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Piero Malguzzi

Abstract

The feedback between large-scale stationary Rossby waves and small-scale high-frequency eddies is computed analytically in the framework of a barotropic and frictionless model atmosphere. The Rossby wave is meant to model blocking situations characterized by high-over-low dipoles and flow splitting, while the eddies represent synoptic disturbances propagating in the deformation field of the blocking pattern. Apart from a westward contribution to the phase speed of the large-scale pattern, the eddy forcing (Reynolds stress), averaged in time, turns out to be a nonlinear function of the large-scale streamfunction amplitude. It is shown that the eddy forcing always determines, in the time average, a steepening of the flow split associated with the block and (possibly) multiple large-scale amplitudes. A comparison between these results and previous studies about eddy forcing of blocking situations is attempted.

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Roberto Benzi
and
Piero Malguzzi

Abstract

The problem of resonance and meridional confinement of planetary-scale waves is investigated in the framework of a quasigeostrophic, two-layer, β-plane model with lateral sponge layers. It is shown that the model resonant wavenumber is meridionally confined despite the predictions of the linear barotropic theory based on the time-mean zonal wind. As a consequence, the fully nonlinear model exhibits a resonant response to orographic forcing significantly higher than that predicted by the linearized (baroclinic) version of the model itself.The model response to a localized (zonally and meridionally) orography is also studied and found to be consistent with previous results observed in spherical geometry. The significance of this result and the mechanisms leading to meridional confinement are discussed. It is suggested that nonlinearity and the inverse energy cascade process are responsible for the observed, enhanced wave confinement.

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Maurizio Fantini
and
Piero Malguzzi

Abstract

Idealized numerical experiments, supported by analytic considerations, are performed to determine the preferred direction of symmetric instability when water loading is considered. It is concluded that the most unstable direction is tangent to a surface of neutral buoyancy, which can be defined numerically from the water content of lifted air, and coincides with the tangent to saturated isentropes only when all condensed water is precipitated out, consistent with the thermodynamic approximations made in the definition of equivalent potential temperature.

In more common situations, when part or all of the condensed water is retained in the cloud, the orientation of symmetrically unstable normal modes is much more slanted toward the horizontal, to the point that regions of the atmosphere, diagnosed as unstable from the consideration of equivalent potential temperature and vorticity, can in fact be stable.

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Daniele Mastrangelo
and
Piero Malguzzi

Abstract

The monthly forecasting system of the Institute of Atmospheric Sciences and Climate of the National Research Council (CNR-ISAC) of Italy is operationally run on a weekly basis in the framework of the Subseasonal-to-Seasonal (S2S) project to produce 41-member ensemble forecasts. The first two years of forecasts, covering 106 weeks from April 2015, are verified against ERA-Interim as weekly averages starting from the first forecast day. Nonprobabilistic scores of 500-hPa geopotential height and 850-hPa temperature anomalies are computed for the extratropical hemispheres. The anomaly correlation coefficient shows enhanced predictive skill during the cold months, when favorable values are occasionally obtained beyond week 2. The root-mean-square error saturates toward the climatological value between weeks 2 and 3. Reliability diagrams are used to evaluate the probabilistic forecast skill of 2-m temperature over Northern Hemisphere extratropical land points, in terms of above- and below-normal events. The forecasting system loses reliability and resolution beyond week 2, but well reproduces the observed 2-yr mean frequency up to week 4, proving to be unbiased. The reliability of the forecasting system systematically outperforms that obtained by persisting the previous week forecast. Beyond week 2, the forecast distribution of below-normal events shows low confidence. However, a reliability diagram based on equally populated bins of forecast probabilities highlights residual resolution up to week 4 at low probabilities. ROC diagrams confirm that the modeling system has greater discrimination capability for below-normal events. The reliability analysis of accumulated precipitation shows minor differences between below- and above-normal events, with lower skill than 2-m temperature.

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Andrea Buzzi
,
Nazario Tartaglione
, and
Piero Malguzzi

Abstract

The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM).

The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband and frontal structure whose evolution determines both intensity and location of the prefrontal warm and moist flow. Almost all of the simulated precipitation over the Alps forms in the middle–low troposphere through forced ascent, whereas part of the secondary maximum, observed over the Apennines, is of convective type.

Sensitivity experiments have been conducted to investigate the effects of orography, surface fluxes, and latent heat exchange processes in the atmosphere. The role of the orography is crucial in determining distribution and amount of precipitation, whereas sensible and latent heat fluxes from the Mediterranean Sea (over the period considered) enhanced only the convective precipitation. Distinct dynamical effects, important for the amount and the spatial distribution of precipitation, are found to be associated with warming due to condensation and cooling due to evaporation and melting of precipitation. The latter process seems to be responsible for the simulated formation of rainbands and complex evolution of the cold front over the western Mediterranean. The multiple front life cycle and propagation feeds back on the simulated precipitation distribution, affecting the location of the prefrontal moist flow. Condensation affects the atmospheric effective stratification where the flow impinges on the orography, determining the flow regime (orographic lifting vs blocking and flow around), which, in turn, has an important impact on precipitation.

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Piero Malguzzi
and
Paola Malanotte-Rizzoli

Abstract

Many recent theoretical and observational studies have been devoted to the understanding of atmospheric patterns that persist beyond the synoptic time scale. These patterns are known as blocking events.

Properties of blocking events emerging from the observational evidence are consistent with the properties of nonlinear, i.e., coherent, localized structures characterized by locking of phases and phase speeds which are amplitude dependent.

In the present paper we develop a nonlinear, analytical theory with solutions in the form of stationary, coherent structures superimposed on a mean westerly wind. The model is the inviscid, quasi-geostrophic potential vorticity conservation equation with a mean zonal wind having vertical as well as horizontal shear. The used mean wind profile is typical of the atmosphere at midlatitude. The stationary, coherent solution is an antisymmetric dipole, with the anticyclone north of the cyclone; it has an equivalent barotropic vertical structure, is meridionally as well as zonally trapped and obeys a nonlinear dynamics in the zonal wave guide.

This pattern, even though idealized, exhibits a strong similarity and is consistent with observations of blocking patterns.

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Piero Malguzzi
and
Paola Malanotte Rizzoli

Abstract

Many recent studies have been devoted to atmospheric Patterns that persist beyond the synoptic time scale, such as those known as blocking events. In the present paper we explore the possibility that blocking patterns can be modeled with a local approach. We propose a truncated model that is a time-dependent, highly nonlinear extension of our earlier analytical theory. In this theory, stationary coherent structures were found as asymptotic solutions of the inviscid, quasi-geostrophic potential vorticity equation with a mean zonal wind with vertical and horizontal shear, in the limit of weak dispersion and weak nonlinearity. The truncated model is obtained by projecting the potential vorticity equation onto the orthonormal basis defined by the lowest order problem of the asymptotic theory and then suitably truncating the number of modes. The time-evolution of the model is investigated numerically with different truncations.

The steady solutions were antisymmetric dipoles, with the anticyclone north of the cyclone; they have an equivalent barotropic vertical structure and are meridionally as well as zonally trapped. We suggest that this solution could model the persistent patterns associated with blocking events that satisfy Rex's definition. An extensive series of numerical experiments is carried out to investigate the persistence of the steady solutions and their stability to different superimposed perturbations. The result is that, in an environment as turbulent as the real atmosphere, a typical estimate of the robustness (predictability) of the solution is of the order of 10 to 12 days. Such persistence is consistent with observations of blocking patterns.

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Anna Trevisan
,
Piero Malguzzi
, and
Maurizio Fantini

Abstract

Lorenz's hypothesis of a quadratic law governing the growth of errors provides an estimate of small error growth in the atmosphere based on the knowledge of large error behavior.

An atmospheric model whose simplicity allows for an independent estimate of small error growth was used to investigate the validity of Lorenz's hypothesis. Only errors that are initially infinitesimally small were found to follow Lorenz's law, while errors of large size exhibit higher growth rates. Nonetheless, the results lead to a substantial confirmation of the estimate, based on experimental data, of the doubling time for small errors given by Lorenz for the atmosphere.

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Paola Malanotte-Rizzoli
and
Piero Malguzzi

Abstract

In this paper we exploit a nonlinear baroclinic theory of atmospheric Rossby waves superimposed on westerly winds with meridional and vertical shear which was proposed in two earlier studies, Parts I and II. In Part I, nonlinear, stationary Rossby wave solutions were found consisting of a localized vortex pair and having an equivalent barotropic structure. These solutions, found in the context of an asymptotic theory for the quasi-geostrophic baroclinic potential vorticity equation, were proposed as a model for atmospheric blocking. In Part II, the theory was extended to the time-dependent, highly nonlinear case, removing the weak-amplitude limitations of the asymptotic theory of Part I. The localized highly nonlinear dipole solution of Part II was found to be remarkably robust to different energetic perturbations, even with a baroclinically unstable mean zonal wind. A typical persistence (predictability) time for the solution of Part II was of the order 10 to 15 days, consistent with observations of blocking patterns.

In this paper we investigate two further aspects of the high-amplitude solution of Part II. First, we study the formation of the coherent dipole starting from rather different initial conditions. We establish a necessary and sufficient criterion for the formation of the coherent structure. This criterion involves the preexistence of a zonal low wavenumber component (wavenumber one) in an antisymmetric meridional mode having a large enough amplitude. If this condition is satisfied, the evolution into the block configuration is assured by the model internal dynamics that is of the Korteweg-deVries type.

Second, we study the effect of short-scale, transient eddies upon the blocking dipole. We include dissipative effects and find that the eddy forcing is such to maintain the coherent structure against both mean advection and dissipation. The eddy forcing pattern resulting from the numerical experiments compares well with the observational evidence, given the high truncation of the model used.

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Silvio Davolio
,
Francesco Silvestro
, and
Piero Malguzzi

Abstract

Coupling meteorological and hydrological models is a common and standard practice in the field of flood forecasting. In this study, a numerical weather prediction (NWP) chain based on the BOLogna Limited Area Model (BOLAM) and the MOdello LOCale in Hybrid coordinates (MOLOCH) was coupled with the operational hydrological forecasting chain of the Ligurian Hydro-Meteorological Functional Centre to simulate two major floods that occurred during autumn 2011 in northern Italy. Different atmospheric simulations were performed by varying the grid spacing (between 1.0 and 3.0 km) of the high-resolution meteorological model and the set of initial/boundary conditions driving the NWP chain. The aim was to investigate the impact of these parameters not only from a meteorological perspective, but also in terms of discharge predictions for the two flood events. The operational flood forecasting system was thus used as a tool to validate in a more pragmatic sense the quantitative precipitation forecast obtained from different configurations of the NWP system. The results showed an improvement in flood prediction when a high-resolution grid was employed for atmospheric simulations. In turn, a better description of the evolution of the precipitating convective systems was beneficial for the hydrological prediction. Although the simulations underestimated the severity of both floods, the higher-resolution model chain would have provided useful information to the decision-makers in charge of protecting citizens.

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