1,721,043 research outputs found
A nudging scheme for the assimilation of precipitation data into a mesoscale model
No full textA nudging procedure for the assimilation of rainfall data into a mesoscale model [the Bologna Limited Area Model (BOLAM)] has been developed in order to improve short-range forecasting. The scheme modifies the model specific humidity profiles at every time step, according to the difference between observed and forecast precipitation. Different relaxation procedures are applied depending on the precipitation type (large-scale or convective rain), as estimated by the model itself. Optimizations of the nudging parameters and assessment of the scheme’s performance was carried out in an idealized framework [(Observing System Simulation Experiment) OSSE-type strategy], implementing a lagged forecast scheme. Two events were selected for this purpose, both characterized by heavy precipitation in the Mediterranean Basin. The first was a severe orographic rainfall event, associated with the passage of a frontal system over the Alps during the Mesoscale Alpine Programme (MAP) field phase, in September 1999. The second was characterized by the development of a very deep low close to the Algerian coast, where heavy precipitation caused a disastrous flood in the city of Algiers in November 2001. The effects of rainfall assimilation were evaluated, both qualitatively and quantitatively, in terms of precipitation forecasts and modification of dynamical fields, with particular attention to the impact on cyclone development. Finally, sensitivity tests were performed in order to assess the dependence of the nudging procedure on rainfall data characteristics (length of the accumulation period and associated error) and model error
Instability of Neutral Eady Waves and orography
No full textA quasigeostrophic Eady model is used to study the instability properties of finite-amplitude Eady waves and relate them to the disturbances generated by isolated bottom topography. It is found that when the amplitude of the primary wave is small the unstable perturbations are slightly deformed three-dimensional Eady modes, with growth rates and phase speed near the values obtained for the instability of the mean zonal state only. When the amplitude of the primary wave is large the most unstable modes are frontal waves, with growth rates increasing with the amplitude of the primary wave and locked in phase with it. The transition between the two regimes occurs for amplitudes of the primary wave around 10 mb. When a wave packet is generated by the interaction of a large-amplitude primary wave with orography, the character of the instability is absolute—that is, the local perturbation grows exponentially—while in the small amplitude, as well as in the zonal, case, the perturbation is advected downstream while growing, so that at any fixed spatial point only the base state remains as t → infinity
Vaia, 29 ottobre 2018: perché un vento così forte?
No full text"Vaia" storm led to to extensive distributed damage in northern Italy and southern Europe. After a short description of its genesis, we focus here on the derived strong wind that led to extensive destruction of mountain forests in north-east Italy. We stress its peculiar genesis, but also the fact that, notwithstanding the unusual conditions, the model forecasts had identified the event several days in advance. It is to be commended that, thank to this, the number of lost lives was limited
High resolution simulations of an intense convective precipitation event
No full textA newly developed non-hydrostatic model (MOLOCH), operating at a resolution of about 2 km, is run for a case of heavy precipitation over southeastern France. The event (8–9 September 2002) was characterized by intense convective activity leading to a severe flash flood in the region of the Gard river, south of the Massif Central. An almost stationary mesoscale convective system (MCS), developing well in advance of an approaching cold front, discharged a huge amount of rainfall over the same area, more than 600 mm in 24 hours.
Several simulations are performed in order to test the model set-up, evaluate the sensitivity on different initial con- ditions, and analyse the case-study. The quantitative precipitation forecasts (QPF) appear to vary widely among the experiments, depending on the initialization time chosen (00, 06, and 12 UTC, September 8). Only the run starting at 06 UTC predicts, with a satisfactory degree of accuracy, the location where the MCS developed and its almost stationary behavior during the first stage (~12 hours) of the event. In all the simulations, the convective system then propagated northward over the Massif Central.
In addition to experiments starting from standard ECMWF analyses, an assimilation procedure, based on Optimal Interpolation, is applied to the initial conditions. Surface observations of temperature, wind and relative humidity have been assimilated. The assimilation produces an improvement in the forecasts of surface fields and leads to a better location of the initial triggering phase.
Further experiments, performed by changing the orography in the model, allow the investigation of the role of the Massif Central in triggering the mesoscale convective system and in controlling its evolution
La previsione di supercelle e trombe d’aria
Full text linkLa disamina di un evento meteorologico significativo è spesso all’origine di interessanti e più ampie considerazioni, dato il notevole impatto - in termini di danni materiali, feriti e vittime - che questi fenomeni pos- sono causare. Pertanto l’analisi, dal punto di vista osservativo, sinottico e a livello di mesoscala atmosferica, dell’inconsueta sequenza di trombe d’aria che il 19 settembre 2021, in poche ore, ha provocato seri danni su varie aree della Lombardia e dell’Emilia-Romagna, offre utili spunti di riflessione riguardo l’attuale capacità di prevedere le trombe d’aria in Italia e sul possibile sviluppo di apposite reti di allerta
Numerical mesoscale simulations of the atmospheric flow over Antarctica
No full textThe atmospheric circulation over Antarctica during January 1994 is simulated using a limited area model, Eta model (version 1993, 55km horizontal resolution, 17 levels). The simulations are performed with initialized analyses from European Centre for Medium Range Weather Forecasts (ECMWF) as initial and boundary conditions. The results are in good agreement with the analyses even if the mesoscale features on the high Plateau are poorly represented. The model deficiency over the Plateau is emphasised in the medium-range simulation. Additional comparisons of model results and AWS observations will be presented
The role of surface fluxes in the development of a tropical-like cyclone in southern Italy
Full text linkNumerical simulations of a tropical-like cyclone in southern Italy have been performed with two different modelling systems (BOLAM-MOLOCH and WRF) with the aim of discussing the role of the surface fluxes in the development of the vortex and evaluating their intensity during the mature stage of the cyclone. Although significant differences emerge in their intensity, both the modelling systems agree in showing that the surface fluxes are more important than the latent heat release associated with convection in the initial phase of the vortex lifecycle, while they are less relevant (although more intense) when the minimum assumes the characteristic of a tropical cyclone
Orographic triggering of long lived convection in three dimensions
Full text linkA significant fraction of the occurrences of intense flash floods is due to quasi-stationary or long-lived convection that may insist on the same place for many hours, producing high values of accumulated precipitation. One of the elements that favour the initiation and anchoring of the convective system (MCS) is the orography. In one of the most severe floods (Gard basin in southern France, 8-9 September 2002), the orography of the Massif Central played a rather unusual role, favouring the onset and maintenance of the MCS at some distance upstream of the main orographic slope. In the present work the initial atmospheric conditions of this event have been largely idealized, taking horizontally uniform values for wind, temperature and humidity profiles, and a simplified isolated orography representing the sole Massif Central. A convective system is initiated in the non-hydrostatic simulations, embedded in a quasi-stationary solution of flow over the orography. It is shown that the triggering of convection occurs in the convergence zone immediately upstream of the orographic obstacle, at an altitude comparable with the mountain height. The subsequent growth of the mesoscale convective system is associated with a slow eastward drift, with the intense precipitation located upstream of the mountain and with the formation of a gust front that propagates against the incoming basic flow. Sensitivity experiments show that the development of convection critically depends on mountain height and moisture content. Although the results obtained in such idealized conditions do not reflect all the observed characteristics of the real event, they contribute to clarify the role of the orography in triggering and maintaining strong convection
Intercomparison of satellite-based and model-based rainfall analyses
No full textFour satellite rain estimations based on microwave (MW), infrared (IR) or combined MW-IR techniques are compared with the BOlogna Limited Area Model (BOLAM) rain forecast for a severe weather event (8–13 November 2001) over the western Mediterranean Sea.
Two of the investigated multi-channel MW rainfall algorithms use data from the Tropical Rainfall Measuring Mission (TRMM). The Frequency Difference Algorithm relies on data from the TRMM Microwave Imager (TMI) and the other one combines data from the Precipitation Radar (PR) with those from the nine-channel radiometer TMI, called PR Adjusted TMI Estimations of Rainfall (PATER) algorithm. The pure IR Rain Estimator uses geostationary IR METEOSAT data and the combined Naval Research Laboratory algorithm uses both MW data from low orbiting satellites and IR data from the geostationary orbit.
Validation results, computed over a common grid, which is independent of the different field of view sizes of the applied data sets, indicate that there is generally a better performance for heavy rain (> 6 mm h−1) than for light rain (<1 mm h−1). Both MW algorithms perform rather similarly, although PATER shows some rain detection problems due to thick aerosol loads originating from the desert. The BOLAM model presents a good agreement with the MW and only a minor location error of a heavy rain area was detected. Both IR-based algorithms have problems in identifying the correct rainy areas compared to MW. Overall, the results suggest that there are advantages in combining both techniques – the well-known rain physics of the MW channels with the high temporal resolution of IR algorithms – to retrieve precipitation from satellite data
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