13. Orlić, M.: Wind-induced currents directed to the left of the wind in the northern hemisphere: An elementary explanation and its historical background.
Every beginning student of geophysical fluid dynamics knows that steady-state wind-driven currents are directed to the right of the wind in the northern hemisphere. In contrast, the fact that wind-induced currents may under some specific conditions be of the opposite direction is mentioned in only a few papers. Here, a succession of simple explicit solutions is used to illustrate that the wind varying at subinertial frequencies always generates currents directed to its right, whereas at superinertial frequencies the wind rotating clockwise (counterclockwise) coincides with currents directed to its left (right). The difference is related to a dynamics differing in the two frequency bands. At subinertial frequencies, friction is primarily balanced by the Coriolis acceleration, and therefore the currents vary in phase with the wind while being directed to its right. At superinertial frequencies, the primary balance is between friction and local acceleration, implying that the currents lag behind the wind that causes them while pointing down the forcing. The former dynamic regime was originally considered by Vagn Walfrid Ekman (in 1905), the latter by Karl Jakob Zöppritz (in 1878). The destiny of the two contributions, however, was completely different: Ekman’s is widely appreciated today, whereas Zöppritz’s is rarely mentioned. Although Zöppritz’s findings were adversely influenced by his consideration of the laminar rather than turbulent viscosity, his neglect of the Coriolis acceleration was acceptable and his solution therefore formally correct at superinertial frequencies.
Keywords: wind-driven currents, subinertial frequencies, Ekman, superinertial frequencies, Zöppritz
14. Basseka, C. A., Y. Shandini and J. M. Tadjou: Subsurface structural mapping using gravity data of the northern edge of the Congo Craton, South Cameroon.
In this work, the subsurface structures in a part of the northern edge of the Congo Craton in south Cameroon between the latitudes 2° 30´ to 4° 30´ N and the longitudes 11° to 13° E, has been estimated by the interpretation of gravity data. For quantitative interpretation of the sources of the anomaly, a 3D density model of the upper crust was designed by means of forward modelling and inversion constrained by surface geology and results from Euler deconvolution and spectral analysis methods. The Bouguer anomaly map of the area is characterized by elongated SW-NE trending negative gravity anomaly which correspond to a collapsed structure associated with a granitic intrusion beneath the center of the region and by various gravity highs associated with an uplift of the Pan-African basement in the north and delimited by a strong gradient in relation with the tectonic boundary between the Congo Craton and the Pan-African belt. Our result demonstrated that tectonic structures associated to observed gravity anomalies in the region were put in place during a major continental collision.
Keywords: Bouguer anomaly, Congo Craton, Euler deconvolution, Spectral analysis, 3D gravity modelling
15. Orešković, J., F. Šumanovac and E. Hegedűs: Crustal structure beneath Istra peninsula based on receiver function analysis.
The structure of the Earth’s crust has been determined at three temporary seismic stations in Istra (Croatia) installed as a part of the passive seismic experiment ALPASS‑DIPS (Alpine Lithosphere and Upper Mantle PASsive Seismic Monitoring - DInarides-Pannonian Segment). The stations were located at the north-eastern edge of the Adriatic microplate. The knowledge of the crustal structure under Istra will help the understanding of the tectonic evolution within the broader region of the contact between Adria and Eurasia. Teleseismic data recorded at three-component stations were analyzed using the P receiver functions method, which allows detecting seismic discontinuities within the crust and upper mantle below the stations. To determine more detailed crustal structure, we have done 1‑D forward modelling of receiver functions. The results of modelling are the S-wave velocity models of the crust beneath the stations. Calculated receiver functions showed three converted phases in the first 5 s of delay time, thus suggesting three seismic discontinuities in the crust, that is, discontinuity in the shallowest part of the upper crust, intracrustal discontinuity, and the Mohorovičić discontinuity. A forward modelling approach at all three stations showed a shallow high‑velocity zone observed at a depth between 2 and 8 km. This zone may probably be related to an anhydrite series with dolomite alternations characterised by high seismic velocity. Intracrustal discontinuity is defined at a depth between 18 and 21 km. Models of the shear velocity at the three stations show a decreasing of the Moho depth from 43 km at the northern Istra to 37 km at the south-eastern part of Istra.
Keywords: Istra, crustal structure, P receiver function, forward modelling
16. Mandeep, J. S.: Comparison of rain rate models for equatorial climate in South East Asia.
Statistics of 1 minute rain rate has a major impact in the design of satellite communication systems at frequencies above 10 GHz. The effect of rain causes serious degradation of radio signals at frequencies above about 10 GHz; therefore, models for the prediction of statistics of excess path attenuation needed for the design of communication propagation paths requires a statistical description of rain-rate occurrences. In this paper, the tasks are tackled by processing 3 years rain rate data for selected sites in the equatorial region. A comparison between rain rate data set with a sampling period of 1 minute and existing rain rate prediction models is presented.
Keywords: rain rate, rain attenuation, satellite communication, radio-wave propagation
Kotal, S. D. and Roy Bhowmik S. K.: A
Multimodel Ensemble (MME) Technique for Cyclone Track Prediction over the North
Indian Sea .
A multimodel ensemble (MME) technique for predicting track of tropical cyclones over the North Indian Sea has been proposed. The technique is developed applying multiple linear regression procedure. Parameters of the ensemble technique are determined from the forecast datasets on the tracks of tropical cyclones over the North Indian Sea during the year 2008-2009. The parameters selected as predictors are: forecast latitude and longitude positions at 12-hour interval up to 72-hours forecast of five operational numerical weather prediction models. The dynamical models included for development of the ensemble technique are: (i) forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), (ii) the National Centers for Environmental Prediction Global Forecast System (NCEP), (iii) the MM5 model, (iv) the Quasi-Lagrangian model (QLM) and (v) the model of Japan Meteorological Agency (JMA). A collective bias correction is included in the ensemble technique in which a multiple linear regression based minimization principle for the model forecast position against to the observed position is applied. These bias factors are described by separate weights at every 12-hours interval up to the 72-hour forecasts for each of the member model. When the technique is tested with the independent samples, forecast skill of the MME technique is found to be reasonably good. The average error ranges from of the order of 74 km to 290 km for forecasts up to 72-hour. Performance of the MME technique shows that there are skill improvements up to 30 km for the position errors over the best model at 72-hour forecast. The forecast skill of the MME technique for forecasts up to 72-hour also shows an improvement as compared to the forecasts from member models and the simple ensemble mean (ENM).
Keywords: tropical cyclone, track prediction, multiple linear regression, regression coefficient, ensemble mean and multimodel ensemble technique
18. Gajić-Čapka, M. and Cindrić, K.:
Secular trends in indices of precipitation extremes in Croatia, 1901–2008 .
In this study, trends in annual and seasonal precipitation amounts in Croatia are discussed. This discussion is followed by trend analysis in seven indices of precipitation extremes, which indicate intensity and frequency of extreme rainfall events. These indices have been proposed by World Meteorological Organisation and are calculated using daily precipitation amounts. The data sets used in this study cover the period 1901-2008 at five meteorological stations distributed among different climate conditions in Croatia: continental, mountainous and maritime. The trends are estimated by a simple least squares fit of the linear model and tested for statistical significance by a non-parametric Mann-Kendall test. The time series with significant trends are identified, and a Sneyers progressive analysis is then performed to determine the beginning of the trend. The time series analysis of coefficients of variation in consecutive 30-year periods indicates the variability in precipitation. The extreme quantiles for annual one-day and five-day precipitation maxima have been estimated by the Generalised Extreme Value (GEV) distribution and discussed in relation to the original time-series. The results show a downward trend in annual precipitation amounts since the beginning of the 20th century throughout Croatia, which agrees with the drying trend observed across the Mediterranean. Precipitation amounts have large interannual variability, on both annual and seasonal scales. By the end of the 20th century, the precipitation variability decreased in the north-western Croatian mountainous and northern littoral regions and the eastern lowlands. The Dalmatian Islands experienced increased variability since the middle of the 20th century. In regions of drying, such as Croatia, there is no evidence of major secular changes in precipitation extremes that are related to the high amounts of precipitation and the frequency of heavy rainfall days over the majority of Croatia.
Keywords: trend analysis, precipitation variability, indices of precipitation extremes, seasonal precipitation, annual precipitation, GEV distribution, Croatia
19. Verbanac, G.: Modeling the geomagnetic field over Croatia.
During the last years investigation of the geomagnetic field in Croatia has started and an effort to establish the geomagnetic observatory has been made. Based on the total field measured over the northern eastern part of Croatia, the suitable position for installing the geomagnetic observatory is proposed.
In this paper the core field behavior from 1961 to 2002 over the entire Croatian territory was investigated by exploiting different global models. Using a regional European model the secular variation over the country was calculated. The evolution of the core field at the potential location for installing the geomagnetic observatory was analyzed in detail. The calculated field variations follow the general core field variations over Europe and there are not variations due to different induced fields or other effects at the future observatory location.
This study contributes to the investigation of the potential observatory location and paves the way for better understanding of the geomagnetic field behavior over Croatia.
Keywords: geomagnetic secular variation, modeling, geomagnetic observatory