DYNAMICAL STRUCTURE OF A WIDE COLD-FRONTAL CLOUDBAND OBSERVED DURING FRONTS-87

Conditional symmetric instability (CSI) is examined as a possible mechanism for generating cloudbands within a cold-frontal system. The data used in this study were collected during the Franco-British field experiment FRONTS 87. A dense network of rawinsounding measurements was used to test the various hypotheses underlying CSI, such as the sign of the moist potential vorticity, the relative slope of absolute momentum and moist isentropic surfaces or the alignment of bands with the vertical shear of the wind inside the unstable domain. Radar data provided from the French dual-Doppler radar system RONSARD was used to retrieve the three-dimensional airflow within a cloudband observed during the last frontal event of FRONTS 87. A new retrieval method, called MANDOP, was used to estimate the circulations within this band and to test whether the retrieved circulations were consistent with the CSI theoretical expectations. A roll-like circulation present in the vicinity of the observed cloudband seems to support the hypothesis that CSI was the formative mechanism.