Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
ISSN-L : 0026-1165
Volume 74 , Issue 6
Showing 1-15 articles out of 15 articles from the selected issue
  • Kyozo Ueyoshi, John O Roads, Francis Fujioka, Duane E. Stevens
    1996 Volume 74 Issue 6 Pages 723-744
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    On the island of Maui, the dispersion of air pollutants associated with the field burning of biomass is complicated because the persistent Maui Vortex in the lee of Haleakala tends to trap smoke making pollution concentration worse in the central valley. In the present study we describe the spatially as well as temporally continuous short-term climatology of the airflow in the central valley of Maui during a summer month under trade wind conditions, simulated by a high-resolution mesoscale model initialized and forced by large-scale objective analyses. These simulations are compared with station observations. Under persistent trade wind conditions, an eddy would form in the central valley, and would remain steady and stationary regardless of the time of the day with little indications of vortex shedding even if the Froude number Fr=U/NH<0.4, where U is the uniform flow speed, H the height of the obstacle, and N the Brunt-Väisälä frequency. However, vortex shedding could occur if the West Maui mountains were removed. Two major factors contributing to formation, maintenance, and steadiness of the Maui Vortex, solar heating and the accelerated northerly flow over the valley resulting from deflection of the trades by West Maui, are discussed. Suggestions are made for preferred synoptic conditions for field burning.
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  • Kozo Ninomiya, Junzo Fujimori, Takako Akiyama
    1996 Volume 74 Issue 6 Pages 745-761
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    Features of the polar air outbreak over the Japan Sea and the northwestern Pacific are studied for the 3-day period of 8-10 January 1987.
    The polar air outbreak from the Asian Continent occurred following the development of a synoptic-scale cyclone over the northwestern Pacific. A comma cloud originated under a polar trough in polar air streams rearward of the synoptic-scale cyclone. The comma cloud exhibited features of the secondary cold front as it developed. The polar air outbreak was intensified when the comma cloud induced an instant occlusion by interacting with the major frontal cloud band.
    Polar air protruded as a wedge of cold air from the Continent toward the southwestern portion of the occluded cyclone. A shear line associated with a cloud band formed between the cold westerly streams in the cold wedge and the relatively warm easterly streams in the northwestern portion of the cyclone. The local frontogenesis took place due to the horizontal differential thermal advection in the shear flow. In the mid-troposphere, a long narrow trough of west-east orientation elongated over the cold wedge in the lower troposphere.
    The meso-scale cyclogenesis occurred in association with the frontogenesis in the shear line. The meso-scale cyclone caused strong gusts of -25m/s in its mature stage. The northerly cold air outbreak was further intensified after the passage of the shear line.
    The present paper thus proposes the concept of the multi-scale features of the polar air outbreak, which will be important in the realization processes of the polar air outbreak over the eastern coast of the Asian continent and the northwestern Pacific.
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  • Kiyotaka Shibata, Osamu Uchino, Takehisa Kamiyama, M. Patrick McCormic ...
    1996 Volume 74 Issue 6 Pages 763-780
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The global distribution of the Pinatubo aerosol in the stratosphere is constructed for optical thickness at 1μm by combining satellite data and lidar data for one year, from June 1991 to June 1992, following the eruption. The distribution, averaged for every two months, is composed of the data from Stratospheric Aerosol and Gas Experiment II between about 70°N and 70°S and the data from other satellites and lidar measurements at high latitudes and in the tropical lower stratosphere. Radiative heating rate due to the Pinatubo aerosol is calculated combining this distribution at 1μm with the radiatioe properties of observed sulfate aerosols.
    Sulfate aerosol does not absorb substantially but only scatters solar radiation, thereby producing very weak warming of about 0.02Kday-1 in the upper stratosphere due to ozone absorption of aerosol-reflected radiation. Terrestrial radiation brings a warming of about 0.1Kday-1 to the aerosol layer in low and middle latitudes, and very weak cooling at high latitudes. The net effect is, then, brought about by terrestrial radiation.
    Stratospheric temperature change is also calculated using a fixed dynamical heating model. In low latitudes, the middle stratospheric temperature rises rapidly by about 2K during the first 40 days, following which the warming slows down and its axis moves northward to about 15°N. The largest warming of 3K appears in the axis latitude after 10 months and the second largest warming of 2.9K at 5°S after 12 months.
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  • Chiu Wai Yuen, Kaz Higuchi, Neil B. A. Trivett, Han-Ru Cho
    1996 Volume 74 Issue 6 Pages 781-795
    Published: December 25, 1996
    Released: September 15, 2009
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    A 3-dimensional dynamical regional atmospheric model was used to investigate the quantitative relationship between the physical processes of long range CO2 transport from the emission region in Eurasia, and the magnitude and duration of a CO2 anomaly measurement over the Canadian Arctic. We chose to simulate a positive CO2 anomaly episode of about 5ppm observed at Alert, located at the northern tip of Ellesmere Island, during early December of 1990. The simulation allowed us to examine the evolution of the spatial distribution of the CO2 anomaly field as it was being transported from the emission region in northern Europe to Siberia, and then across the Arctic Ocean to the Canadian Arctic. The anthropogenic CO2 took more than a week to reach eastern Siberia, and stayed there until a synoptic situation evolved to finally advect it across the pole to the Canadian side.
    With the industrial emission from the northern Eurasia as the only CO2 source, the model was able to simulate an intrusion of a CO2 anomaly field into the Canadian Arctic of little less than 2ppm. With the inclusion of land biospheric CO2 sources, the model was able to account for much of the remaining 3ppm.
    The positive CO2 anomaly observed at Alert on December 1 represents a “classical” case of a low-level transport of CO2 from Eurasia, via northern Siberia, to the Canadian Arctic during a winter season characterized by a very strong inversion in the lower troposphere.
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  • Kazuo Saito, Masataka Murakami, Takayo Matsuo, Hakaru Mizuno
    1996 Volume 74 Issue 6 Pages 797-813
    Published: December 25, 1996
    Released: September 15, 2009
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    Numerical experiments on the orographic snowfall over the mountainous region of northern Japan in winter are conducted using a 2-dimensional non-hydrostatic model with a cloud microphysical parameterization which predicts not only the mixing ratio of water species but also the number density of ice species. The orographic effect on the snowfall is investigated by sensitivity tests from cloud microphysical aspects.
    Modification of air-mass and formation of snow clouds over the Sea of Japan, enhancement of snowfall over the windward slope of the mountainous area and disappearance of clouds on the lee side are simulated by an experiment where a simplified orography and sea-land distribution of northern Japan is given. The snow clouds gradually increase their top heights from sea, off the west coast, to inland, and the number density of cloud ice increases with the decrease of cloud-top temperature. The influence of the differences of roughness and temperature between sea and land on the snowfall distribution is examined as subsidiary factors. According to comparative experiments, the difference of temperature between sea and land contributes to the enhancement of snowfall over the windward side of land area, while the difference of roughness does not have a substantial influence.
    Forced condensation is evaluated assuming that the air-mass is lifted up by the mountain, and precipitation efficiency is defined as the portion of the forced condensation which converts to precipitation over the land. In the sensitivity experiments on the mountain-top height, the precipitation amount over the land increases significantly when the mountain height exceeds the height of cloud base. The precipitation efficiency is about 40% when the mountain height is below 600m, while it reaches about 80% when the mountain height is greater than 1000m. On the other hand, in the warm-rain experiments, the precipitation amount as well as the precipitation efficiency are only about 1/3 those of the experiments with ice phase, because the greater part of the condensed cloud water goes to the lee of the mountain before it converts to rain. Sensitivity experiments where ice nucleation rates are changed over the land are conducted, and it is shown that the snowfall over the land decreases with the suppression of ice nucleation over the mountain, while it increases with the enhancement of ice nucleation over the land, even without mountain. These experimental results suggest that not only the forced condensation but also the natural seeding over the mountainous area-the ice nucleation by the decrease of the temperature in cloud due to the orographic ascent-plays an important rôle in the orographic enhancement of the snowfall over northern Japan.
    A seeding experiment in which the ice nucleation rates are enhanced over a specified zone in the Sea of Japan demonstrates the possibility of artificial modification of the snowfall.
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  • Seita Emori, Kanji Abe, Atusi Numaguti, Shigeki Mitsumoto
    1996 Volume 74 Issue 6 Pages 815-832
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The significance of runoff processes in the atmosphere-land water cycle is investigated using an atmospheric general circulation model with simplified boundary conditions. Two runoff schemes representing two distinct runoff mechanisms, the subsurface drainage runoff and the surface saturation runoff, are incorporated separately in the model, and the simulated results are compared.
    The sensitivity of the water cycle to the runoff scheme is significant in the regions of large runoff, namely the tropics, the rainy-season subtropics, and high latitudes with large snowmelt. The surface runoff scheme produces less runoff than the drainage runoff scheme when the ground is sufficiently wet and the precipitation (or snowmelt) is relatively small, and, conversely, more runoff when the ground is dry and the precipitation is large. This difference in runoff causes a systematic difference in ground wetness during the wet seasons. After that, this difference in ground wetness gradually decays due to the modification of evaporation and runoff. When the atmospheric feedback processes are neglected, the time scale of decay is estimated to be less than one month. With the atmospheric feedback, however, this time scale is significantly elongated and the difference in ground wetness persists for a few months. This modification is considered to be principally due to a strong feedback between evaporation and precipitation.
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  • Zeng-Zhen Hu, Tsuyoshi Nitta
    1996 Volume 74 Issue 6 Pages 833-844
    Published: December 25, 1996
    Released: September 15, 2009
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    Localization of interannual and decadal variations of summer rainfall in North China and India and seasonal mean Southern Oscillation Index (SOI) from 1891-1992, and the favored time scales of the significant correlations among them are investigated with the wavelet transform (WT) analysis method. Strong localization and non-stationary evolution in the interannual and decadal variations of the rainfall in North China and India are demonstrated. The dominating time scales in the rainfall variations in North China and India are mainly located in two time scale bands: shorter than 10 years, and 14-28 years. The correlations between the rainfall variations in North China and India are time-scale dependent. The significant positive correlations are concentrated in two time-scale bands: shorter than 7 years and longer than 14 years. The correlations are insignificant on time scales of 7-14 years.
    El Niño and Southern Oscillation (ENSO) cycle is a nonstationary process. The dominant time scales for the variation of SOI are shorter than 5-7 years, and the decadal variations are obvious in 1891-1915 and 1970-1992. There is strong interaction between the Indian summer monsoon and the ENSO cycle. The significant positive correlations are mainly focused on the time scales shorter than 24-30 years, and the significant negative correlations on time scales longer than 40 years in summer and winter SOI cases. The correlation between the rainfall variations in North China and the ENSO cycle in various seasons are less significant, more scattered and complex, but there are some similarities in the correlation pattern compared with that of India, especially in the shorter time scales.
    The calculation shows that the summer rainfall variations in North China and India at different time scales are related with different general circulation anomalous patterns in middle and high latitudes of Eurasia and the western Pacific. The correlation patterns over Eurasia and the western Pacific between the summer rainfall variations in North China or India and the geopotential height at 500hPa are similar to the correlation patterns between the geopotential height and the WT results of the rainfall on time scales of about 5 years, but different from those on time scales of about 11 years.
    Therefore, the similar behavior between the rainfall variations in North China and India may be caused by the similar associations between the rainfall variations and the ENSO cycle and the general circulation anomalies in middle and high latitudes over Eurasia and the western Pacific, and the difference may be related to the different character of these associations.
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  • Tomoaki Ose
    1996 Volume 74 Issue 6 Pages 845-866
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The atmospheric response to the regional snow mass anomalies in early spring over Tibet, Eastern Europe, and Siberia is compared by general circulation model ensemble experiments. The positive snow mass anomalies over Tibet produce the largest cooling anomalies in the atmosphere from spring to early summer. Almost no significant forcing anomalies are systematically formed by the snow mass anomalies over Eastern Europe and Siberia.
    The model experiments show that the cooling source over Tibet works significantly to delay the seasonal transition from spring to summer in the Northern Hemisphere. It is confirmed by the simulated weak Asian monsoon; a weak lower tropospheric monsoon jet in South Asia, a weak large-scale divergence center at the upper troposphere in Southeast Asia, negative lower-layer geopotential height anomalies in the North Pacific and the North Atlantic, and the weak Walker circulation in the equatorial Pacific are simulated.
    The above noticeable response of the atmosphere to the snow mass anomalies over Tibet is associated with the following characteristics of Tibet as compared with Eastern Europe and Siberia;
    (1) The small snow melt speed over Tibet, probably due to its high elevation, maintains the early spring additional snow mass until almost the end of the climatic snow-melt season.
    (2) The snow albedo is effective because of large solar incidence and relatively small cloudiness over Tibet.
    (3) The existence of the anomalous snow cover works to cut the upward sensible heat flux rather than the evaporation over Tibet because of its dry ground condition.
    (4) The dry ground over Tibet makes it possible that all the anomalous snow-melt water is not drained as runoff, but a part of it is stored in the ground.
    (5) As pointed out by many researchers, the Tibetan Plateau plays an important rôle in the establishment of the Asian monsoon.
    The above conditions (1)-(5) for the effective snow mass anomalies are found in the model Tibet, but these conditions would be satisfied by the real regions where the snow mass anomalies affect the atmosphere efficiently.
    The extended atmospheric responses are found in May for the East European case and in August for the Siberian case. These are accompanied by significant ground condition anomalies in northern Eurasia. Those ground condition anomalies are not directly related to the successive ground condition anomalies followed by the anomalous snow mass melt, and seem to be created after the water and heat of the initial snow mass anomalies are supplied into the atmosphere.
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  • Ryuichi Kawamura, Takio Murakami, Bin Wang
    1996 Volume 74 Issue 6 Pages 867-890
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The tropical Madden-Julian oscillation (MJO), which is of convective origin, systematically propagates northward in the Western North Pacific summer monsoon region (5°-20°N, 110°-160°E). The northward propagating MJO, which is statistically significant, displays strong horizontally and vertically asymmetric circulation. The low-level cyclonic response is pronounced due to the presence of convergent, cyclonically sheared mean monsoon flow, while the upper-level anticyclonic counterpart is weak due to the influence of the divergent, anticyclonically sheared upper-level mean monsoon flow. The upper tropospheric MJO response is largely associated with divergent winds, as manifested by strong equatorward outflows that export anticyclonic vorticity into the upper troposphere of the Southern Hemisphere. The summer mean monsoon flow is responsible for reinforcing the vertical asymmetry of the MJO circulation induced by convective heating, and for generating a barotropic component in the baroclinic MJO through the mean monsoon flow-MJO interaction.
    The mid-latitude intraseasonal oscillation (ISO) displays a barotropic wave train structure along a great circle traversing the North Pacific, and exhibits the largest variability in the westerly jet exit region of the North Pacific (40°-50°N, 180°-150°W). This phenomenon is attributed to the combination of two different dynamic processes. While the MJO is propagating northward, the barotropic MJO component generated by the mean monsoon flow-MJO interaction produces a significant tropical-extratropical interaction on the intraseasonal time scale. Namely, when the 45-day convective activity becomes strongest near the Philippines, the barotropic MJO component acts as an origin of the barotropic Rossby wave dispersion emanating out of the convective forcing, eventually contributing to the development of an extratropical ISO. The enstrophy budget analysis shows, on the other hand, that the largest contribution to ISO is the in situ barotropic mean flow-ISO interaction in and around the jet exit.
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  • Jun Du, Han-Ru Cho
    1996 Volume 74 Issue 6 Pages 891-908
    Published: December 25, 1996
    Released: September 15, 2009
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    It is proposed that the mesoscale convective systems along Baiu (called Mei-Yu in China) fronts are caused by instability of the low-level potential vorticity maximum along the fronts. The growth rate of the most unstable wave depends on the intensity of cumulus heating, with the e-folding time of the order of one day. When the cumulus heating parameter is below a critical value, the wavelength is about 8-15 times the cross-front width scale of the PV anomaly, and the structure of the wave is of the barotropic type but modified by convection. When the heating parameter is above the critical value, the disturbance draws its energy almost entirely from heating and the structure of the wave resembles a system driven purely by cumulus heating. The wavelength of the most unstable wave is about 1700-2100km and bears little relationship to the width of the background PV anomaly.
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  • H. L. Tanaka, Kazuo Kimura, Tetsuzo Yasunari
    1996 Volume 74 Issue 6 Pages 909-921
    Published: December 25, 1996
    Released: September 15, 2009
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    In this study, a long-term (1000 years) integration of a simple barotropic primitive equation model was carried out to investigate the typical magnitude and spectral features of natural variability of the model atmosphere.
    The first experiment without an annual-cycle forcing shows no noticeable ultra-low-frequency variability. The frequency spectrum of the model atmosphere is characterized as a white noise for the low-frequency range beyond the period about 50 days. The spectrum then shifts sharply to a red noise for the period shorter than 50 days, indicating a characteristic -3 power slope over the frequency domain. Although no noticeable spectral peak is detected, we can find intraseasonal variability with a period of about 50 days in the time series of the model atmosphere as a result from the sharp transition from the red to white noise. Since the sole energy source of the system is a parameterized baroclinic instability of frequency about (5-day)-1, we must have a reverse energy cascade from higher- to lower-frequency ranges along the -3 power slope of the red noise spectrum. It is discussed in this study that the spectrum tends to be red over the high-frequency range beyond (50-day)-1 where a linear relation holds between life-time and spatial scales for prominent atmospheric phenomena. Beyond this period, the internal non-linear dynamics of the primitive equation can not sustain large energy because the spatial scale of the Earth is finite. As a result, the very-low-frequency variability results in the white noise spectrum.
    Next, the same simple model is integrated with an annual-cycle forcing for 100 years to investigate the excitation of harmonics and subharmonics expected by the non-linear dynamic modulation of the annualcycle forcing. The results show, however, that the spectral features are not altered by the inclusion of the annual cycle, except for the isolated spectral peak associated with the annual-cycle forcing. We suggest from the results of this study that the harmonics and subharmonics, such as semiannual and biennial oscillations, are not excited solely by the non-linear dynamic modulation of the forced annual cycle in the atmosphere.
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  • Masahiro Watanabe, Masato Shinoda
    1996 Volume 74 Issue 6 Pages 923-934
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    An observational study has been made of the long-term variability of Asian summer monsoon rainfall for the period 1946-1988 and its relationship to global sea-surface temperatures (SSTs) and geopotential height fields.
    The trend analysis of June-September rainfall for the recent four decades shows a distinct long-term trend in rainfall over several Asian regions; increasing trends in southwest India, and decreasing trends in northwest and central India, on the south side of the Himalayas, and in the western Indochina peninsula facing the Bay of Bengal.
    A rotated empirical orthogonal function (R-EOF) analysis was applied to global SSTs for the above 43-year period (Shinoda and Kawamura, 1994). The second R-EOF (R-EOF2) mode exhibits the warming trend of the Indian Ocean. The correlation analysis between the time coefficients of the R-EOF2 mode and monsoon rainfall shows that the trend components which are observed in both the R-EOF2 mode and rainfall for the above areas are responsible for the high correlation between them.
    The R-EOF2 mode is highly correlated not only with monsoon rainfall, but also with tropical 850hPa geopotential heights. This correlation pattern exhibits an east-west contrast having positive correlations over Africa and the western Pacific Ocean and negative correlations over the northern Indian Ocean. The above positive and negative correlations coincide with the increasing and decreasing trends in 850hPa heights.
    Over the Indian subcontinent, areas having negative and positive trends of 850hPa heights coincide with those having increasing and decreasing trends in régional rainfall, respectively. On a monthly basis, for June and August, the Indian subcontinent is extensively covered with areas having increasing trends in rainfall and decreasing trends in 850hPa heights, whereas the areas are limited to southwest India during July and September.
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  • Hirotada Kanehisa
    1996 Volume 74 Issue 6 Pages 935-939
    Published: December 25, 1996
    Released: September 15, 2009
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    An equation of the β-gyre rotation of a cyclone in a non-uniformly straining environmental flow is derived in a barotropic non-divergent system. The environmental flow, which is generally both zonally and meridionally varying, is assumed to be a quadratic function of coordinates. The total flow is assumed to consist of the environmental flow, a symmetric cyclonic circulation and a flow of β-gyres. The resulting equation says the followings. The β effect forces the β-gyre centres to lie perpendicular to the direction of β, i. e., of the gradient of the Coriolis parameter. The symmetric cyclonic circulation advects the β-gyres counter-clockwise. The deformational component of the environmental flow forces the β-gyres centres to lie along the dilatation axis of the defomation matrix. The rotational component of the environmental flow rotates the β-gyres with an angular velocity equal to half the environmental relative vorticity. The second derivative of the environmental flow affects the β-gyre rotation only through the relative vorticity gradient, and the other components of the second derivative have no influence on the rotation.
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  • Yuji Ohsaki, Yoshihisa Masuda
    1996 Volume 74 Issue 6 Pages 941-945
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The wind profiler developed by the Communications Research Laboratory and operating at the Okinawa Radio Observatory is a Doppler radar for measuring the vertical profile of the three-dimensional wind vectors from the echo signal scattered by clear air turbulence. Vegetation motion caused by wind, however, will produce signals cluttering the Doppler spectrum and making it difficult to identify the echo signal in the spectrum. A software-based clutter cancellation method was therefore developed, and its effectiveness has been confirmed by comparing the horizontal wind vectors estimated from the clutter-cancelled wind profiler data with those measured from radiosonde.
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  • H. L. Tanaka, Ritsuko Kanohgi, Tetsuzo Yasunari
    1996 Volume 74 Issue 6 Pages 947-954
    Published: December 25, 1996
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    Walsh (1994) reported that the surface vorticity in the Central Arctic has changed its sign from negative to positive since 1988. This study is devoted to confirm the recent abrupt change in the Arctic documented by Walsh. The analysis is extended not only to the surface but also to the upper air circulation to find the vertical structure in the vorticity change.
    As a result, the decreasing trend in geopotential height since 1988 is confirmed for the polar vortex throughout the troposphere. The vorticity change is thus regarded as barotropic in its structure. The surface Beaufort high has weakened associated with the strengthened upper air vortex. It is anticipated from the result that the wind-driven Beaufort gyre might undergo significant deformation, which would lead to a drastic impact on the export of sea ice to the North Atlantic Ocean through Fram Strait.
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