Wave height statistics for seakeeping assessment of ships in the Adriatic Sea

The paper presents methodology and results of the development of sea states statistics for the Adriatic Sea. Such statistics is still lacking despite a need of the shipping industry. The presented study is based on the Atlas of Climatology containing statistics of sea states observations in the Adriatic Sea made by merchant ships during the period of 15 years. The results, presented in the Atlas in the form of “wave roses”, are digitalized and empirical frequencies of sea state occurrences are obtained. The 3-parametric Weibull distribution is then fitted through empirical data points enabling the “smoothening” of the histogram. The resulting histogram is compared with other studies for the long-term prediction of the sea states in the Adriatic Sea. The paper concludes with the discussion on the accuracy and applicability of the results.     

Dependence of the simulated seeding effects of Cb cloud on the types of the AgI agents

Summary A one-dimensional kinematic model is used to investigate the effects of silver iodide seeding in the region of a Cb cloud between isotherms of –8°C and –12°C. The agent interaction with cloud atmosphere is simulated by an improved microphysical model version which includes phoretic processes. The behaviour of the different agent types is investigated using the maximum values of agent mixing ratios and corresponding agent particle masses and radii.It is shown that the agent residence time in the seeding zone significantly depends on vertical velocity. The residence time is comparable to that previously reported. On the other side, the final graupel production decreases slightly when vertical velocity increases, while the corresponding graupel production is quite different for agents used. The main agent nucleation mechanisms are the Brownian coagulation of cloud droplets, inertial impact of cloud droplets and deposition nucleation.With 9 Figures     

Graupel growth and trajectories in a shallow cb cloud determined by a forced 1‐D model

Abstract

A kinematic model is applied to graupel growth. The vertical velocity and ther‐modynamic field data are taken from the forced 1‐D time‐dependent model of Cb cloud developed by Curie and Jane (1988). The graupel embryo pocket was released at the height of the — 10°C isotherm. The influence of the forced lifting on further graupel growth and its trajectory is analysed by sensitivity experiments based on the amplitude of the forced lifting, and initial graupel radius, density and cloud droplet concentration for the forced lifting initiation time derived from the model and the forced lifting duration time that agreed with observations. In particular, the sensitivity analysis was carried out for the forced lifting initiation and duration times.

It is shown that for large values of the forced lifting amplitudes, the residence time of the graupel within cloud and the final graupel radius may be significantly larger compared with those in the non‐forced case. The residence time in a cloud can also be significantly larger for the smallest amplitude, whereas the final radius is rather insensitive owing to oscillations around the melting level. For some cases the forced lifting causes recycling inside the updraft, contrary to the results of previous non‐forced numerical models. The recycling mechanism is sensitive to the forced lifting duration time and the time interval between the graupel pocket injection in cloud and the initiation of the forced lifting. Initially the observed recycling mechanism is a consequence of the periodic forced lifting mechanism, but then combines with recycling of the Pflaum type (1980).     

Rainfall Variability in Murang'a District, Kenya: Meteorological Data and Farmers' Perception

This study examines meteorological data and farmers' perception of rainfall in the Central Highlands of Kenya. Rainfall data from five meteorological stations during the period from 1947 to 1996 were analysed on an annual and a monthly basis. Daily data exist from 1957 and analyses from 1957 to 1996 were done on a daily basis. Discussions were held with 60 farmers about rainfall and its variation. Ten of these farmers were interviewed with a questionnaire about rainfall. Analysing annual precipitation and rain periods gave no clear trends over the study period. Results from analysing the growing seasons for maize, the main annual crop, and periods during which maize plants are sensitive to drought showed decreasing trends in rainfall amounts for the study period. Results from the interviews indicate that most of the farmers think rainfall has decreased over the last 40 years. Farmers'perceptions of rainfall are related to periods when the main food and annual crop require water and not to the periods scientists often analyse, i.e. annual and rain periods. More research and better advice from agricultural extension workers to farmers are necessary to enable this area to sustain itself in sufficient food production if the decreasing trends during the growing seasons and drought-sensitive periods continue.     

Column Density Distribution of Galactic Lyα AbsorptionSystems

A simple consistency argument for hypothesis of the galactic halo origin of the Lyα forest absorption lines is advanced, based on the recent determination of column-density vs. impact parameter relation for the low-redshift gaseous galactic haloes. It is shown that observations of neutral hydrogen absorption around luminous galaxies are consistent with the index of the power-law column density distribution derived from statistical analyses of large samples of high-redshift Lyα forest lines.PACS: 98.62.Ra, 98.62.Gq, 98.80.Es This revised version was published online in July 2006 with corrections to the Cover Date.     

Land use changes in Central Kenya from the 1950s — A possibility to generalise?

Land use changes in Kenya's central Highlands were examined with the purpose of comparing findings from Murang'a District with a new study carried out in a similar environment, Nyeri District. In addition, a generalisation of the findings was examined and methods were analysed to investigate its use in other areas. Aerial photographs were used to analyse land use and soil and water conservation in form of terraces. Furthermore, farmers were interviewed in order to obtain information about how they perceive environmental changes and how these changes have affected their livelihoods. Interpretation of aerial photographs together with field verification and interviews give information about environmental changes and their effects. Population censuses were investigated and findings were compared. Results from this study show that the two study areas have similar physical conditions with decreasing soil fertility. The main difference is the cultivated cash crops, tea and coffee in Nyeri, but only coffee in Murang'a. Also, more land is terraced in Nyeri than in Murang'a. The reason why there are adequate terraces in Nyeri is because of the multitude of cash crops that require terraces for their establishment. Compared with Murang'a, farmers in Nyeri are more satisfied with their situation, mainly due to regular payment from tea and a lower population pressure. This study shows that generalisation of the findings can be made but in order to extrapolate it further to the whole central Highlands more studies and knowledge about the whole area are required. The study also shows that methods used to examine environmental changes can be used elsewhere. Although the recommendations are to generalise with caution so that the final results are reliable and true.     

Inorganic cobalt species in seawater

Polarographic and precipitation methods were used for Co speciation in seawater taking into account only the major inorganic anionic components of seawater, such as chloride, sulphate, carbonate, hydroxide and bicarbonate. The corresponding constants were experimentally determined at the seawater ionic strength from the shift of the irreversible half-wave potential and solubility limits. The value for the carbonato-complex was obtained for the first time, while the hydroxo- and sulphato-complex values are in a fairly good agreement with the data already published. The value for the chlorocomplex was determined at $\text{I = 2.0}$ and 3.5 mol 1^?1 and extrapolated to the seawater ionic strength.The distribution of dissolved inorganic Co species in seawater was evaluated; free Co ion (hydrated) is the predominant species (about 45%) and CoSo₄⁰ and CoCl⁺ are found in approximately equal amounts (about 22%), while the fractions of CoOH⁺, Co(OH)₂⁰ and CoCO₃⁰ are between 1% and 5%.     

Surfactant production by marine phytoplankton

Electrochemical methods based on adsorption of organic molecules at the mercury electrode-solution interface were used to investigate surfactant production by marine phytoplankton. Six species of marine phytoplankton, representing the classes of Bacillariophyceae, Haeptophyceae, Chlorophyceae and Cryptophyceae, were studied in batch cultures.Our experimental results showed that surfactants were produced in culture media by healthy exponential growing cells. The measured response was found to depend on the particular species and the age of the culture.Total surfactant content in culture media generally increased with cell density, while surfactants per cell showed an inverse relation to cell density. However, we found that in Cryptomonas culture medium, during the exponential growth, excretion of the insoluble surfactant material per cell was independent of cell concentration.In addition to culture experiments, surfactant activity at several northern Adriatic stations was measured during various stages of phytoplankton bloom. It was concluded that a significant part of surfactant activity in a seawater column is due to phytoplankton production.     

Numerical modelling of dynamic consolidation on granular soils

The application of Pastor–Zienkiewicz constitutive model for sands to dynamic consolidation problems is presented in this paper. This model is implemented in a coupled code formulated in terms of displacements for both solid and fluid phases (u?w formulation), which is firstly compared with u?p_w formulation for some simple examples. Its range of validity, previously established for elastic problems and harmonic loading, is explored. Once the suitability of the u?w formulation has been ascertained for this kind of dynamic problems in soils, one‐ and two‐dimensional (plane strain) dynamic consolidation numerical examples are provided, aiming to give some light into the physics of this ground improvement technique. A ‘wave of dryness’, observed at the soil surface during the impact in field cases, is numerically reproduced and justified. Some hints on the influence of the loading zone size are also given. Copyright © 2007 John Wiley & Sons, Ltd.