THE OBSERVATION OF THE SURFACE-ROUGHNESS CHARACTERISTICS OF THE RHINEPLUME FRONTAL BOUNDARIES BY SIMULTANEOUS AIRBORNE THEMATIC MAPPER ANDMULTIFREQUENCY HELICOPTER-BORNE RADAR SCATTEROMETER

In this paper, we describe how high spatial resolution (10 m) multisen sor remote sensing techniques can be used to study the surface roughne ss characteristics of large scale frontal boundaries (in this case ass ociated with the Rhine Plume). The instrumentation employed in the res earch consisted of a Daedalus AADS 1268 Airborne Thematic Mapper (ATM) operated by the UK National Environment Council, the HELISCAT helicop ter-borne multifrequency microwave scatterometer of the University of Hamburg, and research vessels (R.V.s) from the University of Wales and the Dutch Rijkswaterstaat. The data we present were gathered on 24 Ap ril 1991 when calm wind conditions developed within the test area. A s equence of thermal infrared images gathered by the ATM provides a reco rd of the motion of a frontal boundary through this experimental regio n which is then used to identify the frontal signature in the HELISCAT data. ATM sunglint images show that the front is characterized by a z one of reduced surface roughness, some 75 m in width, which is detecte d on the 'upstream' side of the front (as defined relative to the tida l flow direction), where surface current convergence can be expected. Radar backscatter levels at X and C bands are reduced by similar to 10 dB in this region but with increase in radar wavelength, the signatur e weakens and is rarely detected at L band. On crossing the front in t he downstream direction, radar backscatter levels are rapidly restored . The available evidence indicates that the reduced backscatter signat ure is caused by a surface slick which is formed at the frontal interf ace rather than by short gravity wave damping from shear in local surf ace currents.