MESO-BETA-SCALE CIRCULATIONS IN REALISTIC FRONTS .1. STEADY BASIC STATE

This paper investigates the occurrence of conditional symmetric instab ility during MFDP/FRONTS 87 IOP2.A saturated negative equivalent-poten tial-vorticity area is detected by the mesoscale analysis in the upper troposphere, and this is confirmed by dropsounding data. Two-dimensio nal hydrostatic simulations are performed with the Meteo-France PERIDO T model which give rise to the onset of symmetric instability. A compl ete life cycle is obtained as well as the growth rate of the perturbat ion (about 2.5 x 10(-5) s(-1) approximate to (11 hours)(-1)). Particul ar emphasis is put on the meanings and results of the kinetic-energy c onversions. Most of the perturbation energy comes from the basic state through vertical momentum fluxes, which is consistent with previous t heoretical or numerical works. The decay of the perturbation is also s hown to be due to its own dynamics, the symmetric rolls advecting posi tive equivalent potential vorticity into the unstable region. Deformat ions of equivalent potential temperature (theta(e)) surfaces and m-sur faces give an insight into the parcel trajectories: the ascending parc els follow theta(e)-surfaces, leading to a buckling of the m-surfaces, and the descending parcels follow theta-surfaces. The small value of the growth rate is stressed as well as of the maximum intensity (w app roximate to 5 cm s(-1)). Thus, interactions with the frontogenetical f orcing are expected owing to the small growth rate obtained. This is t he subject of Part II of this paper.