1,720,994 research outputs found
The statistical properties of general atmospheric circulation:observational evidence and a minimal theory of bimodality
No full textn this paper we address the problem of constructing a minimal model of atmospheric circulation able to describe the bulk of the statistical properties of the general circulation in middle latitudes. After a brief reexamination of the Charney-DeVore theory of multiple equilibria, we compare the statistical properties that can be deduced from it with those resulting from analysis of observations of the northern hemisphere middle-latitude circulation at 500 mb. While the prediction concerning the amplitude of the waves seems to be confirmed by the presence of a clear bimodality of the statistical distribution, we find no trace of bimodality in the zonal wind. We show how the theory can be brought into agreement with observations by introducing nonlinearity into the wave equation in such a way that the Charney-DeVore resonance curve is bent, producing different states corresponding to the same value of the zonal wind. We finally address ourselves to the question of what physical processes determine the observed unimodal statistics of zonal wind, a subject of future work
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
Effects of finite height topography on non-geostrophic baroclinic instability : implications to theories of lee cyclogenesis
No full textBaroclinic instability in the presence of steep finite amplitude topography is studied in the primitive equation model. The quasi-geostrophic theory of Alpine cyclogenesis of Speranza et al. is reanalyzed and discussed in this context.
The present model is a generalization of the one used by Stone to include topographic effects, lateral shear of the basic wind, and/or lateral walls. We focus in particular on the differences between this formulation and the quasi-geostrophic one when the meridional scale of the topography is very small (of the order of 100 km). We find that only in the primitive equation model does a small-volume mountain, of height and width comparable with those of the Alps, introduce significant large-scale modifications to the baroclinic modes. The most unstable mode attains its maximum amplitude to the southern side of the mountain. We show that these results do not depend upon the specification of the lateral boundary conditions provided the basic state baroclinicity is meridionally confined
Previsione e controllo di eventi idrologici estremi: l'alluvione del novembre 1966 nel Triveneto
No full textOrographic 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
High resolution simulations of a flash flood near Venice
Full text linkDuring the MAP D-PHASE (Mesoscale Alpine Programme, Demonstration of Probabilistic Hydrological and Atmospheric Simulation of flood Events in the Alpine region) Operational Period (DOP, 1 June–30 November 2007) the most intense precipitation event observed south of the Alps occurred over the Venice Lagoon. In the early morning of 26 September 2007, a mesoscale convective system formed in an area of convergence between a south-easterly low level jet flowing along the Adriatic Sea and a north-easterly barrier-type wind south of the Alps, and was responsible for precipitation exceeding 320 mm in less than 12 h, 240 mm of which in only 3 h. <br><br> The forecast rainfall fields, provided by several convection resolving models operated daily for the D-PHASE project, have been compared. An analysis of different aspects of the event, such as the relevant mechanisms leading to the flood, the main characteristics of the MCS, and an estimation of the predictability of the episode, has been performed using a number of high resolution, convection resolving models (MOLOCH, WRF and MM5). <br><br> Strong sensitivity to initial and boundary conditions and to model parameterization schemes has been found. Although low predictability is expected due to the convective nature of rainfall, the forecasts made more than 24 h in advance indicate that the larger scale environment driving the dynamics of this event played an important role in favouring the achievement of a relatively good accuracy in the precipitation forecasts
On the correct surface stress for the prediction of the wind wave field and the storm surge in the Northern Adriatic Sea
Full text linkThis paper discusses which formulation of the surface stress over the sea determines the most accurate prediction of the wind wave field and storm surge in
the Northern Adriatic Sea. The study shows that the results of the storm surge and wind wave models, when compared to the available observations, can be used for the validation of the surface stress and of the expression adopted for the ssr (sea surface roughness). The results are representative of short fetch and young wind sea conditions. The agreement between the results and the measurements shows the feasibility of the wind wave and storm surge predictions in the Adriatic Sea and supports the dependence of the ssr, and, therefore, of the surface stress, on the spectrum of the surface wave
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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