ALADIN Model Research Articles

Cyclonic activity and severe Jugo in the adriatic

A case study on 2–3 April 1996 is presented, as an example of the typical chain of mesoscale events over the Adriatic region. The case is characterised by the extreme amounts of daily precipitation along the Croatian coast of the Adriatic. Besides heavy precipitation, strong jugo, with gusts up to 30 m/s, occurred in southern Adriatic. Analysis of the Meteosat infrared images shows that a shallow cyclonic vortex develops in the Adriatic parallel to the development of the cyclone in the Mediterranean. The ability of the mesoscale ALADIN model to simulate the process is presented. Results of simulation show that the model successfully reproduced the distribution and the intensity of recorded precipitation as well as the jugo strength along the coast.

Contribution of a mesoscale analysis to convection nowcasting

We study the contribution of surface data to convection nowcasting outside mountainous areas and under weak synoptic forcing. The CANARI optimal interpolation mesoscale analysis scheme is used, which combines guess-fields from the fine-mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. A tuning of this scheme is realized in order to fit convective systems scales (meso-β scale) as well as the mesonet scale. Then, these grid point analyses allow the computation of diagnostic fields such as Convective Available Potential Energy (CAPE) and MOisture CONvergence (MOCON), which are relevant in convection nowcasting. These diagnostic fields are compared with radar reflectivities observed from 1 to 5 hours later, so as to estimate their skill in predicting convection triggering. First results on six case studies show that convection generally occurs over areas where high CAPE values and persisting convergence were analyzed 4 to 1 hour before.

An intercomparison between a global variable mesh and two limited area models on a case of rapid cyclogenesis

This study has been carried out in order to investigate the predictability of a rapidly developing storm over the Mediterranean area, by comparing the behaviour of three different meteorological models. The models chosen are the global, variable mesh, model ARPEGE, which is running operationally at Meteo-France, and two limited area models, namely ALADIN (also developed at Meteo-France) and BOLAM (developed in Italy, mainly at FISBAT). This comparison study has been performed by a cooperation among the French Meteorological Service and the National Research Council of Italy (CNR) FISBAT and IFA institutes. In this paper we present the results obtained by integrating the models on a short time-range, corresponding to the developing stage of a strong cyclone that moved rather quickly to the North from Northern Africa, associated with perturbed weather and distinct mesoscale features. In particular, this cyclone was characterized by a low-level warm core, associated with very strong winds and precipitation. All the three models were successful in describing the main characteristics of this cyclogenesis event, in particular the rapid evolution of the low level vortex. However, important differences emerged in “forecasting” the value of the pressure minimum, its position and time evolution, and the associated distribution of precipitation. When the large scales are treated by using analyzed rather than forecast lateral boundary conditions, the two LAMs perform better than the global model in simulating the low evolution but the global high resolution model better describes the general features of the cyclogenesis. Differences in model performance are found especially in the distribution of precipitation forecasts, suggesting two main conclusions: insufficient descriptions of mountains can change the location of the precipitation release, the precipitation scheme of the ALADIN model is sensitive to the lateral boundary conditions. The overestimation of the cyclone in BOLAM seems due to an oversensitivity to the latent heat release that intensifies the circulation in the cyclone core.

Some limits to the variable-mesh solution and comparison with the nested-LAM solution

The solution at high resolution of a global spectral model with a conformally-variable mesh, obtained through a stretched coordinate, is compared as fairly as possible to the ‘classical’ solution of a limited-area nested model. We compare the global, variable-mesh, ARPEGE model at hyper-stretched values (which is integrated operationally at Météo-France with a ‘low’ value of stretching) with its limited-area version, the ALADIN model. Having the same physics, the same grid-point dynamics and a high code-compatibility, they allow clean comparisons on real situations. to conclude on their efficiency and accuracy, comparisons are made choosing configurations of the two models having either the same resolution or the same computational price. The limits of the variable-mesh strategy on the sphere are also investigated through a set of experiments with increasing stretching. We found that there is indeed a limit to the stretching factor in the interval [7, 9]: a stretching factor c = 7 keeps an accurate spectral solution and gives satisfying results, even when compared with the limited-area model at the same computational price. We show that the value c = 10.5 cannot be reached without paying the price of a spectral deformation of the fields which becomes too obvious. Additional tests showed the occurrence of this deformation, even if less pronounced, at the stretching value c = 8.75. At the same resolution ALADIN does not manifest any similar weakness. Although the solution remains stable for at least 72 hours, its accuracy when increasing the stretching above the critical value becomes questionable. This problem seems to be due to a particular behaviour of the truncation error, which does not decrease with increasing resolution through stretching, thus leading to an energy accumulation in the tail of the spectra. As a conclusion of this study, it follows that one can extend the idea of Courtier and Geleyn, currently used in operations at Météo-France for c = 3.5, to ‘medium’ stretched configurations up to c = 7 at least (at the current computer's power). Above a value found to be about c = 9, our results show that increasing the resolution does not produce any improvement and, hence, that the use of a limited-area model appears a better choice for very high resolutions.

Some limits to the variable‐mesh solution and comparison with the nested‐lam solution

AbstractThe solution at high resolution of a global spectral model with a conformally‐variable mesh, obtained through a stretched coordinate, is compared as fairly as possible to the ‘classical’ solution of a limited‐area nested model. We compare the global, variable‐mesh, ARPEGE model at hyper‐stretched values (which is integrated operationally at Météo‐France with a ‘low’ value of stretching) with its limited‐area version, the ALADIN model. Having the same physics, the same grid‐point dynamics and a high code‐compatibility, they allow clean comparisons on real situations. to conclude on their efficiency and accuracy, comparisons are made choosing configurations of the two models having either the same resolution or the same computational price.The limits of the variable‐mesh strategy on the sphere are also investigated through a set of experiments with increasing stretching. We found that there is indeed a limit to the stretching factor in the interval [7, 9]: a stretching factor c = 7 keeps an accurate spectral solution and gives satisfying results, even when compared with the limited‐area model at the same computational price. We show that the value c = 10.5 cannot be reached without paying the price of a spectral deformation of the fields which becomes too obvious. Additional tests showed the occurrence of this deformation, even if less pronounced, at the stretching value c = 8.75. At the same resolution ALADIN does not manifest any similar weakness.Although the solution remains stable for at least 72 hours, its accuracy when increasing the stretching above the critical value becomes questionable. This problem seems to be due to a particular behaviour of the truncation error, which does not decrease with increasing resolution through stretching, thus leading to an energy accumulation in the tail of the spectra.As a conclusion of this study, it follows that one can extend the idea of Courtier and Geleyn, currently used in operations at Météo‐France for c = 3.5, to ‘medium’ stretched configurations up to c = 7 at least (at the current computer's power). Above a value found to be about c = 9, our results show that increasing the resolution does not produce any improvement and, hence, that the use of a limited‐area model appears a better choice for very high resolutions.