Summary
This paper deals with the first results concerning mesoscale ageostrophic circulations as they were presented during the ‘International Meeting on Fronts and Orography’, held in Burghausen, Germany, on June 1991. The aim is to test the importance of some specific small scale and mesoscale processes on the observed ageostrophic circulations. For this, various available theoretical models including these processes are used, in order to put in evidence their effect or to diagnose their presence or not in the observed circulations. These secondary circulations have been observed in the vicinity of two surface cold fronts documented during the European experiment FRONTS87 and they are quantitatively evaluated and compared with the theoretical ones. The diagnoses are performed by numerically solving two versions of the two-dimensional Sawyer-Eliassen equation for the ageostrophic streamfunction: a form based upon the Geostrophic Momentum approximation of along-front geostrophy and a Primitive Equation form that allows the existence of a along-front ageostrophic component. A simple parameterization of the Ekman pumping effect in the secondary circulations is added, through a boundary condition in the Sawyer-Eliassen equation resolution.
This application of the Sawyer-Eliassen equation to measured data from a network of two or three sounding stations allows the identification of the significant frontogenetic terms and the testing of the assumptions underlying this diagnostic tool. Also, the impact of various parameterizations is considered. For instance, the introduction of a boundary layer parameterization scheme permits the reproduction of low-level updrafts observed in reality ahead of the frontal zone and predicted by numerical models found in literature.
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Lagouvardos, K., Lemaître, Y. & Scialom, G. Mesoscale ageostrophic circulations of two fronts observed during the fronts87 experiment: Diagnosis and interpretation. Meteorl. Atmos. Phys. 48, 293–307 (1992). https://doi.org/10.1007/BF01029574
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DOI: https://doi.org/10.1007/BF01029574