In this study, spectral energetics characteristics of FGGE III-b original analyses and the re-analyses are compared in order to assess the achievement of the FGGE project. We compare both of the GFDL and ECMWF versions of the old and new FGGE analyses. It has been noted that the old version of the FGGE GFDL data is noisy, containing excessive small-scale disturbances. The GFDL dataset also appears to be inconsistent with respect to the wind and mass balance. These characteristics result largely from the technique of the continuous data injection as well as from GFDL's philosophy such that the observed data should avoid model bias as much as possible. In contrast, the old version of the FGGE ECMWF data appears to be too smooth, showing a significant damping of divergent wind, especially in the tropics. The data assimilation and initialization cycle at the ECMWF is based on a philosophy such that a successful analysis is one which leads to a successful weather prediction. With this concept, the ageostrophic component of the atmospheric motion is damped to less than 5 percent in magnitude. According to normal mode energetics diagnosis, which separates atmospheric motions into Rossby and gravity modes, it turns out that the gravity mode energy level ECMWF is only 58 percent of that in GFDL for the old analyses. The FGGE original analyses have been revised by the FGGE re-analysis based on knowledge and criticism gained after the FGGE. The GFDL re-analysis is now smoother, and the ECMWF re-analysis contains much divergent wind. In this study we have conducted the same normal-mode energetics analysis for the FGGE re-analyses, and the results are compared with the FGGE original analyses. It is found, however, that the gravity-mode energy level in ECMWF is still only 42 percent of that in GFDL. The significant difference remains in the re-analyses as before The possible causes of this discrepancy are discussed.
Sea surface energy balance was analysed at the ocean weather station-T (OWS-T; 29°N, 135°E), south of Japan, based on direct measurements on board. The intensive observation periods were set up during OMLET (Ocean Mixed Layer Experiment) as two cruises of the R/V Hakuhomaru, University of Tokyo around OWS-T in Apr/May 1988 and Jan/Feb 1991. The first period corresponds to the decaying stage of the ocean mixed layer and the second one is the stage of the development. Radiation measurements and turbulent flux measurements by the eddy correlation method were carried out during the cruises as well as the upper air soundings and the ocean measurements. A part of the results from OMLET'88 was already reported by Tsukamoto et al. (1990). The eddy correlation flux measurement is the most accurate one. However it is not always applicable for the sea-surface flux evaluations for a long term. The bulk aerodynamic formulas are usually accepted for practical purposes. The bulk transfer coefficient should be determined based on the eddy correlation flux measurements. However previous energy budget studies are based on universal bulk models and no confirmation was made of those results. In the present study, the bulk transfer coefficients were determined based on the eddy fluxes as a function of wind speed as in situ values. The bulk coefficients were applied to the continuous surface meteorological data for the evaluation of long-term fluxes. The net heat fluxes into the water were evaluated as +54Wm-2 (downward) in Apr/May and -192Wm-2 (upward) in Jan/Feb. The local time changes of the ocean mixed layer heat content were calculated with JMA buoy data as 356Wm-2 and -297Wm-2, respectively. The difference of these values can be regarded as due to horizontal or vertical heat transfer.
It is of importance to study the regional characteristics of the snowfall-formation mechanism in order to lessen and possibly prevent damage caused by heavy snowfalls. Therefore, snowfalls were observed in areas toward the Japan Sea, Japan, paying attention to the riming growth of snow particles for the purpose of studying the regional characteristics of the snowfall-formation mechanism. The observational results can be summarized as follows. Snowfall, in which the riming process was predominant in the growth of snow particles (snow particles having a mass of rime more than 50% of the total mass), accounted for about 70% of the total snowfalls in coastal areas. On the other hand, the riming process accounted for about 40% of the total snowfalls in inland areas. On the bases of this and other observational results, it was clarified that the riming process plays an important role in the growth of snow particles in coastal areas, whereas in inland areas the role is not so important. It was shown that the contribution of the riming process to the growth of snow particles is controlled by meteorological conditions. Favorable meteorological conditions are found more frequently in coastal areas than in inland areas. These observational facts explain why the riming process is predominant in the growth of snow particles in coastal areas and not in inland areas.
The basic structure of steady baroclinic waves observed in a differentially-heated rotating-fluid annulus is found to be composed of high- and low-pressure vortices, upper-level (eastward) and lower-level (westward) jet-streams meandering through the vortices, and boundary layers. On the basis of this structure, recently, Sugata and Yoden (1994) numerically studied the Lagrangian motion of a fluid particle in the annulus. Stimulated by their results, we conducted experiments on a rotating-fluid annulus by injecting drops of red ink into a jet or a vortex and observing the results in the co-rotating frame of the drifting wave. The observed 3-D ink patterns appearing in the fluid revealed the inner region of the vortices. That is, their structures are composed of a core region, which is rather well isolated and split into separate upper and lower layers, and next to the core a transition zone where fluid particles are frequently transported to and from its outside, but rarely to the core region. Several interesting phenomena observed in the annulus are also presented, such as one suggestive of the Ekman pumping found in the low-pressure vortex.
This study is concerned with the characteristics of acidity of snowfalls brought by the northwesterly winter monsoons, low pressure systems and other disturbances around Sapporo, Hokkaido, Japan. Samplings of daily snowfalls were carried out from January 1 to March 31, 1991 in Sapporo. Falling snow crystals, whole depth of snow cover, depth of snow layers formed by each snowfall, and snowmelt water existing snow cover were measured through a period of accumulation and melting of snow cover. Snowfalls brought by the northwesterly winter monsoon winds had relatively lower pH values and higher electrical conductivity than those brought by low pressure systems. The acidity of the whole depth of snow cover did not vary greatly before the snowmelt period. The acidity of each snow layer, however, indicated characteristic variations closely connected with metamorphism of the accumulated snow cover. During the observation period, the acidity of daily snowmelt water which had run off from snow cover showed the greatest ion concentrations in the first stage of the snowmelt season. Concentrations of chemical constituents of snow cover had drastically decreased and acidity had approached neutrality at the peak of the snowmelt season. The cause of this seems to be the large quantity of chemical constituents which run off snow cover for a short time during the first stage of the snowmelt season.
Comparative experiments of moist convection using hydrostatic and non-hydrostatic models are performed to study the suitability of the hydrostatic approximation for a high-resolution model when the grid size falls below 20km. The moist convection in the models is treated by the use of an explicit warm-rain process predicting cloud water and rainwater as well as by a semi-explicit scheme consisting of the warm-rain process and moist convective adjustment. The differences between the experiments with and without hydrostatic water loading are also examined and quantitatively compared with those between the hydrostatic and non-hydrostatic simulations. When the prognostic explicit scheme is used, the hydrostatic simulation overdevelops moist convection, overestimates the total amount of precipitation, and overexpands the area of precipitation as the grid size decreases. This overdevelopment alters substantially the structure of moist convection and precipitation patterns. The absence of hydrostatic water loading also alters the total amount and structure of precipitation. Hydrostatic water loading exerts more significant influences on simulated precipitation than the hydrostatic approximation. In the 20-km simulations, the hydrostatic simulation with hydrostatic water loading produces results that are comparable to the non-hydrostatic counterpart. The difference in the total amount of precipitation between the hydrostatic and non-hydrostatic simulations was not as large as that of the convective development. This can be explained by considering the total water budget, which includes simulated precipitation and water vapor flux through the lateral boundaries, i. e., the water-vapor flux in the hydrostatic simulation corresponds to that in the non-hydrostatic one. Although moist convective adjustment removes conditional instability and does not produce strong updrafts, the characteristics of the results in the comparative experiments of moist convection using hydro-static and non-hydrostatic models were hardly changed by the incorporation of moist convective adjustment (the semi-explicit scheme). On the other hand, hydrostatic water loading exerts more significant influences on simulated precipitation with the semi-explicit scheme than that with the prognostic explicit scheme. Therefore, in developing 10-20km numerical weather prediction models, hydrostatic water loading should be evaluated in preference to adopting non-hydrostatic models.
Many articles dealing with theoretical and observational aspects of atmospheric blocking have appeared in the literature. So far little attention has been paid to the question of long term variations in blocking frequency. Usually 500-hPa geopotential height data are used in observational studies of atmospheric blocking. We used a 94-year time series of sea-level pressure data in order to extend the analysis back to the first half of the century. Long term variations of frequency of blocked flow as well as individual blocking events have been documented. There are two areas of study, the North Atlantic and Western Europe (40°W to 30°E) and the Pacific (160°E to 130°W). Low-pass filtered data suggest that there was increased blocking activity during the winter and spring seasons in the Atlantic during 1930-1935 and 1950-1960, whereas there was reduced blocking activity during 1910-1915, in particular during the winters. However, a statistical test did not verify that these peaks are real. Data for the Pacific do not show any outstanding periods with blocked flow. Another result is that there are less blocking episodes in the Pacific than in the Atlantic. Data for the whole time period show that the number of episodes in the Pacific is 60% of the number of episodes in the Atlantic during winter and spring. During summer and autumn there are less blocking episodes in both regions than during the other two seasons. Furthermore, the number of episodes in the Pacific is only 40% of the number of episodes in the Atlantic. We also found that there is a factor of almost 2 between the number of blocking episodes during those decades with the highest number of blocking episodes and those with the lowest. This is true for both the Atlantic and the Pacific.
Direct solar radiation data observed since 1933 at the Japan Meteorological Agency (JMA) observing stations have been analyzed. The transformed turbidity factor ln(τ0--1), where τ0 is the Feussner-Dubois' turbidity coefficient, is introduced in order to normalize the frequency distribution. This allows for a better statistical analysis of atmospheric turbidity. The turbidity factor clearly shows the influence of three large volcanic eruptions: Agung (1963), El Chichon (1982), and Pinatubo (1991). These volcanic effects appear in the time series of data as unusual seasonal variations. Direct solar radiation data at global stations should be continuously analysed in order to better understand the trend in stratospheric aerosols.
In this paper, the interannual variability of the East Asian summer monsoon (EASM) rainfall and the tropical sea surface temperature (SST) have been studied. It is found that the EASM rainfall possesses a strong biennial signal, which is particularly pronounced over the southeast China. For the SST, the biennial oscillation is the second most significant quasi-periodic signal over the entire tropical Indian and Pacific Oceans. Results indicate that the biennial variations in the SST and EASM rainfall are closely linked. The SST pattern which is best correlated with EASM rainfall appears in the form of a double see-saw with quasi-stationary centers of action over the Indian Ocean, the Asian monsoon region and the eastern Pacific. The most pronounced SST signals are found in the equatorial eastern Pacific and Indian Ocean about two seasons preceding and following the EASM rainfall. Evidence is presented suggesting that the biennial variability of the EASM rainfall is phase-locked to a global scale biennial oscillation involving the interplay of the Asian monsoon, the Hadley and Walker circulations, and basin wide fluctuations in SST. In particular, the eastward propagation of zonal wind anomalies from the Indian Ocean to the western Pacific, which regulates the moisture fluxes from the western Pacific to the East Asian region, appears to be a key component of the biennial fluctuation associated with EASM rainfall. Results suggest that the relationship between the Asian monsoon and tropical SST is more robust in the biennial than the ENSO time scale, hence raising the possibility that the biennial oscillation may be more fundamentally related to monsoon-ocean-atmosphere interaction than ENSO itself.
The effects of both weak stratification and shear of environmental flows on density currents are analytically investigated in a 2-dimensional model. Consistent with published results, both the propagation speed and depth of density currents increase as the shear in the direction of propagation increases. The lower-level shear is shown to be more effective than the upper-level one. The stable stratification increases the propagation speed but decreases the depth of density currents.
A QBO-like oscillation has been obtained in a simulation using the 1/5 sector three-dimensional model, with moist convective adjustment, derived from a preliminary version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) general circulation model. The basic structure of the oscillation is similar to that of the observed quasi-biennial oscillation (QBO). The amplitude of the oscillation is rather strong, about 30ms-1, which is comparable to the observed oscillation (25ms-1). However, the period of the oscillation is about 400 days, or half as long as that of the observed QBO in the lower equatorial stratosphere. Further, the height of the simulated oscillation is about 15km higher than that of the observed QBO. From a preliminary analysis of the wave behavior, it is suggested that gravity waves are important in the simulation of the QBO-like oscillation.