The channel 3 brightness temperature accompanies poor resolution and large noise at the low temperature. The brightness temperature difference between channels 4 and 5 shows strong dependence on temperature and viewing angle at the low temperature due to the nonlinearity error and variation of snow surface emissivity. An empirical correction is applied to the low temperature data for the automatic cloud detection.
In order to investigate the behavior of aerosol particles near the ground surface, observations of vertical distributions of air temperatures, aerosol concentrations and wind velocities using two separate poles of 6 m in height were carried out in the Second Experimental Farm of Hokkaido University in Sapporo. Aerosol concentrations were found to increase with height over the grass surface, suggesting that the grass surface acts as sink rather than source of aerosol particles. The normalized vertical gradients of aerosol concentration seemed to increase with the increasing stability, however, the gradient was smaller over the snow surface than over the grass surface. Assuming the constant flux layer existed near the ground surface, and using concentration gradient method, the deposition velocity of aerosol particles was calculated. The deposition velocities are distributed in the range between 0.01 and 1cms-1 and they seemed to decrease with the increasing stability, indicating that the turbulent transfer of aerosol particles was suppressed by strong stable stratification. Further, the deposition velocities were also evaluated to be larger over the grass surface than that over the snow surface, suggesting that the grass surface was a stronger sink for aerosol particles than the snow surface.
To investigate the mirror image relation (hereinafter, M.I.R.) observed at the ground surface, a new formulation in which snowflakes and snow particles acquire charges by the selective ion capture process is lead forth and one dimensional numerical experiments are carried out in this paper. The M.I.R. is more clear during snowfalls than during rainfalls and charges on these snow particles are also larger than that of raindrops in which the diameter is equivalent to the melted diameter of snow particles. In their experiments, Asuma and Kikuchi (1983), reported that although snowflakes have numerous pores in themselves, the final charges expected from the selective ion capture process (-12πε0Fa2) are the same as that of a conductive sphere of the same diameter. It is expected therefore that the high porosity of snowflakes leads to the increase of the collection efficiency of ions and it requires a shorter time to attain their final charges. We call this "the penetration effect". The formulation of a charging mechanism for snowflakes including this effect was derived. The modifications of charges on precipitation particles under the cloud base are examined by numerical experiments. Two types of precipitation particles are tested in these experiments, that is, snowflakes and raindrops. In case of snowflakes, the comparison Whipple and Chalmers (1944) formulation with our formulation was discussed. The main results obtained are as follows; (i) There is a tendency that on snowflakes observed on the ground surface hardly keep the electric charges produced in the cloud but raindrops keep the charges produced in the cloud. This tendency would be caused by the differences of their falling speeds. (ii) The penetration effect is more effective near the ground surface where ions are emitted abundantly in the atmosphere by the point discharge. (iii) On the ground surface, the precipitation current and the point discharge current are balanced and this balanced relation brings about the M.I.R.
To investigate the mirror image relation between the electric potential gradient and charges on precipitation particles above the ground surface, a simultaneous observation system of the vertical structure of the electric potential gradient in the lower atmospheric layer and the charge on precipitation particles falling through the layer during a snowfall was developed. This system consists of a tethered balloon, observation stages on the tether and a microcomputer on the ground. Each stage has a field mill and a charge collector and it is possible to measure simultaneously the vertical component of the atmospheric electrical potential gradient and the maximum charge on precipitation particles of at most 4 stages at 5-second intervals at every 50 or 100m in altitude. Data are converted to digital signals at the stage, transmitted by digital optical communications with optical plastic fiber cables and stored in microcomputer on the ground. Several measurements were carried out in March 1985 and some results were obtained. The mirror image relation was observed on the ground but it was not so clear aloft. It was considered that the charges on the precipitation particles were reelectrified during their falling near the ground surface.
Individual nitrate-containing particles were identified with a vapor deposited nitron thin film method in winter season. Information about the mixture of individual nitrate-containing particles was obtained with an electron probe X-ray microanalysis and other thin film method. It is indicated that nitrate did not exist always in the fine particle range, and the particulate nitrate existed in mixed state with sulfate as a solid mixed salt.