VOL. 84, NO. B8 JOURNAL OF GEOPHYSICAL RESEARCH JULY 30, 1979 A Note on the Comparison of Radar Altimetry With IR and In Situ Data for the Detection of the Gulf Stream Surface Boundaries C. D. LEITAO AND N. E. HUANG NASA Wallops Flight Center, Wallops Island, Virginia23337 C. G. PARRA EG&G Washington Analytical Services Center, Inc., Pocomoke City, Maryland 21851 A definition of theGulf Stream boundaries ispresented together with 'its most widely used approxima- tions. Positions of the Gulf Stream boundaries, as obtainedby radar altimetry, are compared with those determinedby infrared and in situ data such as airborne bathythermographs and airborne radiation thermometers. The data used are derived from the Geos 3 satellite, the Experimental Ocean Frontal Analysis, and other experiments conducted by the U.S. Navy Naval Oceanographic Office during Febru- ary 1976and January 1977.Results point out that the radar altimetercan sense the Gulf Streambound- aries with as much precision as the infrared-derived data analysis. Both techniques provide excellent agreement with the in situ data. The two sensor systems complement each other, sinceone is an all- weatherprofiler and the other a synoptic two-dimensional sensor. INTRODUCTION Traditionally, oceancurrent phenomena have been studied primarily usinghydrographic methods. With the introduction of the satellite-borne infrared (IR) radiometer, the situation has changeddramatically. Within the last few years, the I R technique has quickly established itself as one of the most important methods to delineate the ocean surface current boundaries. The principle of the IR technique is to utilize the temperaturecontrast of differentwater masses associated with a current system. The sharpest signature of an I R image is at the location of the largest surfacetemperature gradient. Al- though I R can only detect the very top layer of water, this technique provides a surprisingly good indication of the sur- face current pattern and the boundaries of different water masses and eddies shed by the•Gulf Stream. Recently,a satelliteradar altimetry technique has beende- veloped. The principleof satellite altimetry hasbeendiscussed by McGoogan [1975]. Briefly, radar altimetry provides a direct measurement of the sea surface topographic height. From such measurements the geostrophic component of the surface cur- rent can be computed directly. The purpose of this paper is to present some selected cases of Gulf Stream studies where IR, in situ, and radar altimetry data are all available. It is hoped that through these inter- comparisons the validity of the remotesensing methods can be firmly established. DEFINITIONS OF THE GULF STREAM BOUNDARIES The Gulf Streamcan be defined as a narrow ribbon of high- velocitywater flowingalong a boundarybetween cooler,den- ser slope water to the shorewardside and warmer, lighter water in the Sargasso Sea [Stommel,1965].The Gulf Streamis in geostrophic balance with the pronounced pressure gradient between the two water masses. Fuglister and Worthington [1951] have definedthe boundaries or edges of the current as thosepointswherethe pressure gradientbecomes zero. These authors further point out that to locate these points,deep and closely spaced temperature and salinity measurements are neededto calculate the crosscurrent pressure gradients. Be- Copyright ¸ 1979 by theAmerican Geophysical Union. cause of the obvious difficulties involved in obtaining hydro- graphic data of this type, various approximationshave been developed. Based on these approximations,techniqueshave been adopted which monitor other characteristics associated with the Gulf Stream boundaries. The most widely acceptedapproximation of the northern surface boundary of the Gulf Stream is the location of the 15øC isothermat 200 m (15øC/200 m) as proposed by Fuglis- ter and Voorhis [1965]. It is important to emphasize here that the 15øC at 200 m definition is uniqueto the Gulf Stream and is not a general criterion. For the Kuroshio, the 15øC/200 m line is a good indicator of the main axis rather than the boundary[Uda, 1964]. Even in the Gulf Streamsyste m, the 15øC/200 m line may be positioned in somecases directly un- der the maximum surface current' however, it is alwaysto the north of the main body of the Gulf Stream. This definition still calls for hydrographic data or at least expendable bathythermograph (XBT) data. Infrared remote sensors are now routinely used to locate oceansurface currents from space by obtaining the seasurface temperaturesignature.The maximum surfacethermal gradi- ent is used to define the Gulf Stream northern surface bound- ary [Curtis and Rao, 1969; Warnecke et al., 1971' Rao et al., 1971' Maul and Hansen, 1972]. The surfacethermal gradient has been related to the 15øC/200 m line empirical!y;Hansen and Maul [1970] found that the surface thermal front was, on the average, 14.5 km north of the 15øC/200 m line. The 14.5- km value corresponds rather well to the 15-km value pre- viously reported by Fuglister and Voorhis [ 1965]. Therefore the conclusionthat the northern surface boundary of the Gulf Stream is related to the maximum surface temperaturegradi- ent can be drawn, but the relationship is not exact. For ex- ample, a report by Khedouri et al. [1976] provides somedata indicating that the 15øC/200 m line is a betterindicator of the locusof the maximum surface temperaturethan of the maxi- mum surfacetemperaturegradient. The surface elevation of the ocean is tilted over a strong current to a slope given by Ah g Paper number 8B0960. 0148-0227/79/008 B-0960501.00 3969 AL 2• sin ½ v (1)