Meso‐β‐scale circulations in realistic fronts. I: Steady basic state

Abstract

AbstractThis paper investigates the occurrence of conditional symmetric instability during MFDP/Fronts87IOP2. A saturated negative equivalent‐potential‐vorticity area is detected by the mesoscale analysis in the upper troposphere, and this is confirmed by dropsounding data. Two‐dimensional hydrostatic simulations are performed with the Météo‐France PERIDOT model which give rise to the onset of symmetric instability. A complete life cycle is obtained as well as the growth rate of the perturbation (about 2.5 × 10−5 s−1 ≈ (11 hours)−1). Particular emphasis is put on the meanings and results of the kinetic‐energy conversions. Most of the perturbation energy comes from the basic state through vertical momentum fluxes, which is consistent with previous theoretical or numerical works. the decay of the perturbation is also shown to be due to its own dynamics, the symmetric rolls advecting positive equivalent potential vorticity into the unstable region. Deformations of equivalent potential temperature (θe) surfaces and m‐surfaces give an insight into the parcel trajectories: the ascending parcels follow θe‐surfaces. leading to a buckling of the m‐surfaces, and the descending parcels follow θ‐surfaces. the small value of the growth rate is stressed as well as of the maximum intensity (w ≈ 5 cm s−1). Thus, interactions with the frontogenetical forcing are expected owing to the small growth rate obtained. This is the subject of Part II of this paper.