Impact of the sea surface temperature anomalies (SSTAs) on atmospheric circulations are studied with emphasis on the winter climate in Japan with the use of an atmospheric general circulation model. The empirical orthogonal function analyses are performed for precipitation, geopotential height at 500mb and surface air temperature. It is shown that leading eigenvectors of precipitation are zonally elongated in the tropics and that the distribution of positive and negative precipitation anomalies is dependent on the SST and precipitation field in the control run. Surface air temperature in East Asia is mostly governed by the temperature contrast between Japan-East China region and the Sea of Okhotsk. This is associated with high pressure anomalies in the North Pacific Ocean, which weakens the cold surge from Siberia. This circulation is found in the first eigenvector in the present experiment El(Z500) and is dominant in the run which uses the composited January SSTA observed during warm winter in Japan. The simulated anomalies in mid-latitude circulation correspond well with the observations. An additional run with the SSTA over the equator east of the dateline gives the largest anticyclonic circulation response over the North Pacific Ocean and the warmest surface air temperature anomalies in East Asia. The pattern relevant to warm winter in Japan is not a simple atmospheric response to tropical heating. It is conceivable that inherently there is a dominant circulation mode like El(Z500) and that this mode can be activated either by a direct and/or an indirect orographic effect or by anomalous heating induced by the SSTA. This pattern can be understood as a response to subtropical mass source/sink distribution under the framework of a linear theory.
In this observational study the 30-50 day motion fields, at 850mb and 200mb, for 1980 to 1984 are analysed using the synoptic approach. The present analysis examines their interannual variability. A similar analysis of the 30-50 day motion fields during the FGGE (1979) year showed, a) eastward propagating planetary scale divergent waves around the globe throughout the FGGE year at 200mb and b) a family of northward propagating trough and ridge lines over the MONEX region (30E-150E, 30S-40N) during the summer of 1979 at 850mb. We have performed a careful analysis of the sensitivity of the calculations of low frequency waves to the width of the time filter. Our results show that the phenomenon is quite robust and is not very sensitive to the width of the filters. The 850mb streamline-isotach analysis of the 30-50 day time scale waves over the MONEX region for the years 1980 to 1984 shows the northward propagating trough and ridge systems during May, June and July of 1982 and during May, June and July of 1983. The speed of northward propagation is approximately 0.7 degree latitude/day during the summers of 1979, 1982 and 1983. During the summer seasons of 1980, 1981 and 1984 the trough and ridge lines do not exhibit a steady northward propagation. The direction and the speed of propagation are found to be variable from month to month during these years. The trough and ridge systems during 1980 and 1981 are either stationary or show some northward propagation. During the summer of 1984 the propagation of the trough and ridge system is northward in May and June and southward in July and August. Interannually the onset and break periods of the Indian summer monsoon exhibit a close relationship to the passage of the 30-50 day trough and ridge systems over India. A global analysis of velocity potential at 200mb shows eastward propagating waves during 1979 and between October 1982 to October 1983. During the remaining period the direction of propagation is less well defined. The amplitude of the divergent wave is found to be variable from one year to the next. The 30-50 day wave motions exhibit a similar behaviour during 1979, 1982 and 1983, both at the 200 and the 850mb levels. 1982-83 was an El Nino year and 1979 was characterized by weak warm sea surface temperature anomalies of the order of 1°C over the eastern equatorial Pacific Ocean. Both of these years were characterized by deficient rainfall over India, with somewhat above normal rainfall over the region south of India.
Based on surface winds at 6-h intervals for four Northern Winters and Springs (1970-73), two cases of strong westerly wind bursts were identified in the core of the equatorial western Pacific (-155°E). One case occurred in early April and another in early May 1972, both prior to the maximum sea surface temperature anomalies along the Peruvian coast during the 1972 ENSO event. During the Northern Spring, as an anomalously strong anticyclone moves rapidly from north-central China to its east coast, the surface wind fields to the southeast of the Philippines respond swiftly, turning from an easterly background to northerly. In the meantime, surface pressure in the far western equatorial Pacific tends to rise. These rapid equatorial resposes are probably due to gravity wave-like motions induced by the pressure-wind imbalance in the midlatitudes. The local pressure increase in the extreme western Pacific enhances the west-to-east pressure gradient in the equatorial trough zone and results in a strong westerly wind acceleration in the core of the equatorial western Pacific. This acceleration is also preceded by a west-to-east displacement of the pressure surge in the equatorial trough zone. The enhanced zonal pressure-gradient force and the associated eastward displacement of the equatorial pressure surge are two critical factors for initiating westerly wind bursts. Westerly wind surges detected primarily from fixed-station data compare favorably with those calculated from ship records of an independent source.
Large-scale situations over East Asia associated with the frontal zone extending from China to Japan are examined from late August to the beginning of November on the basis of 5-day averaged fields during 1981 to 1985, mainly in 1983. As a result, the investigated period from late summer to autumn is divided into four stages according to the frontal positions, stagnating feature of fronts and characteristics of large-scale thermal, moisture and height fields. The relationship with the retreat of the Indian summer monsoon is also examined. In Stage 1, the frontal zone was situated in the northern part of Japan. The center of a subtropical anticyclone was located to the south of Japan and an associated ridge extended over South Japan. This Stage is regarded as a part of mid-summer season. The beginning of this stage roughly coincided with the beginning of the retreat of the Indian summer monsoon from Northwest India. In Stage 2, the frontal zone shifted to the southern coast of Japan and stagnated there, although the stagnating nature was less pronounced than the Baiu frontal zone. Over the Eurasian Continent, the mid-latitude westerlies were meandering and a quasi-stationary trough was formed between 160°E and 160°W. The horizontal temperature gradient around the frontal zone over China in Stages 1 and 2 was small (-4°C/1000km), while the moisture gradient was large (-5g/kg/1000km). The temperature gradient was relatively large (6-8°C/1000km) over Japan during these stages. These characteristics are similar to those of the Baiu frontal zone. The frontal zone and this stage are regarded as the Shurin (Akisame) frontal zone and the Shurin season, respectively. The period of this stage was from September 6 to 26 on a five-year average. In Stage 3, the position of the frontal zone was not changed from Stage 2. However, the stagnating nature of fronts was not pronounced compared with Stage 2. The height field of the westerlies over the Eurasia Continent abruptly changed to the zonal pattern at the beginning of this Stage and a quasi-stationary trough was formed between 100°E and 150°E. The temperature gradient along the frontal zone over China strengthened (-10°C/1000km). This stage is the early period of the autumn season. The period was on a five-year average from September 27 to October 21. At the beginning of Stage 4, the frontal zone abruptly shifted southward to the southern part of Continental China and was located away from the Japanese Islands. At the same time, the subtropical jet stream suddenly shifted to the south of the Tibetan Plateau in its eastern part, and the northeast monsoon began in South India. This stage occurs in the late period of the autumn season.
The effect of advection and accumulation of cold air from surrounding mountain slopes on the nocturnal cooling at the bottom of basins was studied with the use of one-dimensional numerical model. In the model, horizontal advective cooling was parameterized by using the results of heat budget analyses and power law profiles of air temperature. Calculated results show that the nocturnal cooling in the basin, the depth of which is 500m for example, is about 15% (in winter) and about 25% (in summer) larger than that at flat terrains while it is about 40% (in winter) and about 90% (in summer) larger than that at the top of mountains or hills. These excesses of the nocturnal cooling in basins are due to the difference in downward long wave radiation which is caused by the advection and accumulation of cold air inside basins. This effect of cold air accumulation on the nocturnal cooling is remarkable in deep basins. On the other hand, the nocturnal cooling at the foot of a mountain or in valleys is small compared with that in basins. This can be explained by the difference in the amount of cooling due to the advection and accumulation of downslope cold air among those topographies. Calculated results agreed well with observed values.
As the formation of graupel is considered to be quite important in the mechanism of snowfall, it was studied by thin-section observations to investigate its generation and growth. Graupel embryos were classified into two types and the internal structures of graupel into three types, respectively. Each type was closely related to the meteorological conditions. Namely, embryo types were related to the temperature at the cloud base and the thickness of mixed cloud, while internal structure types were classified by the wet-bulb potential temperature in clouds and the temperature at the cloud base. By knowing the meteorological conditions, embryo types and internal structure types of graupel may be predicted.
Observations of the fine structure of clouds have been made with a ruby lidar at the Meteorological Research Institute (36.1°N, 140.1°E), Tsukuba, Japan. Case studies of depolarization ratios of middle and high level clouds are presented. The middle level clouds observed on 16 October 1984 and 10 May 1985 had high values of the depolarization ratio at the cloud base and low values at the center of the cloud layer where the backscattered signals of the parallel component were large. For cirrus clouds observed on 24 and 27 February 1987, depolarization ratios were greater than 10 percents through the cloud layers where the atmospheric temperature was below -40°C. On the other hand, cirrus clouds observed on 11 June 1987 showed variable vertical profiles of the depolarization ratios in the cloud layers where atmospheric temperatures were -20°C to -40°C. While there were regions of depolarization ratio of a few percents in the cloud layer, the layers with depolarization ratio greater than 10 percents lowered toward the cloud base with time. The rate of descent varied from about 0.74 to 1.28m/s. Depolarization ratios in high level clouds showed a tendency to increase when the atmospheric temperature decreased.
To develop a monitoring technique estimating the shortwave radiation absorbed in the ocean(Iwabs) from space, sensitivities of Iwabs, and the upward irradiance at the top of the atmosphere(It↑) to atmospheric and oceanic parameters are investigated in a model atmosphere-ocean system.These irradiances are computed using the doubling-adding method in the wavelength region ranging from 0.285μm to 5.0μm for the atmosphere-ocean model in the mid-latitude summer.Under no cloudiness, oceanic type aerosols are considered with their size distribution represented by the log-normal distribution.The ocean surface is simulated by many facets whose slopes are changed according to an isotropic Gaussian distribution with respect to the surface wind speed.The effect of whitecaps is also included in the model.Two types of hydrosols are considered: the one engages in of pure scattering and the other includes partial absorption.The present simulation tells us that (1) the atmospheric aerosols mostly affect Iwabs, and It↑;(2) water vapor and ozone in the atmosphere moderately affect Iwabs, but their effects might be evaluated using their spectroscopic absorption characteristics;(3) Purely scattering hydrosols affect Iwabs and It↑, especially in the case of the turbid condition;(4) The surface state of the ocean shows little effect on Iwabs, and It↑ if the solar zenith angle is less than about 60°.These results may be used for development of space techniques upon deriving Iwabs, with the aid of the model atmosphere-ocean system.Finally, to validate Iwabs, by ship, a sensitivity of radiation just above the ocean surface to atmospheric and oceanic parameters is also discussed.