Large-Eddy-Simulation Study of the Effects of Building-Height Variability on Turbulent Flows over an Actual Urban Area

Large-eddy simulation (LES) is used to investigate the effects of building-height variability on turbulent flows over an actual urban area, the city of Kyoto, which is reproduced using a 2-m resolution digital surface dataset. Comparison of the morphological characteristics of Kyoto with those of European, North American, and other Japanese cities indicates a similarity to European cities but with more variable building heights. The performance of the LES model is validated and found to be consistent with turbulence observations obtained from a meteorological tower and from Doppler lidar. We conducted the following two numerical experiments: a control experiment using Kyoto buildings, and a sensitivity experiment in which all the building heights are set to the average height over the computational region \(h_{all}\). The difference of Reynolds stress at height \(z=2.5h_{all}\) between the control and sensitivity experiments is found to increase with the increase in the plan-area index (\(\lambda _p\)) for \(\lambda _p > 0.32\). Thus, values of \(\lambda _p\approx 0.3\) can be regarded as a threshold for distinguishing the effects of building-height variability. The quadrant analysis reveals that sweeps contribute to the increase in the Reynolds stress in the control experiment at a height \(z= 2.5h_{all}\). The exuberance in the control experiment at height \(z=0.5h_{all}\) is found to decrease with increase in the building-height variability. Although the extreme momentum flux at height \(z=2.5h_{all}\) in the control experiment appears around buildings, it contributes little to the total Reynolds stress and is not associated with coherent motions.     

Analysis and attribution of trends in water levels in the Vietnamese Mekong Delta

In the Vietnamese Mekong Delta (VMD), water levels at some stations have increased. However, the factors that cause this rise in the VMD have not been identified. We considered four factors that may have contributed to the water level rise: (1) increased runoff from upstream, (2) sea‐level rise, (3) land subsidence, and (4) decrease in flood mitigation function because of construction of high dykes. We analysed daily maximum and minimum water levels, and mean daily water levels from 24 monitoring stations from 1987 to 2006. Using daily and annual water level differences, we classified the delta into two groups: one is dominated by flows from upstream, while the other is tide dominated. We then tested the trends of annual maximum and minimum water levels using the Mann–Kendall test, and identified the slope of the trend using the method of Sen. The areas of dyke construction were estimated using the Enhanced Vegetation Index (EVI) and Land Surface Water Index (LSWI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Results show (1) river inflow has little impact on rising water levels in the VMD, (2) the influence of high dykes on water level rise could not be quantified in this study, and (3) both maximum and minimum water levels significantly increased in the tide‐dominated area. Trend of annual minimum water level can be considered as the sum sea‐level rise and land subsidence. Therefore, we attribute 6.05 mm year^?1 (80%) to land subsidence and 1.42 mm year^?1 (20%) to sea level rise, indicating that inundations have been severe in the VMD, caused primarily by land subsidence. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of the introduction of rice on evapotranspiration in seasonal wetlands

Land use changes in wetland areas can alter evapotranspiration, a major component of the water balance, which eventually affects the water cycle and ecosystem. This study assessed the effect of introduced rice‐cropping on evapotranspiration in seasonal wetlands of northern Namibia. By using the Bowen ratio–energy balance method, measurements of evapotranspiration were performed over a period of 2.5 years at two wetland sites—a rice field (RF) and a natural vegetation field (NVF)—and at one upland field (UF) devoid of surface water. The mean evapotranspiration rates of RF (1.9 mm daytime^?1) and NVF (1.8 mm daytime^?1) were greater than that in UF (1.0 mm daytime^?1). RF and NVF showed a slight difference in seasonal variations in evapotranspiration rates. During the dry season, RF evapotranspiration was less than the NVF evapotranspiration. The net radiation in RF was less in this period because of the higher albedo of the non‐vegetated surface after rice harvesting. In the early growth period of rice during the wet season, evapotranspiration in RF was higher than that in NVF, which was attributed to a difference in the evaporation efficiency and the transfer coefficient for latent heat that were both affected by leaf area index (LAI). Evapotranspiration sharply negatively responded to an increase in LAI when surface water is present according to sensitivity analysis, probably because a higher LAI over a surface suppresses evaporation. The control of LAI is therefore a key for reducing evaporation and conserving water. Copyright © 2013 John Wiley & Sons, Ltd.     

Contamination level of mercury in red meat products from cetaceans available from South Korea markets

Levels of total mercury (T-Hg) were surveyed in red meat (n=73) and liver (n=3) from toothed whales, dolphins and porpoises (odontocetes) sold for human consumption in the coastal cities of South Korea. High concentrations of T-Hg were found in the liver products of finless porpoises (18.7 and 156 microg/wet g) and common dolphins (13.2 microg/wet g). The T-Hg concentrations in red meat products were highest in the false killer whale (9.66+/-12.3 microg/wet g, n=9), bottlenose dolphin (10.6+/-12.6 microg/wet g, n=3) and killer whale (13.3 microg/wet g, n=1), and lowest in Cuvier's beaked whale and the harbour porpoise (0.4-0.5 microg/wet g). Thus, most of the products that originated from odontocetes exceeded the safety limit of 0.5 microg/wet g for T-Hg set by the South Korean health authorities for the fishery industry. Pregnant women and other vulnerable sectors of the population living in South Korea should therefore limit their consumption of odontocete products.     

Nutrient and Phytoplankton Dynamics in Harima-Nada, Eastern Seto Inland Sea, Japan During a 35-Year Period from 1973 to 2007

Long-term monitoring of water quality and phytoplankton was conducted at 19 sampling stations in Harima-Nada, eastern Seto Inland Sea, Japan for 35 years from 1973 to 2007. There were two significant long-term changes, an increase in winter water temperatures of 0.042°C year^?1, and a decrease in dissolved inorganic nitrogen (DIN) from about 10 μM in the 1970s to ~5 μM in the late 1990s due to the reduction in nutrient inputs. DIN concentrations and total phytoplankton cell density were both higher during the 1970s to the early 1980s and then exhibited a significant decrease in the mid 1980s and remained relatively constant thereafter. Diatoms were the dominant phytoplankton group (>90%) over the 35-year period, and there was a dramatic shift from Skeletonema dominance (~70%) to Chaetoceros in the mid 1980s. This shift in diatom species may be attributed to differences in the life cycle of Skeletonema and Chaetoceros and the response to the decrease in DIN concentration.     

Flare ribbon expansion and energy release

We report a detailed examination about the relationship between the evolution of the Hα flare ribbons and the released magnetic energy during the April 10 2001 flare. In the Hα images, several bright kernels are observed in the flare ribbons. We identified the conjugated foot-points, by analyzing the lightcurves at each Hα kernels, and showed their connectivities during the flare. Then, based on the magnetic reconnection model, we calculated quantitatively the released energy by using the photospheric magnetic field strengths and separation speeds of the Hα flare ribbons. Finally, we examined the downward motions which are observed at the Hα kernels. We found that the stronger the red-asymmetry tends to be associated with the brighter the Hα kernel.     

GCM simulation of balloon trajectories on Titan

Possible trajectories of passive balloons in Titan's troposphere are simulated with the instantaneous wind field predicted by a GCM (general circulation model). In most areas the basic motion of a balloon is a predominantly eastward or westward drift, depending on altitude, latitude and season of the balloon release point. Some meridional oscillation is always superposed on this basic motion, resulting in a wavy trajectory, with a maximum extent (of 40^°) at high latitudes of the winter hemisphere. As a general rule, the meridional oscillation can be maximised if a balloon is deployed in altitudes and latitudes where the mean zonal wind is eastward and smaller than the phase speed of Saturn's gravitational tide on Titan. A balloon's groundtrack does not repeat as it makes successive circuits around Titan, but rather makes a spiral or braided pattern. The summer pole is rather difficult to access for a balloon not directly introduced there because of small meridional oscillation, while the winter pole can be readily accessed and left several times. A preferred zonal drift direction can be achieved by choosing a proper altitude and hemisphere, but choosing a preferred meridional direction is not possible.     

On the stability of hypothetical satellites coorbital to Mimas or Enceladus

In the present work, we study the stability of hypothetical satellites that are coorbital with Enceladus and Mimas. We performed numerical simulations of 50 particles around the triangular Lagrangian equilibrium points of Enceladus and Mimas taking into account the perturbation of Mimas, Enceladus, Tethys, Dione, Titan and the oblateness of Saturn. All particles remain on tadpole orbits after 10 000 yr of integration. Since in the past the orbit of Enceladus and Mimas expanded due to the tidal perturbation, we also simulated the system with Enceladus and Mimas at several different values of semimajor axes. The results show that in general the particles remain on tadpole orbits. The exceptions occur when Enceladus is at semimajor axes that correspond to 6:7, 5:6 and 4:5 resonances with Mimas. Therefore, if Enceladus and Mimas had satellites librating around their Lagrangian triangular points in the past, they would have been removed if Enceladus crossed one of these first-order resonances with Mimas.