The generalized Lagrangian mean (GLM) theory and the transformed Eulerian mean (TEM) theory were reconstructed into a unified formalism. Application of a basic postulate that reciprocal translation is possible between Eulerian and Lagrangian information revealed the existence of a dual structure of governing equations having two standard forms which respectively correspond to the Lagrangian and Eulerian approaches. Either of these forms provides a complete equation set for the continuum concept, and can easily be transformed from one to the other. Therefore, whenever a new set of continuum equations is introduced which has either standard form, a new hypothetical continuum can be defined. As in the case of the original equations, the governing equations of the new continuum can also be easily transformed into various forms (other standard forms and the material form). The definition of new dependent variables is essential, since the hypothetical coninuum contains particular arbitrariness due to the definition. Additionally the new governing equations are formulated without any physical restrictions such as initial conditions or spatial symmetry. The GLM theory is reconstructed by selecting a form for new dependent variables and their related additional terms in the Lagrangian mean continuum. In this reconstruction, a new particle represent-ing the original particle group exists at its Lagrangian mean position, and moves with the Lagrangian mean velocity. Similarly, the TEM theory is reconstructed as one choice from the Eulerian mean continuum, and also describes a new particle, being representative of a newly labeled domain moving with the transformed velocity. As a by-product of these reconstructions, the non-acceleration theorem in both theories was also investigated and subsequently extended.
The collision between graupel particles observed by a stroboscopic TV camera is classified by the fall velocity and the size distribution, and the total collision rate and the collision rate distribution are strictly derived. The collision was caused by the difference in fall velocity between two graupel particles, and the fall velocity and the size distribution were expressed as a power law and in an inverse exponential form, respectively. Using the fall velocity and the size distribution observed, the total collision rate of all particles and the collision rate distribution were derived strictly using a sweep-out concept. It is shown clearly that the total collision rate can be expressed a product of the sweep-out volume of the particle with mean size and the square of the total number concentration. The derived collision rate for graupel particles has a simple functional form and is valid for other precipitation particles. The applications of this collision rate expression to ice particle multiplication and to charge generation are demonstrated.
Trends in the large-scale tropical circulation during 1948-1883 are studied in relation to the persis-tent drought in Subsaharan Africa, based on ship observations over the tropical oceans and surface records of stations in Western Africa, and using cluster, correlation, and trend analyses. Deficient Sahel rainy seasons are characterized in the Atlantic sector by an equatorward expansion of the North Atlantic subtropical high, enhanced Northeast trades and reduced southerly cross-equatorial flow, and anomalously cold surface waters across the tropical North Atlantic, and in the Indian Ocean by relatively high sea surface temperature. These associations with Sahel rainfall variations are typical both of year-to-year and trend-like changes. Trends of the large-scale circulation setting in the Atlantic sector include a pressure rise over the tropical North Atlantic; southward migration of the near-equatorial trough; acceleration of both the North and South Atlantic trades along with increasing cloudiness in the equatorial zone; increasing cloudiness over the tropical North Atlantic and the Central American Seas; and cooling of surface waters in a band across the tropical North Atlantic contrasting with warming in the South Atlantic. Records at land stations in Western Africa show near-coastal temperature and pressure trends consistent with the adjacent ocean areas. In the interior of Subsaharan Africa warming trends prevail, presumably as direct consequence of reduced inflow of cool maritime monsoon air, precipitation, and in-situ evaporation. Downward pressure trends at continental stations in Subsaharan Africa may reflect the hydrostatic effect of increasing temperature.
The generation of the external-mode ("barotropic" mode) motions from the internal-mode ("baroclinic„ mode) motions which are directly forced by stationary tropical heating is investigated with a two-level global model. The equations are linearized with respect to a solid-body rotation basic flow with vertical shear. We examine in detail the effect of this vertical shear which permits the coupling of the external mode with the internal mode. First, we determine the internal-mode motions forced by internal heating; second, from the vertical shear of the basic flow and the internal-mode motions, we determine the external-mode motions. In this treatment the equations governing internal and external modes can be reduced to the shallow water equations with different values of equivalent depth. The results show that the amplitude of the external mode induced in this way is smaller than that of the internal mode in the tropics, but becomes predominant in the extra-tropics. The external-mode wavetrains emanating northeastward and southeastward from the localized heating region are formed. These wavetrains extend to the antipodes of the emanating region. We examine respective effects of the several terms which express the excitation of the external mode from the coupling between the internal mode and the vertical shear of the basic flow. It is found that the term which expresses an advection of internal-mode vorticity by the basic flow with vertical shear is the most important. By a speculation concerning this forcing term and an inspection of the result of the numerical calculation, we examine in detail the sign of vorticity of the external-mode motions in the region where the internal mode has a large amplitude. Further, we examine the effects of the differential damping and the Ekman pumping on the excitation of the external mode.The external-mode wavetrains are formed by these effects also. The sign of the vorticity of the external-mode motions in the heating region is discussed.
The seasonal change of the water budget in the Amazon river basin during the FGGE period is investigated, using the global objective analyzed data set, precipitation data and river discharge data. Substituting these data into the atmospheric water balance equation and that of the basin, the evapotranspiration and relative value of basin storage are estimated and the relationships of water balance terms are examined. The annual water vapor flux convergence, calculated using the FGGE "main" III-b data set analyzed at the European Centre for Medium-Range Weather Forecasts, is estimated to be smaller than the annual runoff obtained by river discharge data. This is due to the characteristics of this global objective analyzed data set that it is made by the 4-dimensional data assimilation method which expresses the divergence and convergence of wind in the tropics as weaker than in the real state. The seasonal change pattern of precipitation shows good correspondence to that of water vapor flux convergence, which is multiplied by the factor 1.37 in order that its annual value meets the annual runoff. Evapotranspiration, estimated by the atmospheric water balance equation, remains almost constant within a year and the seasonal change of basin storage is very large. This result indicates that evapotranspiration in the entire Amazon river basin is not affected by the seasonal change of basin storage even in the dry season. The evapotranspiration ratio (evapotranspiration/precipitation) of the dry season is larger than that of the rainy season. It is concluded that the role of evapotranspiration on the water cycle in the Amazon river basin is relatively more important in the dry season than in the rainy season.
Rain observations by vertically polarized and horizontally polarized radiowaves, using a dual-wavelength radar with X-band and Ka-band radiowaves, were performed. Though the polarization switching is slow, a feature of the radar, that the beams at the X-band and Ka-band are the same, makes it possible to compare directly the ZDRs at both frequencies. The so-called ZDRs were mea-sured and the difference between those in the X-band and Ka-band was studied. ZDRs in the X-band increase with rainfall rate, which is as expected. ZDRS in the Ka-band slightly decrease with rainfall rates at a longer observation range, while ZDRs increase slightly with rainfall rates at a short ob-servation range. These facts can be explained by two causes. One is the polarization characteristic of raindrops in the Ka-band. Unlike in the X-band, the Ka-band radar cross sections of raindrops do not always increase as the diameter of a raindrop increases, which results in a weak increase of ZDRs with rainfall rates at short ranges. The other is the differential rain attenuation for vertically and horizontally polarized radiowaves. The rain attenuation for horizontally polarized radiowaves is larger than that for the vertically polarized radiowaves. For a long observation range, the differential rain attenuation is large enough to compensate for the difference of the radar cross sections of rain.
Accuracy of the delta-four-stream approximation is investigated in inhomogeneous scattering atmospheres in order to introduce this approximation into a general circulation model (GCM). By using the Gauss hemispheric quadrature and the matrix operator method, the equation for radiative transfer in an inhomogeneous atmosphere can be reduced to a 2×2 matrix equation. Since operations for 2×2 matrices can be analytically made very easily, it does not take so much computation time to obtain the solution for the matrix equation, thereby indicating that this approximation is usable in GCMs. Solar heating rates are compared between the delta-four-stream approximation and the exact calcu-lation (thirty-two-stream approximation) for model atmospheres containing various aerosol particles and clouds. It is found that the delta-four-stream approximation retains its errors within 5% when the solar zenith angle is smaller than cos-10.3. For solar zenith angles larger than this value the errors are overall less than 10%, though the errors tend to increase as the solar zenith angle increases.
Internal structure and time evolution of a cloud cluster in the ITCZ of the western tropical Pacific which appeared on 8-9 June 1990 was analyzed by using meteorological radar data on board of the Keifu Maru, a research vessel of Japan Meteorological Agency. The radar observed the cloud cluster throughout almost all its life cycle. The horizontal size of the cloud cluster was about 500km in diameter and its life time was about 15 hours. The cloud cluster in the mature stage was shown to be composed of four sub-systems of echoes in different stages of development, with earlier stages on the windward side of the low-level wind. The life time of each sub-system was found to be about 8-10 hours. Development and motion of this cloud cluster was characterized by the succesive formation and development of these sub-systems on the windward side of the low level inflow. A schematic model of this cloud cluster is proposed.
Because the statistical hypothesis test (the Lepage test) has successfully shown the regional abrupt changes in precipitation in Japan after 1900, changes of the temperature and sea-level air pressure are studied by means of the same method. The analysis shows that the temperature and sea-level pressure also presented discontinuous changes. Also, it is noted that the changes in the temperature were not fictitious ones caused by changes in observational methods, instrument changes or other factors. Referring to the previous study on the changes in precipitation, the climate changes which occurred in Japan after 1900 can be summarized as follows: (1) Changes detected were discontinuous ones and trends of temperature did not cause any significant changes. (2) Very noticeable abrupt warmings with a magnitude exceeding 1°C of the seasonal mean temperature occurred in 1914 and 1949, and the precipitation amount increased in the successive seasons. (3) The seasonal mean temperature also changed in 1957 and 1967. These recent changes were discontinuous ones, but were neither abrupt warmings nor abrupt coolings. The mean temperature for August-October fluctuated more after 1957 and for April-July it fluctuated less after 1967. (4) Cooling trends were noticed during periods between discontinuous changes but they did not cause any definite changes to be detected by the Lepage test. (5) The sea-level air pressure changed three times. Though the quality of the sea-level air pressure records is questionable, it may be worthy to note that one of the changes occurred in 1924 and in 1924 the annual precipitation amount decreased. The discontinuous changes support the idea that the climate system is a chaotic spontaneous dynamic system. Mechanisms of the regional discontinuous changes and the relations between the very noticeable abrupt warmings and the concentration of greenhouse effect gases should be the subjects of future studies.
In this study, we examine the interdecadal changes over the North Pacific area of sea level pressure and 500-mb height for the winters 1950-1988. Since most of the variance (-95 %) of these two variables' asymmetric component is explained by the long-wave regime (wavenumbers 1-4), interdecadal changes in this wave regime are used for discussion. The major findings are (1) that the interdecadal changes of sea level pressure and 500-mb height were nearly barotropic and took place on the scale of the quasi-stationary long waves; (2) that the pattern of interdecadal 500-mb height change was spatially in quadrature with the 39-winter mean; and (3) that falls/rises of sea level pressure and 500-mb height in the Pacific were associated with the intensification/weakening of the lower-latitude westerlies. Version 1 of the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR) was used to perform two numerical experiments in which the sea surface temperature (SST) anomalies in the North Pacific during the decades 1950-1959 and 1979-1988 (with respect to the 39-winter (1950-1988) mean SST) were imposed on the climatological SST of the NCAR CCM1 perpetual January control experiment. It was demonstrated that with the numerical simulations, the observed interdecadal changes of the atmospheric circulation reported in this study were apparently associated with the interdecadal changes of the tropical and extratropical North Pacific SST.
As a part of Sino-Japanese Joint Research Program on Atmosphere-Land Interaction in the Heihe River Basin (HEIFE), a preliminary observation of turbulent characteristics and turbulent transport in the surface boundary layer over an oasis in the desert area has been made in succession to that on the Gobi desert (Wang and Mitsuta, 1990 and 1991). Different from the conditions over the desert, the latent heat flux is as much as four times of the sensible heat flux and plays a dominant role in the energy budget. The specific humidity shows a peak twice a day, a little before noon and in the evening. The air temperature fluctuation shows its maximum in the evening when its mean value is falling off. The normalized variance of temperature and specific humidity are both well described by a universal function of stability, showing that they follow the Monin-Obukhov similarity.
Significant evening peaks were observed in the diurnal variations of vapor pressure over an oasis surrounded by desert in the north-west part of China. They appeared not only in the surface layer measurement but also in the upper air soundings below 500 m. According to the surface flux measurements, it was observed that sensible heat flux was much stronger over the desert than over the oasis during the daytime. The upper-air sounding data shows that the oasis is a cool and moist island capped by dry and hot air which would be advected from the surrounding desert. In these situations, upper-layer convective mixing is considered to be much stronger than in the lower cool island, and this can lead to water vapor flux divergence in the afternoon. In the evening, convective mixing, which is suggested by the sensible heat flux from the ground, decreases over the desert and in the upper layer over the oasis. It was observed that evapotranspiration from the surface still continues upward. It is suggested that water vapor flux convergence can occur near the oasis surface.
The behavior of air parcel transport between hemispheres and between the troposphere and stratosphere is studied by directly calculating 180-day trajectories based on observed data. It is shown that air parcels from the extra-tropical troposphere of one hemisphere are transported to the other hemisphere through the upper troposphere. The estimated inter-heimispheric exchange time is about one year. The main pathway from the troposphere to the stratosphere is the tropical tropopause. The flow from the equatorial lower stratosphere branches off in two directions, i, e., one is the return flow to the troposphere through the sub-tropical tropopause gaps and the other is the poleward flow in the lower stratosphere.