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

Conditional symmetric instability (CSI) is examined as a possible mech anism for generating cloudbands within a cold-frontal system. The data used in this study were collected during the Franco-British field exp eriment FRONTS 87. A dense network of rawinsounding measurements was u sed to test the various hypotheses underlying CSI, such as the sign of the moist potential vorticity, the relative slope of absolute momentu m and moist isentropic surfaces or the alignment of bands with the ver tical shear of the wind inside the unstable domain. Radar data provide d from the French dual-Doppler radar system RONSARD was used to retrie ve the three-dimensional airflow within a cloudband observed during th e last frontal event of FRONTS 87. A new retrieval method, called MAND OP, was used to estimate the circulations within this band and to test whether the retrieved circulations were consistent with the CSI theor etical expectations. A roll-like circulation present in the vicinity o f the observed cloudband seems to support the hypothesis that CSI was the formative mechanism.