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Piero A. Chessa and François Lalaurette

Abstract

The aim of this work is the evaluation of some basic measures of skill of the Ensemble Prediction System used operationally at the European Centre for Medium-Range Weather Forecasts. Probabilistic forecasts for the occurrence of predefined flow patterns are analyzed and validated throughout the 10-day forecast range. The weather regimes used as fixed patterns are those defined by Vautard for the 700-hPa geopotential height and used during the field phase of the Fronts and Atlantic Storm-Track Experiment as a component in the planning of the intense observing periods.

The evaluation of the ensemble forecasts is performed for three cold seasons, from 1996 to 1999, using the Brier score and the related skill score. In this case the sample climate and a simple probabilistic forecast (poor man's ensemble) have been used as reference forecasts. The results show a positive skill with respect to the reference forecasts up to the 240-h time step.

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Francois Lalaurette, Claude Fischer, and Jean-Pierre Cammas

Abstract

Both upper-air and surface frontogenesis have often been depicted as processes whose dynamics could he reduced to 2D balanced problems in which “self-sharpening” configurations could be highlighted.

This paper reports on a 3D adiabatic simulation of a baroclinic wave life cycle. Great care has been devoted to the vertical resolution, allowing for a good description of both surface and upper-air frontogenesis. The authors introduce a kinematic diagnostic (Q′ vector) that permits the identification of frontogenetic areas in such complex 3D flows where classical, low-Rossby number balance conditions can be violated. Relations and specificity with respect to frontogenetic forcing diagnostics are discussed. First, Q′ is used for surface frontogenesis, where it describes well the actual frontal activity, including the complex warm-frontal seclusion process. Upper-air frontogenesis is also investigated, both in terms of this kinematic diagnostic or in terms of potential vorticity displacements on isentropic surfaces. Both types of diagnostics clearly distinguish between dynamics of the entrance zone of the northerly jet—where 2D concepts may usefully be applied—and those of the strongly curved zone near the trough axis. Classical cyclogenetic terms (stretching and tilting) as well as the separation of ageostrophic circulations in terms of natural components of the wind also lead to a clear dynamical separation.

The cold front is shown to extend from the surface far into the troposphere. This is shown to be related to a singular property of the 3D flow. Parcels undergoing frontogenegis in the northwesterly upper-air flow are advected on top of those that were forced at the surface cold front in a southwesterly flow. The occurrence of a feedback proem between these upper-air frontogenesis processes and the surface ones is then investigated. Stepwise vertical profiles of horizontal diffusion are used to force local frontolysis. The resulting upper-air frontolysis, despite its local efficiency, does not have any remote effect on the surface front, whose frontolysis in turn has no effect on the upper-air front. The feedback process is thus not occurring in our simulation.

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Tiziana Cherubini, Anna Ghelli, and François Lalaurette

Abstract

The demand for verification of forecasting systems to ascertain their strengths and weaknesses is increasing dramatically as models evolve more rapidly. Precipitation forecasts have always been of great interest to forecasters because they influence daily life. The recent flooding over Europe has also shown how important it is to know how models can reproduce these events. The issue of precipitation verification is addressed here, starting from the assumption that model spatial scales have to be verified against data representing similar scales. Only in this way may the skill of forecasting system used herein be determined. The performance of the European Centre for Medium-Range Weather Forecasts model in predicting precipitation is discussed. The study concentrates on the period September to November 1999 during which high-density observations were available for the Alps. The high-resolution observing network over the Alpine region has been used to reconstruct a precipitation analysis that contains smoothed small-scale variability and represents with sufficient accuracy the average behavior of the observed field in the model grid box. The precipitation forecast is verified against both the precipitation analysis and the surface synoptic observations (SYNOP) available in real time via the Global Telecommunication System. Both verification approaches show that for the Alpine region, during autumn 1999, the model overestimates the precipitation amount. Overestimation is smaller when the forecast is compared with the precipitation analysis. It is also shown that verification against irregular and scattered observations (SYNOP data) is highly influenced by the variability of the precipitation in a grid box. A precipitation analysis is, therefore, important if model skill has to be defined.

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Sarah C. Jones, Patrick A. Harr, Jim Abraham, Lance F. Bosart, Peter J. Bowyer, Jenni L. Evans, Deborah E. Hanley, Barry N. Hanstrum, Robert E. Hart, François Lalaurette, Mark R. Sinclair, Roger K. Smith, and Chris Thorncroft

Abstract

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.

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John R. Gyakum, Marco Carrera, Da-Lin Zhang, Steve Miller, James Caveen, Robert Benoit, Thomas Black, Andrea Buzzi, Cliément Chouinard, M. Fantini, C. Folloni, Jack J. Katzfey, Ying-Hwa Kuo, François Lalaurette, Simon Low-Nam, Jocelyn Mailhot, P. Malguzzi, John L. McGregor, Masaomi Nakamura, Greg Tripoli, and Clive Wilson

Abstract

The authors evaluate the performance of current regional models in an intercomparison project for a case of explosive secondary marine cyclogenesis occurring during the Canadian Atlantic Storms Project and the Genesis of Atlantic Lows Experiment of 1986. Several systematic errors are found that have been identified in the refereed literature in prior years. There is a high (low) sea level pressure bias and a cold (warm) tropospheric temperature error in the oceanic (continental) regions. Though individual model participants produce central pressures of the secondary cyclone close to the observed during the final stages of its life cycle, systematically weak systems are simulated during the critical early stages of the cyclogenesis. Additionally, the simulations produce an excessively weak (strong) continental anticyclone (cyclone); implications of these errors are discussed in terms of the secondary cyclogenesis. Little relationship between strong performance in predicting the mass field and skill in predicting a measurable amount of precipitation is found. The bias scores in the precipitation study indicate a tendency for all models to overforecast precipitation. Results for the measurable threshold (0.2 mm) indicate the largest gain in precipitation scores results from increasing the horizontal resolution from 100 to 50 km, with a negligible benefit occurring as a consequence of increasing the resolution from 50 to 25 km. The importance of a horizontal resolution increase from 100 to 50 km is also generally shown for the errors in the mass field. However, little improvement in the prediction of the cyclogenesis is found by increasing the horizontal resolution from 50 to 25 km.

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