Pi-SAR极化数据与K分布指数估算森林生物量与实验验证

用2002年和2003年日本Pi-SAR全极化数据,研究日本北海道苫小牧森林地区的森林生物量.雷达后向散射系数随森林生物量的增大而增大并迅速达到饱和,L波段雷达数据饱和点约为40t/hm2,X波段仅约为20t/hm2.在SAR数据统计分布中,K分布的指数参数在饱和点以上仍随生物量的增大而增大,并且HV极化方式时相关性最高.根据交叉极化数据K分布的指数参数与森林生物量的关系,本文估算了23个观测点的森林生物量,结果表明平均准确率为85%.因此该算法可以作为一种新的估算森林生物量的手段.     

Length-Scale Similarity of Turbulent Organized Structures over Surfaces with Different Roughness Types

We examine the similarity of turbulent organized structures over smooth and very rough wall flows. Turbulent flow fields in horizontal cross-sections were measured using particle image velocimetry, and the characteristics of turbulent organized structures over four types of surfaces were investigated. Measurements were conducted at several measurement heights across the internal boundary layer. The length and width of turbulence structures were quantified using a two-point correlation method. We selected two thresholds of two-point correlation coefficients to consider both large-scale and small-scale structures; the validity of these choices was examined through the analyses using proper orthogonal decomposition. For large-scale structures, the length and aspect ratios (streamwise length/spanwise width) of structures were highly correlated with the velocity gradient for each measurement height and boundary-layer thickness. This relationship was also examined in the results of previous studies, and the scaling of the aspect ratio with the non-dimensional velocity gradient again showed the importance of the velocity gradient, with slight differences found between smooth and rough surfaces. In contrast, the small-scale structures exhibited weak dependency on the velocity gradient and boundary-layer thickness. Instantaneous snapshots of turbulent organized structures at the same shear level also displayed differences in small-scale structures, but the structures of the organized motions resembled each other, as in the results of the two-point correlation method.     

Interannual variability of subsurface temperature in summer induced by the Kuroshio over Bungo Channel,Tosa Bay,and Kii Channel,south of Japan

Using historical in situ temperature record, we investigate interannual variability of temperature in summer over Bungo Channel, Tosa Bay, and Kii Channel (hereafter “BTK”), south of Japan. This study aims to reveal the interregional relation of the subsurface temperature variability between the shelf and slope as well as between BTK, and specify the primary cause of the temperature variability in terms of the Kuroshio path. It is shown that 100-m temperature on the slope of BTK is synchronous with each other and highly correlated with coastal sea levels from Kyushu to Kii Peninsula, suggesting that subsurface temperature over the slope varies simultaneously on spatial scales of 400–500 km in the along-slope direction. The 100-m temperature on the slope is also correlated with the near-bottom temperature on the shelf, implying that the near-bottom temperature is determined mainly by the larger-scale variability over the slope. The Kuroshio state in summer of each year is classified into the large meander (LM), non-large meander (NLM), and intermittent larger meander (IM) propagating eastward along the slope. It is found that remarkable temperature increases are accompanied by the IM propagations south of BTK. Temperatures during the IM and LM period are significantly different, the means of which are the highest and lowest, respectively. Temperature during the NLM period with the second highest mean value exhibits the largest variance. Statistical analysis suggests that this variance is related to the Kuroshio axis shift over Izu Ridge between the nearshore and offshore NLM paths.     

Momentum and Heat Transfer over Urban-like Surfaces

Momentum and heat transfer was examined for the urban-like surfaces used within the Comprehensive Outdoor Scale MOdel (COSMO) experiments. Simultaneous and comparative meteorological measurements were made over a pair of scale models with different block geometries. These data were analyzed to investigate the influence of height variations, obstacle elongation, and packing density, λ _p , of blocks on the aerodynamic properties. In addition, the robustness of theoretical expressions of bulk transfer coefficients for momentum and heat with respect to geometric parameters was examined. Our analyses showed: (1) the theoretical framework for the bulk transfer coefficient for momentum, C _m , and that for heat, C _h , was applicable for homogeneous building arrays, (2) the sensitivity of C _h to the surface geometry was smaller than that of C _m , (3) the transfer coefficients were increased by variations of block heights, but not by elongation of blocks, (4) first-order approximations of C _m and C _h for an array of blocks with two different heights can be made by applying simple theoretical assumptions to include the effects of height variation, and (5) variations of block heights increased the momentum flux significantly, but caused little change in the sensible heat flux. This can be explained by the feedback mechanism of aerodynamic– thermal interaction; aerodynamic mixing decreased both the advective velocity and the vertical temperature gradient.     

The Impact of Orographically-Induced Gravity Wave on the Diurnal Cycle of Rainfall over Southeast Kalimantan Island

Precipitation measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite indicate that the southeastern area of Kalimantan (Borneo) Island receives much less rainfall than elsewhere on the island during the period from July to October.Results from sur-face meteorological observations show that the diurnal cycle of rainfall differs greatly between the eastern and western coasts of the island.Rainfall on the western coast of the island is frequent in the afternoon and evening,whereas almost all rainfall on the eastern coast occurs in the morning.Meanwhile,the Global Positioning System (GPS)-derived precipitable water (PW) on the eastern coast shows a substantial decrease in moisture in the af-ternoon and evening.Numerical experiments with a mesoscale model reveal that gravity waves driven by di-urnal heating of the elevated land surface of the moun-tains on Sulawesi Island,which lies approximately 300 kilometers to the east of Kalimantan Island,significantly affect the diurnal cycle of rainfall over southeast Kali-mantan Island.     

Foldback reflectors near methane hydrate bottom‐simulating reflectors: Indicators of gas distribution from 3D seismic images in the eastern Nankai Trough

Understanding of fluid behavior and gas distribution in the shallow subsurface are important considerations in gas hydrate formation and the global carbon cycle. Estimation of gas distribution based on reflection seismic surveys, however, is difficult because the boundary of a gas‐bearing zone is indistinct and not systematically defined. This study reports distinctive features related to gas‐hydrate distribution and possible fluid migration in high‐resolution 3D seismic‐reflection data from sediments of the eastern Nankai Trough. These features, here termed foldback reflectors (FBRs), descend in accordion shaped reflectors near the edges of bottom‐simulating reflectors (BSRs). FBRs generally correspond to lateral boundaries between two seismic facies, a ‘dimmed’ facies with relatively low amplitude and subdued high‐frequency components beneath the BSR and the contrasting facies around the BSR. The dimmed facies corresponds to areas of anomalously low velocity consistent with a small amount of free gas. FBR is mostly developed in well‐stratified formations in uplifted regions. Dip directions of the FBR appear to be restricted by orientation of the host formations. Edges of the FBR often correspond to high‐amplitude layers. Such occurrences of FBR suggest that regional uplift and layer‐parallel fluid migration are related to the formation of FBR as well as BSR.     

LES Study of the Energy Imbalance Problem with Eddy Covariance Fluxes

The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the `turbulent' heat flux at a grid point from the horizontally averaged `total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks.The temporally averaged `turbulent' flux based on a point measurement systematically underestimates the `total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.     

Diurnal variations of the planetary boundary layer observed with anL-band clear-air doppler radar

Based on continuous observations of the planetary boundary layer (PBL) with anL-band (1357.5 MHz) boundary-layer radar (BLR) at a hilly location in Japan, we have discovered that on clear days, a thin enhanced echo layer corresponding to the top of the PBL (or mixed layer) appeared at about 500 m height in the morning and ascended to about 1500 m in the afternoon. Strong upward velocities were observed below the echo layer (or inside the PBL), reaching 1500 m in the afternoon.     

Local and Global Similarity in Turbulent Transfer of Heat, Water Vapour, And CO 2 in the Dynamic Convective Sublayer Over a Suburban Area

We investigated the ‘local’ and ‘global’ similarity of vertical turbulent transfer of heat, water vapour, and CO₂ within an urban surface layer. The results were derived from field measurements in a residential area of Tokyo, Japan during midday on fair-weather days in July 2001. In this study, correlation coefficients and quadrant analysis were used for the evaluation of ‘global’ similarity and wavelet analysis was employed for investigating ‘local’ similarity. The correlation coefficients indicated that the transfer efficiencies of water vapour and CO₂ were generally smaller than that of heat. Using wavelet analysis, we found that heat is always efficiently transferred by thermal and organized motions. In contrast, water vapour and CO₂, which are passive quantities, were not transferred as efficiently as heat. The quadrant analyses showed that the heat transfer by ejection exceeded that by sweep, and the ratios of ejection to sweep for water vapour and CO₂ transfer were less than that for heat. This indicated that heat is more efficiently transferred by upward motions and supported the findings from wavelet analysis. The differences of turbulent transfer between heat and both CO₂ and water vapour were probably caused both by the active role of temperature and the heterogeneity in the source distribution of scalars     

Spatial representativeness of single tower measurements and the imbalance problem with eddy-covariance fluxes: results of a large-eddy simulation study

A large-eddy simulation (LES) study is presented that investigates the spatial variability of temporal eddy covariance fluxes and the systematic underestimation of representative fluxes linked to them. It extends a prior numerical study by performing high resolution simulations that allow for virtual measurements down to 20 m in a convective boundary layer, so that conditions for small tower measurement sites can be analysed. It accounts for different convective regimes as the wind speed and the near-surface heat flux are varied. Moreover, it is the first LES imbalance study that extends to the stable boundary layer. It reveals shortcomings of single site measurements and the necessity of using horizontally-distributed observation networks. The imbalances in the convective case are attributed to a locally non-vanishing mean vertical advection due to turbulent organised structures (TOS). The strength of the TOS and thus the imbalance magnitude depends on height, the horizontal mean wind and the convection type. Contrary to the results of a prior study, TOS cannot generally be responsible for large energy imbalances: at low observation heights (corresponding to small towers and near-surface energy balance stations) the TOS related imbalances are generally about one order of magnitude smaller than those in field experiments. However, TOS may cause large imbalances at large towers not only in the case of cellular convection and low wind speeds, as found in the previous study, but also in the case of roll convection at large wind speeds. In the stably stratified boundary layer for all observation heights neither TOS nor significant imbalances are observed. Attempting to reduce imbalances in convective situations by applying the conventional linear detrending method increases the systematic flux underestimation. Thus, a new filter method is proposed.