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Badescu V.,Polytechnic University of Bucharest | Badescu V.,Romanian Academy of Sciences | Dumitrescu A.,Meteo Romania National Meteorological Administration
Meteorology and Atmospheric Physics | Year: 2016

Two products provided by the climate monitoring satellite application facility (CMSAF) are the instantaneous Cloud Fractional Coverage (iCFC) and the instantaneous Cloud Type (iCTY) products. Previous studies based on the iCFC product show that the simple solar radiation models belonging to the cloudiness index class nCFC = 0.1–1.0 have rRMSE values ranging between 68 and 71 %. The products iCFC and iCTY are used here to develop simple models providing hourly estimates for solar global irradiance. Measurements performed at five weather stations of Romania (South-Eastern Europe) are used. Two three-class characterizations of the state-of-the-sky, based on the iCTY product, are defined. In case of the first new sky state classification, which is roughly related with cloud altitude, the solar radiation models proposed here perform worst for the iCTY class 4–15, with rRMSE values ranging between 46 and 57 %. The spreading error of the simple models is lower than that of the MAGIC model for the iCTY classes 1–4 and 15–19, but larger for iCTY classes 4–15. In case of the second new sky state classification, which takes into account in a weighted manner the chance for the sun to be covered by different types of clouds, the solar radiation models proposed here perform worst for the cloudiness index class nCTY = 0.7–0.1, with rRMSE values ranging between 51 and 66 %. Therefore, the two new sky state classifications based on the iCTY product are useful in increasing the accuracy of solar radiation models. © 2016 Springer-Verlag Wien Source


Birsan M.-V.,Meteo Romania National Meteorological Administration | Zaharia L.,University of Bucharest | Chendes V.,National Institute for Hydrology and Water Management | Branescu E.,National Institute for Hydrology and Water Management
Hydrological Processes | Year: 2014

Mean daily streamflow records from 44 river basins in Romania with an undisturbed runoff regime have been analyzed for trends with the nonparametric Mann-Kendall test for two periods of study: 1961-2009 (25 stations) and 1975-2009 (44 stations). The statistical significance of trends was tested for each station on an annual and seasonal basis, for different streamflow quantiles. In order to account for the presence of serial correlation that might lead to an erroneous rejection of the null hypothesis, a trend-free prewhitening was applied to the original data series. The regional field significance of trends is tested by a bootstrap procedure. Changes in the streamflow regime in Romania are demonstrated. The main identified trends are an increase in winter and autumn streamflow since 1961 and a decrease in summer flow since 1975. The streamflow trends are well explained by recent changes in temperature and precipitation that occurred in the last 50years. © 2013 John Wiley & Sons, Ltd. Source


Birsan M.-V.,Meteo Romania National Meteorological Administration
Water Resources Management | Year: 2015

Natural and reconstructed monthly streamflow records from 172 river basins in Romania (143 with full records and 29 with less than 10 % missing values) have been analyzed for trends with the nonparametric Mann-Kendall test for the period 1956–2005. The statistical significance of trends was tested for each station on a monthly basis. Changes in the streamflow regime in Romania are demonstrated. The monthly flow presents upward trends from August to January, and predominantly downward trends from February to June. The most important changes are the increasing streamflow trends from September till November, which are well explained by the increase in autumn precipitation. The annual streamflow shows a dependence on latitude, with increasing trends in the northern part, and decreasing in the south. Strong negative correlations between the North Atlantic Oscillation and the mean annual streamflow have been found in western and southern Romania, highlighting the influence of the large-scale atmospheric circulation on Romanian annual streamflow in these areas, as well as the important orographic role of the Carpathian Mountains. © 2015, Springer Science+Business Media Dordrecht. Source


Birsan M.-V.,Meteo Romania National Meteorological Administration
Romanian Reports in Physics | Year: 2013

The spatially distributed rainfall-runoff model TOPKAPI (TOPographic Kinematic APproximation and Integration) has been widely used for continuous modelling of floods. The model utilises three non-linear reservoir differential equations for the drainage in the soil, the overland flow on saturated or impervious soil, and the channel flow along the drainage network, respectively. The reservoirs derive from the integration in space of the non-linear kinematic wave model. The geometry of the catchment is described by a lattice of cells - the pixels of the digital elevation model (DEM) and their slope - over which the equations are integrated to lead to a cascade of non-linear reservoirs. The parameterisation relies on the digital thematic maps of soil, geology and land use. The model was applied on the upper river basin of Someşul Mare, upstream Beclean (4328 km2) for the 2000-2006 interval: the years 2000-2002 were used for calibration, the model being validated for the 2003-2006 period. The soil and the landuse maps were reclassified with respect to hydrological properties (e.g., soil depth, soil texture, surface roughness, canopy interception). For the time-dependent input, precipitation and temperature from eight meteorological stations have been used. The trial-and-error calibration - based on visually matching the modelled streamflow with the observed one - managed to reproduce the behaviour of the catchment while keeping the parameters within their physically meaningful values. The model reproduced well the behaviour of the streamflow, the peak time, the increase and the recession of the floods. In general, the small floods were overestimated in terms of peak flow. However, considering that only one station (out of eight) is located inside the basin, the first modelling results are very satisfactory. Source


Birsan M.-V.,Meteo Romania National Meteorological Administration | Dumitrescu A.,Meteo Romania National Meteorological Administration
International Journal of Climatology | Year: 2014

Daily measurements of snow depth, cumulated precipitation and mean temperature from 105 meteorological stations with continuous record over the 1961-2010 period were analysed for trends with the Mann-Kendall nonparametric test. Trends in the number of days with snow cover, and in the mean snow depth are decreasing at 29 and 18% of the stations, respectively. The decrease in snow depth affects the intra-Carpathian region and Northeastern Romania. The most dramatic change concerns the number of snowfall days, which is decreasing at 82% of the locations. There is a slight decrease in precipitation amount, significant at only 8% of the stations. The mean temperature is increasing at 47% of the stations, while the number of days with temperature over 0 °C shows upward trends at 63% of the stations. Overall, the winter season in Romania has changed substantially. All snow-related parameters show significant negative correlations with the North Atlantic Oscillation (NAO) index for winter. The NAO has a strong impact throughout the country suggesting that the winter variability in Romania is driven by the large-scale circulation over the North Atlantic. © 2013 Royal Meteorological Society. Source

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