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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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C. Pellacani, C. Tebaldi, and E. Tosi

Abstract

The stability properties of a Helmholtz velocity profile in a stratified, Boussinesq fluid are investigated in the presence of a jump in the Brunt- Väisälä (BV) frequency at a level different from the one where the vortex sheet is located. New unstable modes in the range of low horizontal wavenumbers are found with respect to the case where no BV frequency jump exists. The structure of the associated unstable disturbances is similar to that of neutrally propagating gravity waves. The associated growth rates are rather small but significant because they appear in a range of horizontal wavenumbers which are otherwise stable.

A comparison with the results obtained by Lindzen and Rosenthal and by Lalas et al. shows a strict analogy with the study of the stability properties of a Helmholtz velocity profile in the presence of a lower rigid boundary. The generation of such instabilities is interpreted in terms of multiple overreflexions at the shear interface due to the presence of the reflecting BV frequency jump for neutral propagating waves. Applications for values of the relevant parameters suitable to actual atmospheric cases are discussed.

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S. Tibaldi, E. Tosi, A. Navarra, and L. Pedulli

Abstract

Seven years of analyses and forecasts from the operational archives of the European Centre for Medium-Range Weather Forecasts have been analyzed to assess the performance of the model in forecasting blocking events. This paper extends the previous work by Tibaldi and Molteni to the other seasons of the year and to the Southern Hemisphere. The dataset covers the period from 1 December 1980 to 30 November 1987 and consists of 5OO-hPa geopotential height daily analyses and the 120 corresponding forecasts verifying on the same day, a dataset commonly known as the “Lorenz files.” Local blocking and sector blocking have been defined as in Tibaldi and Molteni, using a modified version of the Lejenas and Økland objective blocking index.

The results broadly confirm the conclusions previously reached for the winter season alone, extending their validity to the rest of the year and, mutatis mutandis, to the other hemisphere. The main observational difference between blocking in the two hemispheres is in the number of preferred locations: Atlantic and Pacific blocking in the Northern Hemisphere, and only one broad region in the Southern Hemisphere, around 180° longitude. Forecasting the onset of blocking events is in general a task that the model finds difficult, whereas if the integration starts from an already blocked initial condition, the performance of the model is usually better. The poor observational data coverage in the Southern Hemisphere is likely to produce initial conditions affected by larger errors, making the correct forecast of the onset of a blocking event an even more difficult task than it is in the Northern Hemisphere. In the Northern Hemisphere, although the dynamical characteristics of Atlantic and Pacific blocks are inferred from the respective model errors to be different, their detrimental effects on forecast performance are similar in the two cases.

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