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61.
Fumie Murata Toru Terao Taiichi Hayashi Haruhisa Asada Jun Matsumoto 《Natural Hazards》2008,44(3):399-410
To improve flood forecasting, the understanding of the atmospheric conditions associated with severe rainfall is crucial.
We analysed the atmospheric conditions at Dhaka, Bangladesh, using upper-air soundings. We then compared these conditions
with daily rainfall variations at Cherrapunjee, India, which is a main source of floodwater to Bangladesh, and a representative
sample of exceptionally heavy rainfall events. The analysis focussed on June and July 2004. June and July are the heaviest
rainfall months of the year at Cherrapunjee. July 2004 had the fourth-heaviest monthly rainfall of the past 31 years, and
severe floods occurred in Bangladesh. Active rainfall periods at Cherrapunjee corresponded to “breaks” in the Indian monsoon.
The monsoon trough was located over the Himalayan foothills, and strong westerly winds dominated up to 7 km at Dhaka. Near-surface
wind below 1 km had southerly components, and the wind profile had an Ekman spiral structure. The results suggest that rainfall
at Cherrapunjee strongly depends on the near-surface wind speed and wind direction at Dhaka. Lifting of the near-surface southerly
airflow by the Meghalaya Plateau is considered to be the main contributor to severe rainfall at Cherrapunjee. High convective
available potential energy (CAPE) also contributes to intense rainfall. 相似文献
62.
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
63.
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
64.
Toru OuchiDepartment of Earth Sciences Faculty of Science Kobe University Kobe Japan 《地震学报(英文版)》1997,(6)
ThebvaluespectrumandηvaluebythemomentmethodToruOuchi(大内)DepartmentofEarthSciences,FacultyofScience,KobeUniversity,Kobe,657,... 相似文献
65.
Minoru Kitamura Toru Kobari Makio C. Honda Kazuhiko Matsumoto Kosei Sasaoka Rie Nakamura Kazuyuki Tanabe 《Journal of Oceanography》2016,72(3):387-402
Seasonal changes in mesozooplankton biomass and their community structures were observed at time-series stations K2 (subarctic) and S1 (subtropical) in the western North Pacific Ocean. At K2, the maximum biomass was observed during the spring when primary productivity was still low. The annual mean biomasses in the euphotic and 200- to 1000-m layers were 1.39 (day) and 2.49 (night) g C m?2 and 4.00 (day) and 3.63 (night) g C m?2, respectively. Mesozooplankton vertical distribution was bimodal and mesopelagic peak was observed in a 200- to 300-m layer; it mainly comprised dormant copepods. Copepods predominated in most sampling layers, but euphausiids were dominant at the surface during the night. At S1, the maximum biomass was observed during the spring and the peak timing of biomass followed those of chlorophyll a and primary productivity. The annual mean biomasses in the euphotic and 200- to 1000-m layers were 0.10 (day) and 0.21 (night) g C m?2 and 0.47 (day) and 0.26 (night) g C m?2, respectively. Copepods were dominant in most sampling layers, but their mean proportion was lower than that in K2. Mesozooplankton community characteristics at both sites were compared with those at other time-series stations in the North Pacific and with each other. The annual mean primary productivities and sinking POC fluxes were equivalent at both sites; however, mesozooplankton biomasses were higher at K2 than at S1. The difference of biomasses was probably caused by differences of individual carbon losses, population turnover rates, and trophic structures of communities between the two sites. 相似文献
66.
Peter G. Brewer Edward Peltzer Izuo Aya Peter Haugan Richard Bellerby Kenji Yamane Ryuji Kojima Peter Walz Yasuharu Nakajima 《Journal of Oceanography》2004,60(4):751-758
We have carried out a small-scale (∼20 l) CO2 sequestration experiment off northern California (684 m depth, ∼5°C, background ocean pH ∼7.7) designed as an initial investigation
of the effects of physical forcing of the fluid, and the problem of sensing the formation of a low pH plume. The buoyant CO2 was contained in a square frame 1.2 m high, exposing 0.21 m2 to ocean flow. Two pH electrodes attached to the frame recorded the signal; a second frame placed 1.9 m south of the CO2 pool was also equipped with two recording pH electrodes. An additional pH electrode was held in the ROV robotic arm to probe
the fluid interface. Local water velocities of up to 40 cm sec−1 were encountered, creating significant eddies within the CO2 box, and forcing wavelets at the fluid interface. This resulted in rapid CO2 dissolution, with all CO2 being depleted in a little more than 2 days. The pH record from the sensor closest (∼10 cm) to the CO2 showed many spikes of low pH water, the extreme value being ∼5.9. The sensor 1 m immediately below this showed no detectable
response. The electrodes placed 1.9 m distant from the source also recorded very small perturbations. The results provide
important clues for the design of future experiments for CO2 disposal and biogeochemical impact studies. These include the need for dealing with the slow CO2 hydration kinetics, better understanding of the fluid dynamics of the CO2-water interface, and non-point source release designs to provide more constant, controlled local CO2 enrichments within the experimental area.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
67.
Naoto Ebuchi Yasushi Fukamachi Kay I. Ohshima Kunio Shirasawa Masao Ishikawa Toru Takatsuka Takaharu Daibo Masaaki Wakatsuchi 《Journal of Oceanography》2006,62(1):47-61
Three High Frequency (HF) ocean radar stations were installed around the Soya/La Perouse Strait in the Sea of Okhotsk in order
to monitor the Soya Warm Current (SWC). The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are
3 km and 5 deg., respectively. The radar covers a range of approximately 70 km from the coast. The surface current velocity
observed by the HF radars was compared with data from drifting buoys and shipboard Acoustic Doppler Current Profilers (ADCPs).
The current velocity derived from the HF radars shows good agreement with that observed using the drifting buoys. The root-mean-square
(rms) differences were found to be less than 20 cm s−1 for the zonal and meridional components in the buoy comparison. The observed current velocity was also found to exhibit reasonable
agreement with the shipboard ADCP data. It was shown that the HF radars clearly capture seasonal and short-term variations
of the SWC. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s−1, in summer and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately
40 km. The surface transport by the SWC shows a significant correlation with the sea level difference along the strait, as
derived from coastal tide gauge records at Wakkanai and Abashiri.
Deceased. 相似文献
68.
Fifteen day records of current velocity, water temperature and salinity were obtained at a coastal station in the Seto Inland Sea. It was observed that water movement was characterized by an exchange of two distinctive water masses east and west of the station. Cross-correlation analysis shows that, at the semi-diurnal period, the motion of these characteristic waters causes temperature and salinity variations. At frequencies higher than 2 cycles per day where power spectra of current velocity have a slope of ?5/3, coherence between current velocity and both water temperature and salinity decreases rapidly. 相似文献
69.
Monitoring of position of the Kuroshio axis in the Tokara Strait using sea level data 总被引:1,自引:0,他引:1
Properties of the index of position of the Kuroshio axis in the Tokara Strait, named the Kuroshio position index (KPI), were examined using sea-level data during 1984–92. The index is KPI=(X+M
x
)/(Y+M
y
whereX(Y) is the anomaly of sea-level difference of Nakanoshima (Naze) minus Nishinoomote from the 1984–92 meanM
x
(M
y
). The correlation with the latitude of the Kuroshio axis in the Tokara Strait concluded that the KPI withM
x
/M
y
=0.83 and realisticM
y
(100±40 cm) best indicates the position of the Kuroshio axis in the strait. The KPI withM
x
=83 cm andM
y
=100 cm was newly called the KPI as the best index. Using daily values of this KPI, the relation between the position of the Kuroshio in the strait and the large meander of the Kuroshio shown by Kawabe (1995) was confirmed and studied in detail. A large meander forms (ends) 3.3 (5.1) months after a northward (southward) shift of the Kuroshio in the Tokara Strait. Yet, a temporary southward shift with a duration of ten to twenty days does not finish the large-meander (LM) path. At the LM formation, a small meander southeast of Kyushu begins to move eastward associated with the northward shift. The processes of LM formation and decay are started by the meridional move of the Kuroshio axis in the Tokara Strait. The Kuroshio axis at the FES line during the LM path is located farther north by 7 latitude than that during the non-large-meander (NLM) path. The latitude during the LM formation (decay) stage is a little higher (lower) than that during the LM (NLM) period, though the Kuroshio still takes an NLM (LM) path. 相似文献
70.
Functional roles of interzonal migrating mesozooplankton in the western subarctic Pacific 总被引:1,自引:0,他引:1
Grazing experiments and production estimation based on life-history analysis of Neocalanus copepods (N. cristatus, N. plumchrus and N. flemingeri) were carried out in the Oyashio region to understand the carbon flows associated with the interzonal migrating copepods. These copepods, and also Eucalanus bungii, fed on nano- and micro-sized organisms non-selectively throughout the season. However, diatoms were the dominant food resource until May and organisms, such as ciliates were the major resource after May. Daily growth rate was estimated from the Ikeda–Motoda, Huntley–Lopez and Hirst–Sheader models. Since the growth rates were considered to be overestimates for the Huntley–Lopez model and underestimates for the other two models, we applied the weight-specific growth rates previously reported for these species in the Bering Shelf. Surface biomass of Neocalanus increased rapidly in June during the appearance of C5, and a successive increase of overwintering stock was evident in the deeper layer. The deep biomass decreased gradually from September to May during the dormant and reproduction period. N. cristatus has the largest annual mean biomass (2.3 gC m−2), followed by N. plumchrus (1.1) and N. flemingeri (0.4). Daily production rate of Neocalanus varied from 0.4 to 363.4 mgC m−2 day−1, to which N. cristatus was the largest contributor. Annual production was estimated as 11.5 gC m−2 year−1 for N. cristatus, 5.7 for N. plumchrus and 2.1 for N. flemingeri, yielding annual P/B ratio of 5 for each species. The annual production of Neocalanus accounted for 13.2% of the primary production in the Oyashio region. Their fecal pellets were estimated to account for 14.9% (0.7 gC m−2 year−1) of sinking flux of organic carbon at 1000-m depth. Moreover, their export flux by ontogenetic vertical migration, which is not measured by sediment trap observations, is estimated to be 91.5% (4.3 gC m−2 year−1) of carbon flux of sinking particles at 1000-m depth. These results suggest the important role of interzonal migrating copepods in the export flux of carbon. 相似文献