Appendix F 



Detection of Ocean Fronts in the 



Gulf Stream / Labrador Current System 



by Side-Looking Airborne Radar 



LTJG N, B. Thayer, USCGR 



D. L. Murphy 



Introduction 



The Gulf Stream probably 

 reaches its greatest complexity in 

 the region south and southeast 

 of Newfoundland where it 

 interacts with complex bathymetry 

 and the southward-flowing 

 Labrador Current to produce an 

 ever-changing system of fronts, 

 eddies and associated features. 

 This complex current system is 

 responsible, in large part, for the 

 distribution of icebergs in much of 

 the Intemational Ice Patrol's (IIP) 

 operating area. 



The IIP iceberg drift model, an 

 integral part of the IIP operations, 

 relies primarily on historically time- 

 averaged cun-ents. Using these 

 currents can lead to substantial 

 drift errors, particularly in regions 

 with large current fluctuations. To 

 address this problem, IIP uses the 

 drift of satellite-tracked drift 

 buoys, deployed by IIP aircraft, to 

 provide near real-time cun-ent 

 data to the model. Although this 

 program is successful, the 

 updates to the current field are 

 limited temporally 

 and spatially to the period for 

 which a buoy is drifting through a 

 specific region. 



Renrx>te sensing techniques hold 

 the nrx>st promise for providing 

 current data for future IIP 

 operations. For example, 



satellite infrared imagery is used 

 successfully under certain 

 cond'rtbns to define ocean frontal 

 boundaries and, thus, infer 

 circulation patterns for several 

 ocean areas. Unfortunately, 

 infrared imagery is of limited 

 operational use in the IIP area due 

 to persistent fog and cloud cover. 

 However, active microwave 

 systems (radars) are capable of 

 penetrating clouds and, under 

 the right circumstances, 

 detecting frontal features. 



In 1985 IIP began investigating 

 the feasibility of using imagery 

 from a side-looking airtjome radar 

 (SLAR) to map ocean fronts in the 

 IIP area. This report desaibes 

 some of the preliminary results of 

 that investigation. 



Background 



The International Ice Patrol 

 deploys one week out of two to 

 Gander, Newfoundland, during 

 the icebergs season, typically 

 March through August. Using 

 U.S. Coast Guard HC-130 aircraft, 

 IIP conducts iceberg 

 reconnaissance flights within the 

 area bounded by 40°-52°North 

 and 39°-57°West. 

 Reconnaissance flights are made 

 each day during the 

 deptoyments, each flight 



approximately 3,1 50 km long 

 (1 ,700 nm), covering 

 approximately 65,000 square km. 



Since the spring of 1983, IIP has 

 used SLAR as the main method 

 of iceberg reconnaissance, 

 replacing visual reconnaissance. 

 SLAR is an X-band radar that 

 scans the sea surface in a plane 

 normal to the flight path. The 

 radar image is displayed on a 

 narrow CRT that produces a 

 negative image on photographic 

 film (Figure F-1 ). The standard 

 altitude for IIP reconnaissance is 

 8,000 feet, with a SLAR swath 

 width of 1 00 km, 50 km to each 

 skje of the aircraft with an 

 unimaged swath directly betow 

 the aircraft of about 5 km. SLAR 

 is largely unaffected by weather, 

 with only heavy precipitation 

 otscuring the view of the surface. 



Review of IIP SLAR films for 1 983- 

 1985, representing some 200 

 flights, has revealed that SLAR is 

 capable of detecting the fronts of 

 the Gulf Stream and Labrador 

 Current, with the water nnasses of 

 different temperatures showing 

 up as different shades on the 

 SLAR film's negative image, warm 

 water appearing dark and cooler 

 water appearing light. These 

 con'espond to high radar 

 backscatter and tow radar 



85 



