An application of Spartan spatial random fields in environmental mapping: focus on automatic mapping capabilities

This paper investigates the potential of Spartan spatial random fields (SSRFs) in real-time mapping applications. The data set that we study focuses on the distribution of daily gamma dose rates over part of Germany. Our goal is to determine a Spartan spatial model from the data, and then use it to generate “predictive” maps of the radioactivity. In the SSRF framework, the spatial dependence is determined from sample functions that focus on short-range correlations. A recently formulated SSRF predictor is used to derive isolevel contour maps of the dose rates. The SSRF predictor is explicit. Moreover, the adjustments that it requires by the user are reduced compared to classical geostatistical methods. These features present clear advantages for an automatic mapping system. The performance of the SSRF predictor is evaluated by means of various cross-validation measures. The values of the performance measures are similar to those obtained by classical geostatistical methods. Application of the SSRF method to data that simulate a radioactivity release scenario is also discussed. Hot spots are detected and removed using a heuristic method. The extreme values that appear in the path of the simulated plume are not captured by the currently used Spartan spatial model. Modeling of the processes leading to extreme values can enhance the predictive capabilities of the spatial model, by incorporating physical information.     

The eclipsing binary BW Draconis

In this paper theB- andV-light curves of the eclipsing binary BW Draconis made at Kryonerion Station of the National Observatory of Athens during 1980 and 1981 are presented.     

Ubvr light curves of 44i Bootis during 1978

In this paper theU, B, V, R light curves of the eclipsing variable 44i Boo, obtained during 1978, are represented and discussed.     

A new methodology for the extension of the impact of data assimilation on ocean wave prediction

It is a common fact that the majority of today's wave assimilation platforms have a limited, in time, ability of affecting the final wave prediction, especially that of long-period forecasting systems. This is mainly due to the fact that after “closing” the assimilation window, i.e., the time that the available observations are assimilated into the wave model, the latter continues to run without any external information. Therefore, if a systematic divergence from the observations occurs, only a limited portion of the forecasting period will be improved. A way of dealing with this drawback is proposed in this study: A combination of two different statistical tools—Kolmogorov–Zurbenko and Kalman filters—is employed so as to eliminate any systematic error of (a first run of) the wave model results. Then, the obtained forecasts are used as artificial observations that can be assimilated to a follow-up model simulation inside the forecasting period. The method was successfully applied to an open sea area (Pacific Ocean) for significant wave height forecasts using the wave model WAM and six different buoys as observational stations. The results were encouraging and led to the extension of the assimilation impact to the entire forecasting period as well as to a significant reduction of the forecast bias.     

Overview of Overland Satellite Rainfall Estimation for Hydro-Meteorological Applications

This paper reviews current techniques on rainfall estimation from satellite sensor observations. The sensors considered in this study are the Precipitation Radar (PR) and radiometer (TMI) onboard TRMM (Tropical Rainfall Measuring Missio) satellite, the Special Sensor Microwave/Imager (SSM/I) onboard Defense Meteorological Satellite Program (DMSP) platforms, and infrared (IR) sensors onboard geostationary satellites. We present the physical basis and mathematical formulation of a newly developed combined radar-radiometer (PR/TMI) retrieval for TRMM and its application for overland rain estimation. Subsequently we discuss the current state-of-the-art in overland passive microwave (TMI and SSM/I) rain estimation techniques, and outstanding issues associated with the inverse problem. The significance of lightning information in advancing high-frequency rainfall estimation from passive microwave-calibrated IR retrieval techniques is discussed on the basis of newly developed techniques. Finally, current approaches are presented on merging the infrequent passive microwave-based rainfall estimates with the high-frequency, but lower accuracy, rainfall fields derived from proxy parameters (e.g., lightning and IR). The paper provides useful insights on satellite rainfall estimation and discusses issues and applications.     

Spatial variability estimation and risk assessment of the aquifer level at sparsely gauged basins using geostatistical methodologies

The spatial variability evaluation of the water table level of an aquifer provides useful information in water resources management plans. Three different approaches are applied to estimate the spatial variability of the water table in the study basin. All of them are based on the Kriging methodology. The first is the classical Ordinary Kriging approach, while the second involves information from a secondary variable (surface elevation) and the application of Residual Kriging. The third calculates the probability to lie below a certain groundwater level limit that could cause significant problems in groundwater resources availability. The latter is achieved by means of Indicator Kriging. A recently developed non-linear normalization method is used to transform both data and residuals closer to normal distribution for improved prediction results. In addition, the recently developed Spartan variogram model is applied to determine the spatial dependence of the measurements. The latter proves to be the optimal model, compared to a series of models tested, which provides in combination with the Kriging methodologies the most accurate cross validation estimations. The variogram form is explained with respect to the radius of influence of the pumping wells representing the spatial impact of the pumping activity. Groundwater level and probability maps are developed providing the ability to assess the spatial variability of the groundwater level in the basin and the risk that certain locations have in terms of a safe groundwater level limit that has been set for the sustainability of the groundwater resources of the basin.     

The eclipsing binary AB Andromedae

In this paper the two newB- andV-light curves of the eclipsing variable star AB Andromedae, obtained during 1979 with the 48 in reflector of the National Observatory of Athens, are represented and discussed. General information of the system is given in the Introduction, while in Section 2 some observational and reductional details are given and the new light curves are represented. Section 3 deals with the period of the system and, finally, in Section 4 a general discussion is given.     

DEM assessment of impact forces of dry granular masses on rigid barriers

In the design of sheltering structures/embankments for the mitigation of the risk due to rapid and long spreading landslides, a crucial role is generally played by the assessment of the impact force exerted by the flowing mass on the artificial obstacle. This paper is focused on this issue and in particular on the evaluation of the maximum impact force on the basis of the results obtained by performing an extensive numerical campaign by means of a 3D discrete element code, in which a dry granular mass is schematised as a random distribution of rigid spherical particles. The granular mass is generated just in front of the obstacle: its initial volume, velocity distribution, height, length and porosity are arbitrarily assigned, and the impact process is exclusively analysed. The initial conditions are varied to take a large variety of geometrical/mechanical factors, such as the initial front inclination, its height, the initial void ratio, the length of the impacting mass and the inter-particle friction angle, into consideration. A design formula is also proposed on the base of the obtained results and critically compared with the literature data.     

Necessary and sufficient conditions for reaching and maintaining critical state

According to classical critical state theory (CST) of granular mechanics, two analytical conditions on the ratio of stress invariants and the void ratio are postulated to be necessary and sufficient for reaching and maintaining critical state (CS). The present work investigates the sufficiency of these two conditions based on the results of a virtual three-dimensional discrete element method experiment, which imposes continuous rotation of the principal axes of stress with fixed stress principal values at CS. Even though the fixity of the stress principal values satisfies the two analytical CST conditions at the initiation of rotation, contraction and abandonment of CS occur, which proves that these conditions may be necessary but are not sufficient to maintain CS. But if fixity of stress and strain rate directions in regard to the sample is considered at CS, the two analytical conditions of CST remain both necessary and sufficient. The recently proposed anisotropic critical state theory (ACST) turned this qualitative requirement of fixity into an analytical condition related to the CS value of a fabric anisotropy variable A defined in terms of an evolving fabric tensor and the plastic strain rate direction, thus, enhancing the two CST conditions by a third. In this way, the three analytical conditions of ACST become both necessary and sufficient for reaching and maintaining CS. In addition, the use of A explains the observed results by relating the stress-strain response, in particular the dilatancy, to the evolution of fabric by means of the relevant equations of ACST.     

Dynamical downscaling of IPSL-CM5 CMIP5 historical simulations over the Mediterranean: benefits on the representation of regional surface winds and cyclogenesis

The Mediterranean region is identified as one of the two main hot-spots of climate change and also known to have the highest concentration of cyclones in the world. These atmospheric features contribute significantly to the regional climate but they are not reproduced by the Atmosphere–Ocean General Circulation Models (AOGCM), due to their coarse horizontal resolution, which have recently been run in the frame of the 5th Climate Model Intercomparison Project. This article investigates the benefit of dynamically downscaling the Institut Pierre Simon Laplace (IPSL) AOGCM (IPSL-CM5) historical simulation by the weather and research forecasting (WRF) for the representation of the Mediterranean surface winds and cyclonic activity. Indeed, when considering IPSL-CM5 atmospheric fields, the dramatic underestimation of the cyclonic activity in the most cyclogenetic region of the world jeopardizes our ability to investigate in-depth the Mediterranean regional climate and trend in the context of global change. The WRF model shows remarkable skill to reproduce regional cyclogenesis. Indeed, cyclones occurrence is quasi-absent in IPSL-CM5 data but when applying dynamical downscaling their spatial–temporal variability is very close to the re-analysis. This is a clear benefit of dynamical downscaling in regions of strong topographic forcing. This “steady” source of forcing allows the production of lee cyclogenesis and the development of strong cyclones, whatever the quality of the large-scale circulation provided at the WRF’s boundaries by IPSL-CM5. However, dynamical downscaling still presents disadvantages as for instance the fact that large-scale inaccurate features of the IPSL-CM5 regional circulation are replicated by WRF due to the boundary controlled (small domain) simulation. The advantages and disadvantages of dynamical downscaling are thoroughly discussed in this paper revealing its importance for climate research, especially in the context of future scenarios and wind impacts.