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71.
Meso-scale characteristics of disturbances that bring about atmospheric disasters in pre- and mature monsoon seasons in Bangladesh
are analyzed. Several types of meteorological instruments capable of observations with high temporal and spatial resolutions
were introduced for the first time in this area to capture the meso-scale structure of rainfall systems. We installed an automatic
weather station (AWS) and several automatic raingauges (ARGs) and utilized the weather radar of Bangladesh Meteorological
Department (BMD). From the radar image in the summer of 2001 (16–18 July), a striking feature of the systematic diurnal variation
in this area was elucidated. In these 3 days, the diurnal evolutions of convective activity were remarkably similar to each
other, implying that this pattern can be understood as a typical response of local cloud systems to the diurnal variation
of insolation under some summer monsoon situations. The ARG data show the difference in characteristics of rainfall between
pre- and mature monsoon seasons. The short intense downpour tends to occur more frequently in the pre-monsoon season than
in the mature monsoon season. The pre-monsoon rainfall also has clear diurnal variation with a peak that is more strongly
concentrated in time. In the northern part the rainfall peak is found in between midnight and early morning, while it is observed
in the daytime in central to western parts of the country. Two disaster cases caused by meso-scale disturbances are analyzed.
Although they occurred in the same season, the structures of the cloud systems were largely different from each other. The
disturbance brought about tornadoes on 14 April 2004, consisting of many spherical cloud systems of approximately 20 km size.
On the other hand, another one that caused the tragic river water transport accident on 23 May 2004 had meso-scale rain band
structure. The latter case was captured by the AWS located at Dhaka. Sudden changes in temperature, wind and pressure were
observed clearly, showing the typical structure of convective rain bands. 相似文献
72.
Hidekazu Yoshida Koshi Yamamoto Yuki Murakami Nagayoshi Katsuta Toru Hayashi Takeshi Naganuma 《Environmental Geology》2008,55(6):1363-1374
Takashikozo is a phenomenon of Quaternary sediments in Japan. They are cylindrical Fe-oxyhydroxide nodules that form as plaques round
plant roots, where Fe is preferentially concentrated to develop a solid wall. Structural features suggest that after the roots
have decayed, the central space where the roots were situated acts as a flow path for oxidized water. Analysis of microbial
16S rDNA extracted from the nodules identified iron-oxidizing bacteria encrusted round the roots where they are the likely
initiators of nodule formation. Direct microscopic observation revealed an accumulation of Fe-oxyhydroxides that fill the
pore spaces and is also likely to be linked with the encrusting microbial colonies. Geological history and nanofossil evidence
suggest that these Fe-nodules may have been buried at a depth of up to several tens of meters for at least 105 years in reducing Quaternary sediments. Thus Fe-oxyhydroxide nodules that have formed in a geological environment at the
interfaces between water and rock by microbial mediation can persist under reducing conditions. If this is the case, the phenomenon
is significant as an analogue of post-closure conditions in radioactive waste repositories, since it could influence nuclide
migration. 相似文献
73.
Advanced argillic (AA) alteration is developed over a vertical interval of 500 m, above (and enclosing) Late Devonian quartz
monzodiorite intrusions that accompany porphyry-style Cu–Au mineralization at the Hugo Dummett deposit. The AA alteration
is mainly in basaltic rocks and locally extends into the overlying dacitic ash-flow tuff for about 100 m. The AA zone overprints
porphyry-style quartz veins associated with quartz monzodiorite intrusions, but at least partly precedes high-grade porphyry-style
bornite mineralization. Mineralogically, it consists of andalusite, corundum, residual quartz, titanium oxides, diaspore,
alunite, aluminum phosphate-sulfate (APS) minerals, zunyite, pyrophyllite, topaz, kaolinite, and dickite, as well as anhydrite
and gypsum, but is dominated by residual quartz and pyrophyllite. Alteration zonation is not apparent, except for an alunite-bearing
zone that occurs approximately at the limit of strong quartz veining. Whole-rock geochemistry shows that the AA alteration
removes most major elements except Si, Al, Ti, and P, and removes the trace elements Sc, Cs, and Rb. V, Zr, Hf, Nb, Ta, U,
and Th are relatively immobile, whilst light REEs (La to Nd), Sr, Ba, and Ga can be enriched. Middle REEs (Sm to Gd) are moderately
depleted; Y and heavy REEs (Tb to Lu) are strongly depleted except in two unusual samples where middle to heavy REEs are enriched. 相似文献
74.
An artificial cloud seeding experiment was performed over the Japan Sea in winter to show how massive seeding could be effective to mitigate heavy snowfall damage. The results showed that 20 min after cloud seeding, a portion of the radar echo beneath the seeding track was weakened to divide the radar echo into two parts. In order to analyze the results, a numerical simulation was conducted by using the Weather Research and Forecasting model verion 3.5.1. In this simulation, the seeding effects were represented as phenomena capable of changing rain particles by accreting cloud ice and snow to form graupel particles and by changing cloud liquid water to snow particles. The graupel particles fell rapidly, thus temporarily intensifying the rainfall, which subsequently decreased. Therefore, the weakened radar echo in the field experiment is deemed to have been caused by the increase in rapidly falling graupel particles. 相似文献
75.
Variations of water and flow in Sagami Bay in relation to the Kuroshio path variations are examined by using 100m-depth temperature and salinity data from 25 stations as well as sea level data from five stations (Minami-Izu, ItÔ, Ôshima, Aburatsubo, Mera). In regard to temperature, anomalies from the mean seasonal variations are used. Results show that water properties are clearly different between the three typical paths of the Kuroshio. The difference is more remarkable in temperature than in salinity; temperature is higher during the typical large-eander (LM) path, and lower during the offshore non-large-meander (NLM) path, compared with the nearshore NLM path. Temperature anomaly and salinity distributions, as well as the Ôshima minus Minami-Izu and Ôshima minus Mera sea-level differences strongly suggest that the flows during the typical LM path are distributed as hitherto described in past studies, that is, water in the mouth region of the bay flows clockwise around Ôshima from the west channel to the east channel, and a counterclockwise eddy exists in the interior. On the other hand, flows during the nearshore and offshore NLM paths seem to be quite different from those during the typical LM path; velocities are very weak, and the directions of circulation is frequently reversed. This tendency also can be seen during parts of LM period in which the Kuroshio takes a non-typical LM path.Water properties in Sagami Bay are most characteristic during transitions between nearshore and offshore NLM paths. During transitions from nearshore to offshore NLM paths, temperatures are extremely high as a whole in the bay, while during reverse transitions, both temperatures and salinities are very low in the entire region. 相似文献
76.
Sea level changes south of Japan associated with the non-large-meander path of the Kuroshio 总被引:1,自引:0,他引:1
Masaki Kawabe 《Journal of Oceanography》1989,45(3):181-189
Sea levels south of Japan from 1964 to 1975 are examined in terms of the nearshore and offshore non-large-meander (NLM) paths of the Kuroshio and the transitions between them.The sea-level anomalies from the annual variations on the south coast of Japan are much larger during the transition from the nearshore to offshore NLM paths than during the reverse transition by 9 cm on average. This characteristic can be seen only in the coastal region of the Kuroshio-flowing area, so that the sea-level difference of Naze minus Nishinoomote (indicator of Kuroshio velocity) during the offshore to nearshore transition is larger by 15 cm than during the reverse transition.The transition from the offshore to nearshore NLM paths occurs when the velocity of the Kuroshio is large or increasing, while the nearshore to offshore transition occurs when it is small or decreasing. The former transition occurs whenever the velocity increases greatly, whereas the latter one does not always occur even though the velocity decreases.The sea-level difference between Kushimoto and Uragami is highly coherent with the alternate appearance of the nearshore and offshore NLM paths. Offshore NLM paths longer than 2.5 months appear during large falls of the sea-level difference of Kushimoto minus Uragami, while large rises of the sea-level difference correspond to long-lasting nearshore NLM paths. The mean sea-level difference during the nearshore NLM path is larger by 4 cm than that during the offshore NLM path. 相似文献
77.
Variability of Kuroshio velocity assessed from the sea-level difference between Naze and Nishinoomote 总被引:1,自引:0,他引:1
Masaki Kawabe 《Journal of Oceanography》1988,44(6):293-304
Variations of current velocity of the Kuroshio are examined using the 1965–1983 sea-level difference between Naze and Nishinoomote, located on the offshore and onshore sides of the Kuroshio in the Tokara Strait south of Kyûshû.Interannual variations of Kuroshio velocity are large, especially at periods longer than five years and around 2.1 years. They are almost determined by those of sea level on the offshore side of the Kuroshio. They are highly coherent with the offshore sea level at periods longer than 1.7 years, and incoherent with the onshore sea level at periods longer than 2.8 years.The mean seasonal variation averaged for 19 years is at its maximum in July and at its minimum in the second half of October, with a sharp decrease in August and September. However, such a variation does not repeat every year. Amplitude, dominant period and phase are greatly different by year, and they can be roughly divided into four groups: small-amplitude group, semiannual-period group, and two annual-period groups with different phases. The only feature found in almost all years is a weak velocity from September to December.The amplitude of seasonal variation tends to be large in the formation years of the large meander (LM) of the Kuroshio and small during the LM period. It is also large in the years preceding El Niño, and diminishes remarkably in El Niño years.Kuroshio velocity in the Tokara Strait is incoherent with position of the Kuroshio axis over the Izu Ridge, but highly coherent with 70-day variations of coastal sea levels which are dominant during the LM period. 相似文献
78.
The barotropic instability of a boundary jet on a beta plane is considered with emphasis on the effect of internal viscosity.
An eigenvalue problem for the disturbance equations and its inviscid version are solved by the aid of numerical methods, and
instability characteristics are determined as functions of the Reynolds numberR for various values of the beta-parameter. Typical disturbance structures (eigenfunctions) are also computed.
Numerical examples show that the minimum critical Reynolds numberR
cr
for instability is smaller than 100. At a Reynolds number of the order of hundreds, there appears a second mode of instability
in addition to the first unstable mode originating atR
cr
; a kind of ‘resonance’ between the first and second eigenvalues occurs at the particular value ofR. The neutral stability curves are accordingly multi-looped. Although each of the two unstable modes asymptotically approaches
its inviscid counterpart asR→∞, the asymptotic approach to the inviscid limit is rather slow and the effect of varyingR is conspicuous even atR∼O (104). It is thus demonstrated that the Reynolds number is an essential stability parameter for real boundary jets.
The main part of the material contained in this paper was presented at 1981-Autumn Assembly of the Oceanographical Society
of Japan. 相似文献
79.
Index species of zooplankton of the Oyashio water are found in and beneath the salinity minimum layer in Sagami Bay. In order to clarify the intrusion path of the intermediate Oyashio Water (or the water of the Mixed Water Region), the oceanographic conditions off the Boso Peninsula are studied by using available hydrographic data obtained mainly by Japan Meteorological Agency. The cross-sectional salinity distribution along KJ line which extends southeastward from off the tip of the peninsula always indicates the existence of a low salinity patch just off the coast in the salinity minimum layer. This water is well separated from the offshore low salinity water which is considered as the water in the western margin of the so-called North Pacific Intermediate Water. We refer to the former water as the coastal salinity-minimum-layer (SML) water and to the latter as the offshore SML water. The coastal SML water is usually bounded by the current zone of the Kuroshio. The existence of the coastal SML water seems to indicate the possible pathway of the intermediate Oyashio water along the Boso Peninsula into Sagami Bay. The detailed water type analysis is made in T-S plane, S-st plane, and O2-st plane. There is no significant difference in distribution ranges of the water types between the coastal SML water and the offshore SML water. However, the water types of the coastal SML water is not uniformly distributed, and the water can be classified into two groups: group A with relatively high oxygen content and relatively low salinity value and group B with relatively low oxygen content and relatively high salinity value. Group A is thought to be associated with strong event-like intrusions, the details of which will be discussed in Part II. 相似文献
80.
A series of batch tests have been presented to investigate the effects of solid-solution contact time, solid:solution ratio, and pH of solution on sorption of Cd2+ and Pb2+ on Ariake clay from Kyushu region of Japan. The results show that, among the three investigated factors, solid:solution ratio (SD/SN) seems to be the crucial controlling factor. The Freundlich sorption parameters determined from the batch tests were applied to a typical field landfill in which the Ariake clay was used as soil barrier. The impacts of the landfill were assessed by numerical analysis. Based on the analysis result, the significance of selecting proper solid:solution ratio for determining sorption parameters from batch test has been discussed. 相似文献