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Alain Joly

Abstract

The adjoint method for finding optimal or singular modes is employed for studying the finite time stability of steady, tw0-dimensional atmospheric fronts as represented by the uniform potential vorticity semigeostrophic model.

The most unstable singular models over a given period of time are computed for a wide range of scalar products. The reference scalar products are relevant to physical space and include total, kinetic, or potential energy; geopotential variance; and enstrophy.

A front inspired by observations from FRONTS 87 and including a surface potential temperature anomaly is examined first through the usual linear results. The validity of the linear approximation is considered as a function of amplitude. The modes are also integrated in nonlinear simulations and their life cycles am shown.

Results indicate that each norm and wave has its own preferred spatial scale. This severely restricts the concept of scale selection. Energy and geopotential variance modes increase mostly by improving the energy collection by barotropic processes. Enstrophy modes favor baroclinic processes. The linear approximation is more restrictive for the former than for the latter. In the nonlinear regime, the enstrophy mode exhibits faster deepening rates and larger vertical velocities.

Similar conclusions arise for the Hoskins-Bretherton deformation front in the same range of wavelengths, although this front is stable in the sense of Charney and Stern. The discussion examines the scale selection process inherent to the different scalar products.

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Alain Joly
and
Alan J. Thorpe

Abstract

A methodology suitable for assessing the stability of any time-dependent basic state is presented. The equivalent of the normal modes for steady basic states are the eigenvectors of the resolvent matrix; this matrix incorporates the evolution of the large-scale flow, and growth rates are replaced by amplification rates. This method is applied to the three-dimensional stability of two-dimensional fronts undergoing frontogenesis in the presence of latent heat release in a semigeostrophic model. Disturbances developing in this flow are therefore geostrophically balanced. The concepts are first illustrated in a dry time-dependent uniform shear and potential vorticity flow. At any time during the evolution of the basic flow the stability can be compared to that obtained by assuming that the frontogenesis has, at that instant, ceased. Although differences between the results from the two methods exist, general conclusions as to the scales and structure of the modes are not altered; only large-scale waves are unstable. The situation in moist baroclinic waves is dramatically different. Growth rates are enhanced compared to the steady state analysis, but the possibility for frontal waves on the 1000-km scale to amplify most rapidly depends on the rate of development of the parent wave. Such waves dominate the spectrum only when that rate is slow and then only when the frontal ascent takes on a small cross-frontal width and the vorticity maximum penetrates over a deep layer. The short-wave growth is mostly due to latent heat release in the wave. This heating is shown, in a simplified case, to modify the necessary conditions for instability. It is concluded that shearing deformation does not intrinsically inhibit frontal instability, but paradoxically it greatly favors two-dimensional growth in the early stages due to the more rapid frontogenesis in the presence of latent heating. The role of stretching deformation may be substantially different but is not considered here.

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Alain Joly
and
Alan J. Thorpe

Abstract

The stability of the steady two-dimensional horizontal shear front to geostrophic disturbances in the along-front direction is examined within the framework of semi-geostrophic theory. The basic state corresponds to the geostrophic along-front flow at any time during the nonlinear evolution of a two-dimensional Eady wave. The matrix resulting from the stability analysis can be transformed into a weakly nondiagonal form. Its structure shows that the selection of the most unstable along-front wavenumber is independent of the “intensity” of the front. The growth rate is a linear function of this amplitude. The most unstable along-front mode is a modified Eady mode stationary with respect to the front. It draws a fraction of its energy from the shear. For smaller along-front wavelengths, the solution is dominated by propagating modes near the boundaries. These are also baroclinic, with a larger contribution from the basic kinetic energy and much smaller growth rates. It is apparent that the existence of a vorticity maximum at fronts, however large, is not sufficient to produce the observed small scale of frontal waves. Anomalous potential vorticity at the front is necessary to provide a deep zone of large horizontal shear and hence the reduced horizontal scale of waves.

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Matthieu Plu
,
Philippe Arbogast
, and
Alain Joly

Abstract

Midlatitude cyclogenesis as interpreted in the framework of either baroclinic development or potential vorticity thinking heavily relies on the concept of synoptic-scale anomaly. Given the existence of potential vorticity inversion and attribution, what is at stake to provide a mathematical definition for this concept is a complete finite-amplitude alternative to the linear-based theory of cyclogenesis. The existence of a reasonably objective way to represent anomalies in both real and idealized flows would not only help understanding cyclogenesis, it would also have many other applications for both theory and in practical forecasts. Inspired by the recent theory of wavelet representation of coherent structures in two-dimensional fluid mechanics, a wavelet representation of three-dimensional potential vorticity anomalies is built. This algorithm relies on the selection of the appropriate two-dimensional wavelet coefficients from the stationary wavelet transform in order to guarantee the critical translation-invariance property. The sensitivity of the algorithm to the position, size, and shape of the structures is assessed.

The wavelet extraction is then applied to the upper-level precursor of a real-case storm of December 1999 and is compared to a basic monopolar extraction. Using potential vorticity inversion and forecasts with a primitive-equation model, it is found that both anomalies have similar implications on the development of the surface cyclone. However, the coherence in time of the extracted wavelet structure in the forecast and analysis sequence is more satisfactory than the extracted monopole: this suggests that the underlying mathematical description of an anomaly proposed here does, indeed, point toward the direction of an actual physical reality of the concept.

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Thierry Bergot
,
Gwenaëlle Hello
,
Alain Joly
, and
Sylvie Malardel

Abstract

The feasibility of the recently proposed concept of adaptive observations is tested on a typical case of poorly forecast North Atlantic cyclogenesis. Only numerical tools are employed, no special observations. Although based on simulated data, this study addresses both theoretical and practical problems of adaptive observations.

In the first stage of this study, the role of the data assimilation processes is neutralized; the correction is done by forcing correct continuous fields within the target area. These experiments prove that it is necessary to correct the projection of the initial errors on the first unstable plane (the first two leading singular vectors) in order to significantly improve the forecast. These results also clearly demonstrate that the quality of the initial conditions on a limited, but quite large, area could be a major factor influencing the forecast quality.

In a second stage, the focus is on operational aspects. The correction is done through the assimilation of a discrete set of simulated profiles using a 3DVAR analysis system. This leads to studying the impact of the assimilation scheme and to testing different sampling strategies. These experiments suggest that the concept of adaptive observations shows great promise in situations comparable to the one studied here. But the current assimilation systems, such as 3DVAR, require that all the structure of the target has to be well sampled to have a significant beneficial effect; sampling only the extremum does not suffice.

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Béatrice Pouponneau
,
Franck Ayrault
,
Thierry Bergot
, and
Alain Joly

Abstract

A case study of an eastern Atlantic cyclogenesis event is conducted. The focus is on the impact of aircraft data on both the analysis and forecast of the event. The case takes place between 1 and 3 February 1994 and involves a phase of explosive deepening.

Several new techniques are brought to bear on this problem. An automatic tracking algorithm of vorticity maxima allows the construction of the cyclogenesis scenario in a relatively objective way. It also provides a clear depiction of some of the problems met in a sample of test forecasts. The origin of these problems is determined by employing the adjoint technique in order to point out which parts of the initial conditions are important in explaining the divergences between forecasts.

The cyclogenesis is shown to result from successive baroclinic interactions of a surface vorticity maxima with two upper-level structures in a finite-length baroclinic zone. The largest impact on the forecast is shown to result not from a direct influence of the upper-level observations, but rather from an indirect effect via the observation selection algorithm. While the cyclone development clearly involves upper-level–low-level interaction, the most detrimental difference in initial conditions originates at low levels.

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Alain Joly
,
Dave Jorgensen
,
Melvyn A. Shapiro
,
Alan Thorpe
,
Pierre Bessemoulin
,
Keith A. Browning
,
Jean-Pierre Cammas
,
Jean-Pierre Chalon
,
Sidney A. Clough
,
Kerry A. Emanuel
,
Laurence Eymard
,
Robert Gall
,
Peter H. Hildebrand
,
Rolf H. Langland
,
Yvon Lemaître
,
Peter Lynch
,
James A. Moore
,
P. Ola G. Persson
,
Chris Snyder
, and
Roger M. Wakimoto

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as well as the number of drifting buoys. These facilities will be activated during multiple-day intensive observing periods in order to observe the same meteorological systems at several stages of their life cycle. A central archive will be developed in quasi-real time in Toulouse, France, thus allowing data to be made widely available to the scientific community.

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