Upper- and lower-troposphere coupling processes involved in the FASTEX IOP16 frontal cyclone

Abstract

This paper investigates processes involved in the development of the secondary low sampled on 17 February 1997 during the field phase of the Fronts and Atlantic Storm-Track EXperiment (FASTEX). This low is a fast- moving frontal wave that emerged from the trailing cold front of a parent low situated over Greenland. The dynamics of this cyclone and its environment are described using the European Centre for Medium- Range Weather Forecasts analysis corresponding to Intensive Observing Period 16 of FASTEX. The ageostrophic circulation is retrieved and analysed in the upper and in the lower levels. In the upper levels it is found that the curvature of the upper-level jet plays a crucial role in suppressing or amplifying the vertical motions. In the lower levels, in the region where the cloud head emerges, the vertical motion is scrutinized in the light of forcings which are acting. It is found that the vertical motions are the response to a geostrophic forcing reinforced by diabatic processes. The conjunction of vertical motions in the lower levels associated with vertical motions in the upper levels leads, in the developing period, to a coupling process that seems to be (following theoretical calculations) at the origin of the subsequent development of the cyclone. The instability processes involved in the development of the studied secondary cyclone are also investigated. The atmosphere appears neutral for upright convection while the region of the coupling corresponds to a region which is potentially unstable with respect to slantwise convection. The tendency of the atmosphere is to relax this instability. The vertical motions of the coupling processes are then reinforced by the relaxation of slantwise instability. Copyright © 2002 Royal Meteorological Society