Numerical reconstruction of trajectory of small-size surface drifter in the Mediterranean sea

In this study, we addressed the effects of wind-induced drift on Lagrangian trajectories of surface sea objects using high-resolution ocean forecast and atmospheric data. Application of stochastic Leeway model for prediction of trajectories drift was considered for the numerical reconstruction of the Elba accident that occurred during the period 21.06.2009–22.06.2009: a person on an inflatable raft was lost in the vicinity of the Elba Island coast; from the initial position, the person on a raft drifted southwards in the open sea and later he was found on a partially deflated raft during rescue operation. For geophysical forcing, we used high-resolution currents from the Mediterranean Forecasting System and atmospheric wind from the European Centre for Medium-Range Weather Forecasts. To investigate the effect of wind on trajectory behavior, numerical simulations were performed using different categories of drifter-like particles similar to a person on an inflatable raft. An algorithm of spatial clustering was used to differentiate the most probable search areas with a high density of particles. Our results showed that the simulation scenarios using particles with characteristics of draft-limited sea drifters provided better prediction of an observed trajectory in terms of the probability density of particles.     

Stage level,volume and time‐frequency information content of Lake Tana using stochastic and wavelet analysis methods

Lake Tana is the largest fresh water body situated in the north‐western highlands of Ethiopia. In addition to its ecological services, it serves for local transport, electric power generation, fishing, recreational purposes, and source of dry season irrigation water supply. Evidence shows that the lake has dried at least once at about 15,000–17,000 before present owing to a combination of high evaporation and low precipitation events. Past attempts to understand and simulate historical fluctuation of Lake Tana based on simplistic water balance approach of inflow, outflow, and storage have failed to capture well‐known events of drawdown and rise of the lake that have happened in the last 44 years. This study tested different stochastic methods of lake level and volume simulation for supporting Lake Tana operational planning decision support. Three stochastic methods (perturbations approach, Monte Carlo methods, and wavelet analysis) were employed for lake level and volume simulation, and the results were compared with the stage level measurements. Forty‐four years of daily, monthly, and mean annual lake level data have shown a Gaussian variation with goodness of fit at 0.01 significant levels of the Kolmogorov–Smirnov test. The stochastic simulations predicted the lake stage level of the 1972, 1984, and 2002/2003 historical droughts 99% of the time. The information content (frequency) of fluctuation of Lake Tana for various periods was resolved using Wigner's Time‐Frequency Decomposition method. The wavelet analysis agreed with the perturbations and Monte Carlo simulations resolving the time (1970s, 1980s, and 2000s) in which low frequency and high spectral power fluctuation has occurred. The Monte Carlo method has shown its superiority for risk analysis over perturbation and deterministic method whereas wavelet analysis reconstructed historical record of lake stage level at daily and monthly time scales. Copyright © 2012 John Wiley & Sons, Ltd.     

Electric and magnetic components of ballooning perturbations in the magnetotail plasma sheet before breakup

Using data from THEMIS spacecraft we investigated transverse to the magnetic field mutually perpendicular electric and magnetic components of ballooning type perturbations with periods 60–240 s, which are observed in the magnetospheric plasma sheet during the period preceding substorm onset. With applying Hilbert transform, we analyzed the phase relations between them. It is shown that the perturbations are dominated by radial electric and azimuthal magnetic (that is, toroidal) components which are usually in phase or out-of-phase. Along with them, approximately 2.5 times less intense azimuthal electric and radial magnetic components are present, which are more often phase-shifted by π/2. It is concluded that the observed perturbations are not a simple consequence of the development of plasma sheet ballooning instability, leading to the growth of strongly elongated along the magnetotail ballooning structures. It is pointed out that this conclusion is confirmed by simultaneous ground-based observations of magnetically conjugate auroral structures.     

The June 2013 Alberta catastrophic flooding event – part 2: fine‐scale precipitation and associated features

Data obtained from a variety of sources including the Canadian Lightning Detection Network, weather radars, weather stations and operational numerical weather model analyses were used to address the evolution of precipitation during the June 2013 southern Alberta flood. The event was linked to a mid‐level closed low pressure system to the west of the region and a surface low pressure region initially to its south. This configuration brought warm, moist unstable air into the region that led to dramatic, organized convection with an abundance of lightning and some hail. Such conditions occurred in the southern parts of the region whereas the northern parts were devoid of lightning. Initially, precipitation rates were high (extreme 15‐min rainfall rates up to 102 mm h^?1 were measured) but decreased to lower values as the precipitation shifted to long‐lived stratiform conditions. Both the convective and stratiform precipitation components were affected by the topography. Similar flooding events, such as June 2002, have occurred over this region although the 2002 event was colder and precipitation was not associated with substantial convection over southwest Alberta. Copyright © 2016 Her Majesty the Queen in Right of Canada. Hydrological Processes. © John Wiley & Sons, Ltd.     

Higher tritium concentrations measured in permafrost thaw lakes in northern Alberta

Tritium concentrations were measured in a survey of 24 lakes, 15 wetlands, and 133 groundwaters in the oil sands region of northeastern Alberta and compared with both recent precipitation and precipitation sampled during the 1960s tritium peak caused by atmospheric thermonuclear weapons testing. Water samples from lakes included a group of 14 thaw lakes that had higher runoff attributed to melting of permafrost in peat plateaus within their watersheds. While tritium in all lakes was found to be intermediate between recent and 1960s concentrations, the thaw lakes were found to be significantly enriched in tritium compared with other lakes, as were unfrozen wetlands characterized by a thick sequence of low‐hydraulic conductivity peat. The results provide further evidence of different water sources to the thaw lakes and may indicate that melting of modern permafrost in part formed since the 1950s is occurring in these systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Energy partitioning over a semi‐arid shrubland in northern China

During the last decade, the widely distributed shrublands in northern China have shown significant signs of recovery from desertification, the result of widespread conservation practices. However, to support the current efforts in conservation, more knowledge is needed on surface energy partitioning and its biophysical controls. Using eddy‐covariance measurements made over a semi‐arid shrubland in northwest China in 2012, we examined how surface energy‐balance components vary on diurnal and seasonal scales, and how biophysical factors control bulk surface parameters and energy exchange. Sensible heat flux (H) exceeded latent heat flux (λE) during most of the year, resulting in an annual Bowen ratio (β, i.e. H/λE) of 2.0. λE exceeded H only in mid‐summer when frequent rainfall co‐occurred with the seasonal peak in leaf area index (LAI). Evapotranspiration reached a daily maximum of 3.3 mm day^?1, and summed to 283 mm yr^?1. The evaporative fraction (EF, i.e. λE/R_n), Priestley–Taylor coefficient (α), surface conductance (g_s) and decoupling coefficient (Ω) were all positively correlated with soil water content (SWC) and LAI. The direct enhancement of λE by high vapour pressure deficit (VPD) was buffered by a concurrent suppression of g_s. The g_s played a direct role in controlling EF and α by mediating the effects of LAI, SWC and VPD. Our results highlight the importance of adaptive plant responses to water scarcity in regulating ecosystem energy partitioning, and suggest an important role for revegetation in the reversal of desertification in semi‐arid areas. Copyright © 2015 John Wiley & Sons, Ltd.     

Displacement and rectification of planetary fluids

Abstract

Laboratory experiments on the decay (spin-up) of fluid motion on the β-plane are compared with theory. Under weakly dissipative conditions, some particles conserve potential vorticity during the entire decay. We also study the rectified mean flow which is produced by the lateral Reynolds Stress when a low frequency force is applied to the planetary fluid. The possible connection of effects with oceanic phenomena is briefly discussed.     

流速对浮游藻类生长和种群变化影响的模拟试验

采集夏季崇明岛中心湖原水,在环形有机玻璃水槽开展了不同流速对浮游藻类生长和种群变化影响的研究.相对于静止水槽,实验前期不同的流速条件均对浮游藻类的生长产生了一定的抑制作用,实验后期浮游藻类叶绿素a含量均稳定在一定水平,且流动水槽中叶绿素a含量略大于静止水槽,这可能与静止围隔中出现大量枝角类浮游动物有关,而流动条件则抑制了浮游动物的生长,减轻了对浮游藻类的捕食压力.水体流动导致了浮游藻类种群的变化,蓝藻的迅速消失、绿藻和硅藻形成生长优势是主要特征,表明持续的流动条件是导致浮游植物种属减少和群落结构变化的直接原因.该研究可为调水引流在河道、湖泊和水库中的富营养化控制和水华防治提供基础依据.     

Experimental synthesis of seismic horizontal free-field motion of soil in finite-domain simulations with absorbing boundary

In this paper, a fundamental assessment of the method of physical wave-absorbing boundary and centrifuge modeling is presented in the context of experimental simulations of seismic free-field ground motion. Focusing on the characteristics of a sand stratum, a series of seismic tests on models of uniform density and a large width-to-depth ratio with Duxseal as the side boundary were performed using an in-box shake-table system. By means of the transfer function approach in the frequency domain, the complex three-dimensional nature of the dynamic response of the finite soil model with the boundary treatment is demonstrated in terms of its variable resonant frequency distribution at different g-levels. Apart from being helpful in quantifying the difficulty in interpreting the finite-domain response simulations using one-dimensional theories or homogenized representations, the measured data substantiates the need and usefulness of coupling the Duxseal boundary approach with a three-dimensional elastodynamic synthesis. With the aid of a corresponding boundary element implementation, the feasibility of identifying the soil's in-flight shear modulus variation, Poisson's ratio and horizontal-to-vertical earth pressure ratio from the centrifuge model's free-field measurements is also explored.     

Influence of the mean period of ground motion on the inelastic dynamic response of single and multi degree of freedom systems

This paper focuses on examining the effects of frequency content of the ground motion on the inelastic demands imposed on both single degree of freedom (SDF) and multi degree of freedom (MDF) steel‐framed systems. A detailed literature review is conducted to identify the indicator that best represents the frequency content of ground motion. The mean period (T_m) of ground motion is selected owing to its ability to distinguish between various spectral shapes of ground motion, and its relationship with magnitude, distance and site characteristics. Inelastic displacement demands on SDF systems for target ductility levels are first studied in the light of T_m, using a suite of 128 ground motion records. The study is then extended to MDF systems with the help of incremental dynamic analysis by employing the same ground motion ensemble to assess the influence of T_m on various engineering demand parameters. The results obtained indicate that, for SDF systems, the amplification of displacements occurs when the period ratio between elastic period (T_e) and T_m is lower than unity. For MDF systems, the results demonstrate that the influence of higher modes on the base shear and maximum storey drift profile becomes more pronounced, as T_m approaches the higher mode periods of the structure. These observations, for both SDF and MDF systems, tend to be more evident for higher levels of inelasticity. The significance of the results, with particular reference to European seismic design procedures, is highlighted. Copyright © 2010 John Wiley & Sons, Ltd.