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.     

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.     

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 peculiar eclipsing star BD+13°3496 (V508 ophiuchi): New BV observations and light curve analysis

New BV photoelectric observations of the W UMa-type system V508 Ophiuchi, made in 1986 at the Kryonerion Astronomical Center of the National Observatory of Athens, Greece, are given. The corresponding mean light curves, as well as those obtained in 1981 at the same station using the same instruments, are analysed using frequency-domain techniques. The geometrical and photometric elements of the system have been found. These are combined with the now available spectroscopic data of the system to get its absolute elements. A general discussion for the light variations between the old and the new observations is made. From the light curve analysis it was found that V508 Oph is an A-type W UMa binary with both of its components to fill their corresponding Roche lobes.     

Seasonal and spatial variations of water quality,substrate and aquatic macrophytes based on side scan sonar,in an eastern Mediterranean lagoon (Kaiafas,Ionian Sea)

Temporal and spatial variations of environmental and water quality parameters and their relations with macrobenthic flora were investigated in an eastern Mediterranean lagoon. Kaiafas is a mesohaline lagoon, which is influenced by point and diffused sources from the adjacent agricultural land and the nearby city. Water samples were analyzed for physicochemical parameters, microbial load and primary production, on a seasonal and spatial scale, while heavy metal concentrations were measured into two different sediment cores. An overall analysis of seasonal dynamics of water parameters and Chl-a based on trophic index TSI pointed out the lagoon as eutrophic. According to water quality parameters the lagoon was separated into two parts. The southern sector of the lagoon which was more affected by human activities showed higher nutrients, Chl-a, heavy metals and total Coliforms concentrations. Sediment texture and distribution of macrophytes were detected through the acoustic side scan sonar method, which proved to be a promising tool for defining and monitoring vegetation coverage of shallow lagoons. A number of distinct echo types and three different sediment types were revealed on the basis of the backscatter level and the variability of low and high backscatter areas. Furthermore, the extension and the coverage of Potamogeton pectinatus and Chara hispida f. corfuensis were successively depicted by combining the data of SSS and ground truthing samples. Meadows of P. pectinatus were established to the lagoon limits where high turbidity and silty substrate occurred, while C. hispida f. corfuensis was limited in deep and high transparent waters with sandy substrate.     

Study of the contact binary AK Herculis: Light-curve analysis and orbital period investigation

AK Her is a very interesting contact binary exhibiting variable lightcurves and an obvious O'Connell effect, with max I greater than max II. Inthe present work an extensive study of this system is made covering notonly its light curve analysis, but also its orbital period changes in anattempt to give a definite answer regarding the apsidal motion of AK Her.Photoelectric observations of AK Her, obtained between 1985–1987 at theNational Observatory of Athens Greece, are presented, analysed anddiscussed. The light curve analysis was made with the W-D code and thegeometric and photometric elements of the system were derived. TheO'Connell effect was faced with a two dark spot model. Moreover, theorbital period of the system was examined and two periodicities weredetected. Since the first one corresponds to the time interval for whichobservational data exist and the second was found to be half of the first,it turns out that none of them is true. Two other significant results thatcame from the present analysis are that the orbital period of AK Her doesnot follow a sinusoidal variation and that, from the times of minimumlight at primary and secondary eclipses, it seems that there is no apsidalmotion.     

A new methodology for using buoy measurements in sea wave data assimilation

One of the main drawbacks in modern sea wave data assimilation models is the limited temporal and spatial improvement obtained in the final forecasting products. This is mainly due to deviations coming either from the relevant atmospheric input or from the dynamics of the wave model, resulting to systematic errors of the forecasted fields of numerical wave models, when no observation is available for assimilation. A potential solution is presented in this work, based on a combination of advanced statistical techniques, data assimilation systems, and wave models. More precisely, Kalman filtering algorithms are implemented into the wave model WAM and the results are assimilated by an Optimum Interpolation Scheme, in order to extend the beneficial influence of the latter in time and space. The case studied concerns a 3-month period in an open sea area near the South-West coast of the USA (Pacific Ocean).     

The West African monsoon onset in 2006: sensitivity to surface albedo, orography, SST and synoptic scale dry-air intrusions using WRF

In order to test the sensitivity of the transitional phase of the 2006 West African monsoon (WAM) onset to different mechanisms, weather research and forecasting (WRF) model simulations have been carried out addressing the role of the Saharan heat low (SHL) and its sensitivity to the albedo field and to the northern Africa orography, and the role of the sea surface temperature (SST) in the eastern tropical Atlantic and Mediterranean. Lowering albedo over the desert region induces a northward location of the inter-tropical convergence zone (ITCZ), while removing mountains in North Africa reduces rainfall over West Africa. Shifting SST forward by 15?days leads to a northward location of the ITCZ before the WAM onset. However none of these factors modifies the timing of the WAM onset in 2006. The transitional phase of the 2006 WAM onset has been examined in more detail. The enhancement of SHL intensity, combined with the development of the oceanic cold tongue in the Guinea gulf, leads to low-level moisture flux divergence in the ITCZ reducing rainfall and increasing low-level humidity over the Sahel. However, weakening of convection can be clearly attributed to dry-air intrusions in mid-levels, originating from the subtropical westerly jet and associated with Rossby wave pattern over North Africa. Sensitivity tests on the synoptic scale forcing outside of the WRF model domain confirm the dominating role of large-scale dynamics to control the transitional phase of the WAM onset and its timing. However it is shown that the regional factors can modulate this larger scale forcing.