Linear responses to localized heating, which moves westward or eastward uniformly, are analytically studied on the equatorial beta plane. Although Lau and Lim (1982) solved similar problems by assuming a long-wave approximation, the present model is primitive, and response patterns produced by all the equatorial waves are examined. First, response patterns are examined for a simple heating profile having one vertical mode, a Gaussian meridional mode, and a Gaussian longitudinal profile with scale comparable to the equatorial radius of deformation. For the case of fast eastward-moving heating, the response pattern is mainly produced by the Kelvin wave, and no wave train appears. For the case of slowly eastward-moving or stationary heating, the response pattern due to the Kelvin wave is dominant to the east of the heating, while vortical motions due to the Rossby wave are found to the west. These features are similar to those of the long-wave approximation model. On the other hand, the long-wave approximation does not hold valid for the case of westward-moving heating. In particular, a wave train produced by the westward-propagating inertio-gravity wave appears when the moving speed is sufficiently large. It comes from the difference in meridional structure that by this heating, the westward-propagating inertio-gravity wave is strongly excited, while the eastward-propagating one is not. Eastward-propagating growing (EG) and westward-propagating non-developing (WD) modes were obtained in the positive-only wave-CISK model on the equatorial beta plane (Yoshizaki, 1991). The EG mode corresponds to eastward-propagating super cloud clusters in tropical intraseasonal (30-60-day) oscillations. Then, why does the selection of the EG mode occur? To examine this problem, analytic response solutions are applied from the standpoint of the response problem. The WD modes can not develop, because a wave train of the inertio-gravity wave with higher vertical mode is induced to the east and new modes are excited there. Meanwhile, the EG mode can develop as a single mode, because the response pattern of the Kelvin wave is dominant and no wave train appears. The selective amplification of the EG mode is realized by these differences of the response patterns.
"Coupling development", or the rapid cyclogenesis with a coupling process of the upper and the lower tropospheric vortices is studied numerically using a β-plane channel, dry primitive equation model. The "coupling development" occurred when a lower vortex is initially located at the latitude of the jet axis, and an upper vortex to the northwest of the lower vortex. We displayed the following three stages of the "coupling development". 1) Deformation of the upper vortex associated with the "upper-level frontogenesis" (from day 0-day 2). 2) The northward penetration of the lower vortex through advection of surface high-PV (from day 2-day 3). 3) Continuous rapid growth of the vortex near the surface triggered by the "coupling" of the upper and the lower vortices (day 2.5-day 3.5). We have also made a brief data analysis of the 25-28 April 1986 cyclone developed around the Japan Islands. We observed the following two characteristics in that cyclone. 1) The northward penetration of the lower vortex before the "coupling". 2) The downward intrusion of the upper vortex along the sloping isentropic surface, and the "cou-pling" with the lower vortex at the beginning of the rapid development. These observed features are consistent with our numerical results, which indicate the importance of the "coupling development" mechanism in the mid-latitudinal cyclone development.
A new thin film chemical method has been developed for the simultaneous detection of individual sulfate-containing and nitrate-containing aerosols. It is based on the reactions of the sulfate ion with barium chloride and of the nitrate ion with nitron. Multiple thin films are prepared by successive depositions of nitron and barium chloride. Both reactions of sulfate with barium chloride and of nitrate with nitron proceed in the octanol saturated atmosphere. Under this reaction condition, existence of sulfate and nitrate in individual particles is indicated by the appearance of the Liesegang rings and of a bundle of needle-like crystals, respectively. These reactions are reproducible and not interfered with carbonate and chloride ions. The method is specific for sulfate and nitrate ions and can be applied to soluble sulfate and nitrate. Examples of the method applied to atmospheric aerosols are shown. Particular interest is concerned with the observation of the mixed salt of sulfate-nitrate in the atmosphere.
A bulk parameterization scheme of cloud microphysics which predicts not only the number concen-trations of cloud ice and snow but also that of graupel, in addition to the mixing ratios of six water species (water vapor, cloud water, rain, cloud ice, snow and graupel), is developed. This scheme is applied to the 3-dimensional simulation of the convective snow cloud observed over the Sea of Japan on Feb. 4, 1989, with a cloud top temperature at -20°C. In the simulation where conventional parameters are used for Fletcher's deposition/sorption nucle-ation, Bigg's freezing of cloud droplets and the Hallet-Mossop rime-splinter secondary production of ice crystals, the maximum of number concentration of ice particles is about 5×104/m3, 1/4 of the observed counterpart (2×105/m3). Radar reflectivity is larger by 10 dBZ than observation. Other aspects appear to be well reproduced by the model at least qualitatively. Precipitation formation in the simulated cloud is examined, and it is found to be consistent with observational studies. Sensitivity experiments to the amplification of various ice nucleation rates are conducted to simulate the cloud more realistically and to see the effects of the increase of the number concentration of ice crystals on the precipitation formation of the convective cloud. The increase in the number concentration of ice crystals to the value of 105-106/m3 results in the increase of snow in mass and number and the decrease of radar reflectivity. The further increase in the number concentration of ice crystals to the value (107/m3) results in suppression of precipitation, glaciation of cloud with little cloud water and suspension of abundant snow in the air. These experiments give some support to the possibility of suppressing the precipitation by overseeding. Some merits of predicting number concentrations of ice particles are discussed. In the simulation, the intercept parameters of the inverse exponential size distribution functions for snow and graupel, Nos and Nog, are found to be more dependent on the stage of the cloud development than on the precipitation rate. This feature cannot be reproduced by the model with the prescribed Nos and Nog. For a more realistic simulation of clouds, a more reliable knowledge about ice nucleation is crucial.
Polar low development over the eastern coast of the Asian Continent (northeastern part of the Japan Sea) on 9-11 December 1985 is studied. The low-level strong thermal gradient was maintained along the east coast of the Asian Continent. When a large-scale trough including a cold cut-off vortex approached the east coast, a synoptic-scale low developed over the northwestern Pacific to the east of Japan, and the cyclonic circulation of the synoptic-scale low predominated over the coastal zone. The easterly winds in the northwestern quadrant of the synoptic-scale low advected the warm air from the Pacific to the Continent, while the westerly winds in the southwestern quadrant of the low advected the cold air from the Continent. Consequently, the N-S thermal gradient was enhanced in the western part of the low. This was the frontogenesis in horizontal shear flows, and a W-E oriented trough (shear line) formed in the lower troposphere in the western part of the synoptic-scale low. The polar low of the meso-α-scale formed in the W-E trough under the influence of the cut-off cold vortex aloft. In the aforementioned process, the synoptic-scale low played the role of the parent circulation for the polar low formation.
Two datasets of monthly-mean wind stress, estimated from Ships-of-Opportunity data covering the North Pacific (O°-50°N) and the Pacific trade wind region (30°S-30°N), are used to describe the spatial character of interannual variability. Two characteristic time scales related to the occurrence of ENSO events are given by period bands around 3.6 years (EN) and 2 years (QB), being associated with spectral peaks found in sea surface temperature (SST) in the eastern equatorial Pacific area. Spatial features of wind stress variations in the two period bands are described by patterns derived from correlations between SST in the area and the zonal and meridional components of the wind stress in the datasets. The variation in the EN-band around 3.6 years is coherent over a large portion of the study areas covering the extratropics as well as the tropics and has a Pacific-basin scale. The wind stress pattern in the maximum phase of the SST corresponding to the peak phase of an El Nino event has not only symmetric features, such as westerly anomalies near the equator, but also asymmetric features such as easterly anomalies covering the western porion north of the equator. These spatial features are similar to those in composite fields for the peak phase of El Nino events, which suggests that the EN-period variation is a principal element in ENSO variability. On the other hand, the variation in the QB-band around 2 years is dominant in the tropical area, especially in the western portion north of the equator, being incoherent over most of the study areas. It is suggested that the QB-period variation is characteristic of the tropical zone covering the Indian Ocean and that its phase relationship with the EN-period variation is important for the occurrence of ENSO events.
It has been considered that the unevenness of cloud top and cloud base has a great influence on solar and atmospheric radiation. Thus, to understand the cloud base structure of a wide-spread stratocumulus cloud, observations of stereoscopic photographs were made in which the optical axis of two wide-lens cameras was pointed upwards in a vertical direction. The observations were carried out in the summer and fall of 1986 and 1987. A set of stereophotographs was processed by an image analyzer and the edge lines of unevenness of the cloud base were obtained. Using the edge lines of unevenness of a pair of stereophotographs, 100 to 200 corresponding points were selected on the edge lines and the heights of points were obtained. As a result, it was recognized that the stratocumulus clouds analyzed have an unevenness of 300 m or more at the cloud base. Furthermore, a correlation between the height of cloud base and brightness of cloud base using a set of stereophotographs was obtained. Using the correlation, the cloud base structure, that is, the variation in cloud base height, was analyzed. As a result, it was demonstrated that the relationship between the size and depth of unevenness of the stratocumulus clouds resembled a hemisphere when the wind velocity was weak and wind shear was not strong up to cloud base height. It was recognized, however, that when the wind velocity and wind shear were strong up to the cloud base height, the unevenness at the cloud base changed from a hemispherical shape to a flattened shape.
Using a new type of photomultiplier (PMT) tube R3236 cooled to -30°C, and a photon counting method, lidar measurements of stratospheric aerosols and tropospheric aerosols above an altitude of 2.25 km were made at the fundamental wavelength ofλ=1064 nm of a Nd:YAG laser which is sensitive to aerosol scattering. The PMT R3236 was found to be useful even for measurements of stratospheric aerosols near the background level. The average of the integrated backscattering coefficient above tropopause was 3.3×10-5 sr-1 at Tsukuba (36.1°N, 140.1°E) during November through December 1988. Tropospheric aerosol layers in the free atmosphere were frequently observed and the altitude of the maximum scattering ratio of the aerosol layer varied for every measurement, compared with the stable stratospheric aerosol layer. Some tropospheric aerosol layers were confined by two inversion layers.
According to Nigam (1990), the first mode of his EOF analysis mainly represents variabilities in the zonal-mean zonal wind related to the displacement of the tropospheric subtropical jet. The present results of a lagged correlation analysis reveal that perturbations in zonal-mean zonal winds first occur in the stratosphere and then propagate into the subtropical troposphere. Numerical experiments with a general circulation model also indicate that the initial perturbations in the stratosphere can cause appreciable changes in the subtropical tropospheric jet.
A new method of nesting a high-resolution limited area model (LAM) in a low-resolution global model (GCM) is proposed for the purpose of conducting.regional climate simulations. It is tested with a few feasibility studies of month-long time integrations of the LAM. The conventional method of embedding a LAM in a larger area model is to couple the inner to the outer model in terms of lateral boundary conditions in real space. In the present method, the concept of a spectral boundary in wavenumber space is the key to joining a high-resolution LAM to a low-resolution GCM. In practice the new technique of combining the two different resolution models is to replace the large-scale fields of the LAM with the corresponding large-scale fields supplied externally from the GCM at a selected regular time interval. The present experiments showed that the time integration was very smooth over a period of one month, and that the mesoscale features associated with the large-scale supplied from the GCM were well simulated, suggesting that the new method is encouraging for regional mesoscale climate simula-tions.