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A. Trevisan
and
A. Buzzi

Abstract

Stationary solutions and their stability properties of nondissipative barctropic flow in a narrow, longitudinally periodic channel on a βplane with a sheared basic current and bottom topography are investigated. An analytical treatment is applied which has been used in studies of solitary Rossby waves and is allowed by the choice of geometry. This leads to an ordinary nonlinear differential equation for the longitude-dependent part of the solution. The equation which is obtained in the present case is formally that of an anharmonic oscillator with external forcing and weakly variable natural frequency. Approximate analytical and numerical solutions are obtained under quasi-resonant conditions. Three possible states are found in a certain range of the parameters. Of these, only two are found to be stable. The implications of the existence of multiple solutions, also found by other authors in various contexts, for the large-scale atmospheric circulation and the phenomenon of blocking are discussed.

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A. Buzzi
and
A. Speranza

Abstract

The theory of cyclogenesis in the lee of the Alps presented by Speranza et al. in Part I is reexamined here (Part II) in the context of models dealing with finite amplitude topography. This generalization leads to the specification of the limits of validity of the approximations made in Part I and, at the same time, allows us to calculate the stability properties and normal mode structure of the model atmosphere with realistic topography. The effects of finite slope and finite geometrical height of the mountain are considered separately and their relative importance is evaluated. In the case of a three-dimensional obstacle with an idealized shape simulating the Alps, the modifications induced by the orography on the free, baroclinically unstable modes show essentially the same features observed in numerical experiments, including the orientation of the dipolar structure in the pressure perturbation across the mountain.

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A. Buzzi
and
E. Tosi

Abstract

We examine the nature and spatial structure of meteorological high-frequency variability in two selected areas, one including the Alps and the other the Rocky Mountains. Seven years of geopotential height data, derived from ECMWF analysis set, have been filtered in order to remove periods longer than 6–7 days. Statistical analysis performed on the filtered series include one-point lag correlations and composites that reveal changes in the typical horizontal structure and path of traveling eddies as they move from the oceans to the adjacent mountainous regions. The important modifying action of the orography is shown, and lee cyclogenesis is interpreted as one aspect of such action. Cross-correlation maps between upper- and lower-tropospheric levels reveal the effect that mountains also exert on the vertical structure of baroclinic eddies.

These statistical results are discussed and compared with those predicted by theories of cyclogenesis in the lee of the Alps and the Rocky Mountains, and by theories of propagation of disturbances in the vicinity of large-scale mountains. It is shown that the normal mode theory of baroclinic waves in the presence of mountains is capable of predicting most of the observed features, considering both lee cyclogenesis and eddy propagation and deformation near mountains as different aspects of the interaction of high-frequency eddies with orography.

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S. Davolio
and
A. Buzzi

Abstract

A 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.

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A. Buzzi
,
A. Trevisan
, and
A. Speranza

Abstract

The presence of bottom topography in a baroclinic flow modifies the properties of the propagating baroclinic unstable modes and allows for the appearance of new unstable modes which are nonpropagating, as first shown by Charney and Straus. Mountain form-drag, which provides a coupling mechanism between the zonal flow and the waves, is the essential ingredient for topographic instability. In this paper, the properties of instability for both zonally symmetric and asymmetric baroclinic basic states in the presence of topographic forcing are investigated. The results in a two-layer and a continuously stratified atmosphere are also compared and discussed. We find that two different types of topographic instability exist, one which is essentially baroclinic and is present in symmetric and asymmetric basic states, the other which is mixed barotropic-baroclinic and is present only in asymmetric basic states.

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A. Speranza
,
A. Buzzi
,
A. Trevisan
, and
P. Malguzzi

Abstract

Observational and numerical studies on Alpine cyclogenesis have shown that a developing baroclinic wave approaching the mountain region gives rise to a disturbance of dipolar structure, extending throughout the troposphere with horizontal scales comparable to the Rossby deformation radius. It is possible to interpret such disturbances as modifications of baroclinically unstable modes, induced by localized topography.

In the present approach, the effect of the mountain is introduced in a perturbative sense, in the framework of quasi-geostrophic theory. Even in this simple approach the spatial structure of the unstable modes is modified by a localized topography in the direction required in order to explain the observed features. In the case of a continuously stratified fluid, the basic characteristics of the observed vertical structure are also reproduced.

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S. Tibaldi
,
A. Buzzi
, and
P. Malguzzi

Abstract

Cyclogenesis induced by an isolated mountain chain in a baroclinic flow is simulated in a channel version of the HIBU (Mesinger-Janjić) primitive equation model. The main features characteristic of cyclogenesis in the Ice of the Alps am reproduced when the mountain interacts with a finite-amplitude baroclinic wave. The distribution of derived quantities like vertical velocity and potential vorticity compare well with those analyzed in case studies.

The intercomparison of the evolution of the various fields and the analysis of energetics in experiments with and without mountains highlights the nature of the physical processes involved.

A small-scale baroclinic process is responsible for an amplification of the initial disturbance produced by the mountain when the cold front, associated with the large-scale wave, moves over it. This process, though enhancing the local energy conversion, reduces the efficiency of the baroclinic conversion over the whole domain.

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Piero Lionello
,
P. Malguzzi
, and
A. Buzzi

Abstract

This work studies the two-way coupling between the atmospheric circulation and the ocean surface wave field, as it is described by the recent observations and theories on the dependence of the sea surface roughness on the ocean wave spectrum. The effect of the coupling on the atmospheric variables and the ocean wave field is analyzed by implementing both the atmospheric and the ocean wave models in a periodic channel and simulating a wide range of different situations. In a strong atmospheric cyclone, in comparison to the one-way coupling, the two-way coupling attenuates the depth of the pressure minimum and significantly reduces the wave height and surface wind speed while it increases the momentum flux. The heat and moisture fluxes are increased if they are computed using the same wave-dependent roughness that is used for the momentum flux, while they are decreased if they are computed using the Charnock relation. The effects are proportionally larger for extreme storms because the time required for the deepening of the low pressure is much shorter than the time required by the windsea to reach a well-developed state.

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Andrea Buzzi
,
Damià Gomis
,
Michael A. Pedder
, and
Sergio Alonso

Abstract

We describe a simple and economic method for reducing the errors that can result from the irregular distribution of data points in linear interpolation schemes that use prescribed, isotropic weighting (IW) functions. The method can be applied to single-step analysis as well as to schemes consisting of more than one step.

The starting point of the analysis algorithm is the generation of two datasets (with an IW scheme), one by interpolating the observed field onto the collocated observing sites, the other by interpolating the observed field onto a regular grid. These two datasets are then used independently to estimate two new gridpoint fields as outputs from the same IW analysis scheme. It is assumed that the difference between these two new gridpoint fields is a measure of the error field that results from applying the IW scheme to an inhomogeneous distribution of observing sites, and that this error field is not very different from that associated with the initial gridpoint field analysis. It is therefore used as a basis for correcting the initial gridpoint field analysis. This procedure can be applied iteratively, and is shown to converge when applied to realistic data distributions sampling both real and simulated meteorological fields.

Each step of the iterative scheme is described in terms of a frequency response function in the presence of irregularly spaced data points, in order to illustrate its general convergence properties. The performance of the analysis algorithm has also been investigated in the context of the two-step Barnes analysis scheme and its application to scale separation analysis. Applications of the method to simulated and observed data show that the deviations between analyses and original fields are substantially reduced following a small number of iterations.

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K. Lagouvardos
,
V. Kotroni
,
A. Koussis
,
H. Feidas
,
A. Buzzi
, and
P. Malguzzi

Abstract

Since November 1999, the hydrostatic meteorological Bologna Limited-Area Model (BOLAM) has been running operationally at the National Observatory of Athens. The assessment of the model forecast skill during the 2-yr period included (a) calculation of the root-mean-square errors (model vs gridded analyses) of geopotential height and temperature at 850 and 500 hPa, (b) evaluation of the model's quantitative precipitation forecast skill for the most important events, and (c) evaluation of the model skill in the prediction of surface winds in comparison with buoy observations. Comparison of the verification results with those provided in the literature showed that BOLAM has a high forecast skill, even for precipitation, which is the most difficult parameter to forecast. Especially for precipitation, the comparison between coarse (∼21 km) and fine (∼6.5 km) grid spacing forecasts showed that for the low and medium precipitation amounts, the finer-grid forecasts are not as good as the coarse-grid forecasts. For the large precipitation amounts, the calculated statistical scores provide only little support of the idea that the fine-grid forecasts are better than those of the coarse grid because the fine-grid forecasts give better scores only for the quantity bias and the mean absolute error.

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