Search

Modelling forecast uncertainty is a difficult task in any forecasting problem. In weather forecasting a possible solution is the use of forecast ensembles, which are obtained from multiple runs of numerical weather prediction models with various initial conditions and model parametrizations to provide information about the expected uncertainty. Currently all major meteorological centres issue forecasts using their operational ensemble prediction systems. However, it is a general problem that the spread of the ensemble is too small compared to observations at specific sites resulting in under‐dispersive forecasts, leading to a lack of calibration. In order to correct this problem, various statistical calibration techniques have been developed in the last two decades. In the present work different post‐processing techniques were tested for calibrating nine member ensemble forecasts of temperature for Santiago de Chile, obtained by the Weather Research and Forecasting model using different planetary boundary layer and land surface model parametrizations. In particular, the ensemble model output statistics and Bayesian model averaging techniques were implemented and, since the observations are characterized by large altitude differences, the estimation of model parameters was adapted to the actual conditions at hand. Compared to the raw ensemble, all tested post‐processing approaches significantly improve the calibration of probabilistic forecasts and the accuracy of point forecasts. The ensemble model output statistics method using parameter estimation based on expert clustering of stations (according to their altitudes) shows the best forecast skill. [ABSTRACT FROM AUTHOR]

Nitrogen is one of the two most important elements in the metabolism of aquatic ecosystems. At low concentrations it can limit primary productivity and when present at very high concentrations it can participate in the eutrophication process of these environments. The mechanism of nitrogen transport in sediments is almost unknown, nevertheless it is of vital importance for establishing mass balances in aquatic systems. In the study presented here, we assessed the nitrogen flux in sediments of the central part of tropical Lake Maracaibo, Venezuela, in particular with regard to dissolved oxygen concentrations. Experiments were performed under laboratory conditions in a batch system, and at varying dissolved oxygen regimes (aerobic and anaerobic). Every two days, during 3 months, overlying water samples were taken to analyze nitrite, nitrate, ammonium and Kjeldahl total nitrogen. Average release rates of total nitrogen were 0.86 in aerobic, and 1.06 mmol N m-2 d-1 in anaerobic systems corresponding to 41.7% of total N input to the lake. The behavior of nitrogen was strongly influenced by nitrate concentrations under aerobic conditions, and by organic nitrogen under anaerobic conditions during the course of the experiment. A major trend for the release of organic nitrogen during anaerobic conditions, and of nitrate, during aerobic conditions, was observed. Also fluxes of NO3-, NH4+, organic N and Total N across the sediment-water interface were measured. In anaerobic conditions, which are predominant in the hypolimnetic cone of Lake Maracaibo, denitrification was estimated to be 0.02 mmol N m-2 d-1, which corresponds to 1.89% of the total N released from sediments. This is to our knowledge the first study of nitrogen fluxes in sediments from Lake Maracaibo. The laboratory data presented here reflects conditions in the lake when major nutrients accumulation occurs. [ABSTRACT FROM AUTHOR]