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In the field of hydrological prediction for medium-sized watersheds, characterized by complex orography and short response times, forecasts cannot rely only upon observed precipitation: predicted rainfall is in this case an essential input for hydrological models. However, the quality and reliability of deterministic numerical precipitation forecasts driving a hydrological model are often unsatisfactory, because uncertainty in Quantitative Precipitation Forecasts (QPFs) is considerable at the scales of interest for hydrological purposes. The uncertainty inherent in precipitation forecast can be accounted for better estimating the uncertainty associated with the flood forecast, in order to provide a more informative hydrological prediction. The methodology proposed and adopted in this work is based on a hydrological ensemble forecasting approach that uses multiple precipitation scenarios provided by different high-resolution numerical weather prediction models, driving the same hydrological model. In this way, the uncertainty associated with the meteorological forecasts can propagate into the hydrological models and be used in warnings and decision making procedures relying upon a probabilistic approach. In the framework of RISK AWARE, an INTERREG III B EU project, a detailed analysis of two cases of intense precipitation affecting the Reno river basin, a medium-sized catchment in northern Italy, has been performed. One case study has been performed using lateral boundary values derived from analysed fields, the other simulating a real time forecast, i.e., using forecasted boundary conditions. Four different meteorological models (Lokal Modell, RAMS, BOLAM and MOLOCH), operating at different horizontal resolutions, provide QPFs which are used to force the hydrological model. The discharge predictions are obtained by means of the physically based rainfall-runoff model TOPKAPI. The results provide examples of the uncertainties inherent in the QPF and show that the hydrological response of the Reno river basin, as simulated by the TOPKAPI model, is highly sensitive to the correct space-time localization of precipitation, even if the total amount of rainfall is, on average, well forecasted. The system seems able to provide useful information concerning the discharge peaks (amount and timing) for warning purposes. [ABSTRACT FROM AUTHOR]

The precipitation forecasts of four operational numerical weather prediction models over the Alpine region are evaluated and intercompared for two periods of interest to the Mesoscale Alpine Programme (MAP). The new analysis of Alpine rainfall of Frei and Schaär (1998) is used to validate the models. It is found that the models have a tendency to overestimate the total precipitation and the frequency of intense rain events over high orography. The skill scores show good consistency between models, except for the ability to forecast light rain or heavy rain events. The partition between convective and stratiform rainfall is rather variable between the models. [ABSTRACT FROM AUTHOR]

In this paper we describe the results of several numerical experiments performed with the limited area model LAMBO, based on a 1989 version of the NCEP (National Center for Environmental Prediction) ETA model, operational at ARPA-SMR since 1993. The experiments have been designed to assess the impact of different horizontal resolutions and initial conditions on the quality and detail of the forecast, especially as regards the precipitation field in the case of severe flood events. For initial conditions we developed a mesoscale data assimilation scheme, based on the nudging technique. The scheme makes use of upper air and surface meteorological observations to modify ECMWF (European Centre for Medium Range Weather Forecast) operational analyses, used as first-guess fields, in order to better describe smaller scales features, mainly in the lower troposphere. Three flood cases in the Alpine and Mediterranean regions have been simulated with LAMBO, using a horizontal grid spacing of 15 and 5 km and starting either from ECMWF initialised analysis or from the result of our mesoscale analysis procedure. The results show that increasing the resolution generally improves the forecast, bringing the precipitation peaks in the flooded areas close to the observed values without producing many spurious precipitation patterns. The use of mesoscale analysis produces a more realistic representation of precipitation patterns giving a further improvement to the forecast of precipitation. Furthermore, when simulations are started from mesoscale analysis, some model-simulated thermodynamic indices show greater vertical instability just in the regions where strongest precipitation occurred. [ABSTRACT FROM AUTHOR]

In this paper the problem of computing large-scale trajectories of atmospheric parcels is examined in the framework of a numerical limited-area model used for weather prediction. The parcel of air is considered to be a completely passive tracer, having no interaction with the environment, even when going through an area of deep atmospheric convection. The usual method of computing the trajectory after the integration, using model-produced wind fields every six hours, is compared with the method of computing the trajectory in an almost continuous fashion, during the integration of the model itself, providing in this way an estimate of the time-truncation errors associated to the first, more usual, method. This second method is found to be quite superior and can, in selected instances, lead to largely different results, as documented. Examples of beams of trajectories going through a strongly diffluent flow-field, as well as through an area of strong convection are shown and found to be very realistic, within the limitations imposed by model resolution. In addition, the sensitivity of trajectories to the definition of the vertical coordinate is explored and found to be large. [ABSTRACT FROM AUTHOR]

A number of forecast experiments were performed in order to assess the capability of reproducing, by means of a limited-area numerical model, the highly structured mesoscale circulations occurring in the Po Valley of Northern Italy during a north-westerly cold front passage across the Alpine chain, with particular attention to the modelling of the effects of organized convection. The case-study occurred during summer 1987 and the model used throughout was the 1989 version of the UB/NMC Limited Area Model (University of Belgrade, National Meteorological Centre of Washington). The model was integrated both with eta, η, and sigma, ϑ, as vertical coordinates and ECMWF initialized analyses were always used as initial conditions. ECMWF initialized analysis or operational forecast fields were also used for updating in time the lateral boundary conditions. Experiments show qualitative and quantitative agreement with observations, both in upper-air geopotential height fields, in MSLP and in cumulated precipitation. Several modelling issues were also investigated, e.g. sensitivity of the results to horizontal and vertical model resolution and to the influence of the lateral boundaries poitioning, finding large effects of the latter on quantitative precipitation fields. Difficulties in modelling very localized mesoscale phenomena, e.g. organized convective thunderstorms in the Po Valley and Alpine North Foehn in the Milan area, were generally encounted. [ABSTRACT FROM AUTHOR]