Q. J. R. Meteorol. SOC. zyxwvutsr (1993), 119, zyxwvuts pp. 1321-1345 551.515.81 (441.1/.5) Importance of diabatic processes on ageostrophic circulations observed during the FRONTS 87 experiment By K. LAGOUVARDOV, Y. LEMAITRE and G. SCIALOM Centre de Recherches en Physique de I’Environnement, France (Received zyxwvutsr 17 August 192; revised 31 March 1993) SUMMARY The impact of condensation and evaporation on real cross-frontal circulations is studied, using data from the FRONTS 87 field experiment. The Sawyer-Eliassen equation is solved numerically in its primitive equation form, for one intensive observing period of FRONTS 87. Three different schemes of parametrization of condensation heating are used. Comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone. In particular, when a parametrization scheme for the low-level moisture convergence is applied, a cell of strong ascending motion is formed ahead of the frontal zone, in good agreement with the observations. Further, a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front, as observed in regions where precipitation originating from the strong line convection evaporates. In comparison with their adiabatic counterparts, the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone. 1. INTRODUCTION The impact of non-conservative processes on frontogenesis was raised in the very early stages of frontal research. When Sawyer (1956) demonstrated that frontogenesis is necessarily accompanied by a vertical circulation system (i.e. the frontal secondary circulation), he also underlined the insufficiency of the calculated upward motions to account for the observed vertical velocities ahead of a frontal zone. According to him, latent heat release has to be taken into account within the saturated prefrontal atmosphere. Later on, the influence of the condensation of water vapour was studied thoroughly by several authors, using either time-dependent models (Hoskins and Bretherton 1972; Ross and Orlanski 1978; Williams et al. 1981; Mak and Bannon 1984; Thorpe and Emanuel 1985; Knight and Hobbs 1988; Chan and Cho 1991) or diagnostic formulations (Thorpe 1984;Thorpe and Nash 1984). Comparison with adiabatic solutions in these studies revealed that latent heat release strengthens frontogenesis and can account for the intense updraughts observed ahead of a frontal discontinuity. The magnitude and the extent of the unstable conditions ahead of the frontal zone were found to be of major importance in these numerical studies. Consequently, evaluation of the relative importance of water vapour condensation varies, from points of view which consider the contribution of latent heat release to be secondary to the frontal formation (Hoskins and Bretherton 1972) to results that demonstrated that convection-induced circulation can completely overshadow the original frontal circulation (Ross and Orlanski 1978). In a similar manner, Huang and Emanuel (1991) underlined the effect of evap- oration of rain on frontal circulations, which was found to be responsible for the appearance of a concentrated sloping downdraught beneath the frontal zone. On the other hand, Clough and Franks (1991) underlined the impact of snow evaporation at the rear of a cold front and claimed that this provides a very sufficient mechanism for the maintenance of saturated descent, above or in the melting layer, in the cold sector of the front. Lagouvardos zyxwvuts et zyxwvu ai. 1992 (hereafter referred to as LLS) compared observed frontal z * Corresponding author: CRPE/CNET, 38-40 rue du GCnCral LeClerc, 92131 Issy les Moulineaux, France. 1321