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2000 Meteorology and Atmospheric Physics
Background and key results of the EU-funded project HERA are presented. The identification of the growing possibilities to use forecast precipitation fields as input for hydrological models is followed by a broad assessment of the state of the art regarding the determination of the atmospheric part of the hydrological cycle, with the geographical focus on the mountainous region of the Alps in the heart of Europe. This includes the construction of (north)Alpine radar composites for nine episodes; a first systematic cross-validation on a daily basis of four operational forecasting models with a trans-national gridded observat ional dataset; several detailed case studies involving research versions of operational forecast models and the latest advances in model resolution and microphysical parameterizations; new algorithms for Doppler radar retrievals over complex terrain; and synthetic modelling studies with governing parameters derived from the selected cases to investigate some basic processes in isolation. Finally, implications for proto-type applicatio ns of forecast areal precipitation fields in the hydrological modelling of mountainous catchments are given.
Hagen, M.; Schiesser, H.-H.; Dorninger, M.
2000 Meteorology and Atmospheric Physics
Mesoscale convective precipitation systems in the Alpine region are studied by analyzing radar and rain gauge data. The data from weather radars in Austria, France, Germany, and Switzerland are combined into a composite. Availability of radar data restricts the study mainly to the northern part of the Alpine region. Mesoscale convective systems (MCS) occur often in this region and are comparable to large systems observed in the USA. Seven precipitation events lasting one to six days from the years 1992–1996 are examined in detail. They all moved west to east and showed no diurnal preference in formation or dissipation. They reach sizes of 2 − 6 · 104 km2. MCS with leading-line trailing-stratiform structure tended to be larger and more intense. A 25-year set of rain gauge data indicates that a giant MCS (covering more than 4 · 104 km2 with more than 30 mm/day) occurs every 6 years in the northern Alpine region. MCS occur more frequently in the southern Alpine region.
Steinacker, R.; Dorninger, M.; Wölfelmaier, F.; Krennert, T.
2000 Meteorology and Atmospheric Physics
A new automatic tracking method for convective cells and cell complexes is introduced. The method uses a simple Gaussfilter and a selection procedure to define displacement vectors of the specific system. The change of the half width of the filter results in a separation of the system’s scale. The proposed method is applied on two data sources: lightning density and RADAR reflectivity. They describe different properties of convective cells and cell complexes. A strong connection of the storm tracks to topography becomes also evident.
Mladek, R.; Barckicke, J.; Binder, P.; Bougeault, P.; Brzovic, N.; Frei, C.; Geleyn, J. F.; Hoffman, J.; Ott, W.; Paccagnella, T.; Patruno, P.; Pottier, P.; Rossa, A.
2000 Meteorology and Atmospheric Physics
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.
2000 Meteorology and Atmospheric Physics
Several episodes of heavy precipitation, which occurred in the region south of the Alps, have been simulated by means of the mesoscale model BOLAM3. Each case was run at 3 different resolutions, up to 4 km grid interval. The quantitative precipitation “forecast” fields are compared with available rain data. In general, satisfactory results are obtained in terms of spatial distribution and timing of precipitation, except in cases in which pre-frontal convection is dominant. The diagnostics of phenomena involved in orographic precipitation identify the different mesoscale atmospheric features associated with the interaction with topography, like the formation of low level jets, convergence zones, rainbands, and organized convective systems. These appear as “ingredients” common to all the cases considered and are shown to be sensitive to orographic forcing, as well as to the latent heat exchange processes.
Cacciamani, C.; Cesari, D.; Grazzini, F.; Paccagnella, T.; Pantone, M.
2000 Meteorology and Atmospheric Physics
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.
2000 Meteorology and Atmospheric Physics
Quality and limitations of current quantitative precipitation simulations are determined using the Deutschland-Modell of Deutscher Wetterdienst (DWD). The model independent validation data comprise the regular rain-gauge network of DWD, taylored composites of an international radar network, Doppler winds from the DLR research radar, and meteorological timeseries at a ground station. Area skill scores are compared for convectively dominated as well as more synoptically forced situations in the northern Alpine region. Inspection of the temporal evolution of the components comprising the atmospheric water budget and of the precipitation partition gives insight in the different nature of excitation mechanisms of heavy precipitation. The simulations are found to exhibit growing potential for follow-on hydrological applications, while a real break-through appears to necessitate the coming generation of non-hydrostatic operational forecast models with increased spatial resolution.
Hofinger, S.; Mayr, G. J.; Dreiseitl, E.; Kuhn, M.
2000 Meteorology and Atmospheric Physics
¶Intra-mountain summertime precipitation was studied in the Alps in a 40×20 km2 area centered around Innsbruck, Austria, from June through September 1997. An observational network with a mean separation distance of 9 km and forecasts from the ECMWF model were used to examine the role the strong forcing from the lower boundary plays in creating “hot spots” for the formation of thunderstorms and the location of heavy precipitation as well as systematic precipitation patterns for different weather situations, which can be used to downscale forecasts from global scale routine numerical weather prediction models.
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