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11.
Koji Matsuki Katsuya Nakatani Takashi Arai Kazuo Ohmura Ryuji Takeuchi Yasushi Arai Shinji Takeuchi 《Journal of Hydrology》2008,360(1-4):217-229
By modifying a previous method with constant elements, we developed a quadratic element method for more accurately estimating groundwater flow by the inversion of tilt data. In this method: (1) a region of groundwater flow is divided into quadratic elements in which the change in groundwater volume per unit volume of rock (Δv) and the Skempton coefficient (B) vary in a quadratic manner with the coordinates, (2) the values of Δv are set to zero at the boundaries of the region of groundwater flow and (3) the sum of the squared second derivatives of Δv is adopted as a constraining condition that is weighted and added to the sum of the squared errors in tilt. First, analyses were performed for a flow model to determine the accuracy of this method for estimating groundwater flow and also to clarify the effect of the assumed size of a region of groundwater flow. These analyses showed that the quadratic element method proposed in this study gives a much better estimation of Δv than the constant element method and that a large region of groundwater flow should be assumed, rather than a small region, since the values of Δv at points outside of the actual region of groundwater flow are estimated to be nearly zero when a large region is assumed while these values are greatly overestimated when an excessively small region is assumed. Finally, the quadratic element method was applied to the site of the Mizunami Underground Research Laboratory in the Tono area, Japan. Inverse analyses were performed for tilt data measured by four tiltmeters with a resolution of 10−9 radians during the excavation of two shafts under the assumption that the rock mass is an isotropic and homogeneous half- space. The results showed that the method proposed in this study reproduced the tilt data very accurately. Thus, the distribution of Δv was estimated without sacrificing the reproducibility of the tilt data. The contour maps of B(1 + ν)Δv (ν: Poisson’s ratio) showed that the heterogeneous flow of groundwater occurred at the site and that groundwater volume decreased mainly in the area surrounded by two faults. The latter result is consistent with the finding obtained by previous investigations that these faults have low permeability in the direction perpendicular to the strike and may act as a flow barrier. 相似文献
12.
Annual cycles of monthly albedos simulated with a general circulation model (GCM) are compared with surface observations.
The data observed at 35 stations are retrieved from the Global Energy Balance Archive (GEBA) and drawn from the soil moisture
and meteorological observations in the former Soviet Union. The model data are obtained with the ECHAM4 GCM in a ten-year
simulation of the present-day climate at T106 resolution. The model calculated albedo values are modified before they are
compared with the surface observations: They are interpolated to the stations and adjusted to account for altitude differences
and fractional forest area. During the snow-free period, the model underestimates the albedo by up to 0.05 at the stations
(with values between 0.2 and 0.25 measured over short grass) because the albedo for grassland is too low in the model. During
the period with seasonal snow cover, the model underestimates the albedo by up to 0.2 at stations in Russia, Scandinavia and
Canada, which experience severe winters. This underestimation is due to an oversimplified parameterization of the snow covered
grid fraction and an inadequate linear relation between snow albedo and temperature. The derivative of albedo with respect
to the forest fraction implemented in ECHAM is in line with the observations, although a small overestimation of the model’s
gradient has been detected.
Received: 3 July 1998 / Accepted: 24 December 1998 相似文献
13.
Atsumu Ohmura 《Surveys in Geophysics》2011,32(4-5):537-554
Glacier mass balance and secular changes in mountain glaciers and ice caps are evaluated from the annual net balance of 137 glaciers from 17 glacierized regions of the world. Further, the winter and summer balances for 35 glaciers in 11 glacierized regions are analyzed. The global means are calculated by weighting glacier and regional surface areas. The area-weighted global mean net balance for the period 1960?C2000 is ?270 ± 34 mm a?1 w.e. (water equivalent, in mm per year) or (?149 ± 19 km3 a?1 w.e.), with a winter balance of 890 ± 24 mm a?1 w.e. (490 ± 13 km3 a?1 w.e.) and a summer balance of ?1,175 ± 24 mm a?1 w.e. (?647 ± 13 km3 a?1 w.e.). The linear-fitted global net balance is accelerating at a rate of ?9 ± 2.1 mm a?2. The main driving force behind this change is the summer balance with an acceleration of ?10 ± 2.0 mm a?2. The decadal balance, however, shows significant fluctuations: summer melt reached its peak around 1945, followed by a decrease. The negative trend in the annual net balance is interrupted by a period of stagnation from 1960s to 1980s. Some regions experienced a period of positive net balance during this time, for example, Europe. The balance has become strongly negative since the early 1990s. These decadal fluctuations correspond to periods of global dimming (for smaller melt) and global brightening (for larger melt). The total radiation at the surface changed as a result of an imbalance between steadily increasing greenhouse gases and fluctuating aerosol emissions. The mass balance of the Greenland ice sheet and the surrounding small glaciers, averaged for the period of 1950?C2000, is negative at ?74 ± 10 mm a?1 w.e. (?128 ± 18 km3 a?1 w.e.) with an accumulation of 297 ± 33 mm a?1 w.e. (519 ± 58 km3 a?1 w.e.), melt ablation ?169 ± 18 mm a?1 w.e. (?296 ± 31 km3 a?1 w.e.), calving ablation ?181 ± 19 mm a?1 w.e. (?316 ± 33 km3 a?1 w.e.) and the bottom melt-21 ± 2 mm a?1 w.e. (?35 ± 4 km3 a?1 w.e.). Almost half (?60 ± 3 km3 a?1) of the net mass loss comes from mountain glaciers and ice caps around the ice sheet. At present, it is difficult to detect any statistically significant trends for these components. The total mass balance of the Antarctic ice sheet is considered to be too premature to evaluate. The estimated sea-level contributions in the twentieth Century are 5.7 ± 0.5 cm by mountain glaciers and ice caps outside Antarctica, 1.9 ± 0.5 cm by the Greenland ice sheet, and 2 cm by ocean thermal expansion. The difference of 7 cm between these components and the estimated value with tide-gage networks (17 cm) must result from other sources such as the mass balance of glaciers of Antarctica, especially small glaciers separated from the ice sheet. 相似文献
14.
Enhanced temperature variability in high-altitude climate change 总被引：1，自引：1，他引：0
Atsumu Ohmura 《Theoretical and Applied Climatology》2012,110(4):499-508
In the present article, monthly mean temperature at 56 stations assembled in 18 regional groups in 10 major mountain ranges of the world were investigated. The periods of the analysis covered the last 50 to 110?years. The author found that the variability of temperature in climatic time scale tends to increase with altitude in about 65?% of the regional groups. A smaller number of groups, 20?%, showed the fastest change at an intermediate altitude between the peaks (or ridges) and their foot, while the remaining small number of sites, 15?%, showed the largest trends at the foot of mountains. This tendency provides a useful base for considering and planning the climate impact evaluations. The reason for the amplification of temperature variation at high altitudes is traced back to the increasing diabatic processes in the mid- and high troposphere as a result of the cloud condensation. This situation results from the fact that the radiation balance at the earth??s surface is transformed more efficiently into latent heat of evaporation rather than sensible heat, the ratio between them being 4 to 1. Variation in the surface evaporation is converted into heat upon condensation into cloud particles and ice crystals in the mid- and high troposphere. Therefore, this is the altitude where the result of the surface radiation change is effectively transferred. Further, the low temperature of the environment amplifies the effect of the energy balance variation on the surface temperature, as a result of the functional shape of Stefan?CBoltzmann law. These processes altogether contribute to enhancing temperature variability at high altitudes. The altitude plays an important role in determining the temperature variability, besides other important factors such as topography, surface characteristics, cryosphere/temperature feedback and the frequency and intensity of an inversion. These processes have a profound effect not only on the ecosystem but also on glaciers and permafrost. 相似文献
15.
16.
M. W. Rotach M. R. Marinucci M. Wild P. Tschuck A. Ohmura M. Beniston 《Theoretical and Applied Climatology》1997,57(3-4):209-227
Summary Simulated temperature and precipitation changes over western Europe for a scenario of doubled atmospheric concentrations of CO2 are presented. The simulations are performed using a Limited Area Model LAM (RegCM2) nested into a General Circulation Model (ECHAM3). Both model components are operated at very high spatial resolutions — approximately 120 km for the GCM and 20 km for the LAM; the LAM domain encompasses a region of 1100 × 1100 km squared. Climatologies for five January and five July periods have been simulated. Average surface (2 m) temperatures are found to increase by 1.4 K in winter (January) and 3.9 K in summer (July); this latter figure is, however, largely dependent on a positive bias in the summer temperature fields of the driving GCM. Average precipitation changes are generally small in absolute values, but exhibit considerable spatial variability. Large precipitation amounts are seen to be shifted towards higher elevations with a corresponding reduction in the upwind areas. The results are discussed taking into account the predictive skill of the modelling system, which is derived from comparing the simulated present day temperature and precipitation fields to the corresponding climatological information. A method is introduced to assess the reliability of climate scenario predictions — such as those discussed here — on the basis of this predictive skill.With 14 Figures 相似文献
17.
Peter D. Phillips Hans Richner Jürg Joss Atsumu Ohmura 《Pure and Applied Geophysics》1980,119(2):259-277
This paper describes an intercomparison between the Väisälä, VIZ and Swiss (used at upper-air station 06610) radiosondes. The field experiment was supported by a series of carefully implemented laboratory tests. The results show that the temperature sensor data from the SMI and VIZ sondes both require additional radiation compensation for daylight ascents whilst the Swiss sonde's pressure sensor has particularly large errors in the 200–100 mb region. New correction parameters have been determined for the SMI and VIZ sondes and these have now been built into the systems' software. 相似文献