Role of a Low Level Jet in Triggering and Organizing Moist Convection in a Baroclinic Atmosphere. A Case Study: 18 May 1984

Abstract

This paper presents the kinematic and thermodynamic structure of a vigorously precipitating convective line associated with a low level jet (LLJ) observed during the French cooperative field experiment LANDES-FRONTS 84. The data used originate primarily from Dual-Doppler radars, upper air soundings and a meteorological surface network. The convective line appeared to be quasi-stationary during 6 hours. A distinct feature of the present observation as compared to previous ones is the relatively weak effect of the frontogenetical forcing in the low-troposphere. Thus, the role played by the LLJ in triggering and organizing the vertical motions at mesoscale and small scale can be specified. This observation appears to be consistent with the following mechanisms implying the LLJ. The transverse and vertical variations of the flow create slantwise motions in the lefthand flank of the jet, and the horizontal swear instability interacts with convective instability induced by warm/cold air differential advection effects.