High-Resolution Infrared Ocean Imagery

Introduction: When viewed in the infrared at meter-scale resolution, a normally bland ocean surface becomes covered with complex and fascinating patterns of intersecting dark streaks and elongated bright patches that reveal the underlying small-scale circulation of the ocean. Remarkably, because of the presence of a millimeter-thick 'cool skin,' these patterns are expected to occur, even if the underlying water has a uniform temperature. Understanding such patterns can lead to more realistic models of ocean currents, horizontal dispersion and vertical mixing, air-sea exchange processes, and acoustic propagation.

Physics of the Ocean Cool Skin: At small scales, it is hypothesized that infrared imagery reveals horizontal variations in currents through their interaction with the water's cool skin. The cool skin is a thin thermal boundary layer through which heat flows upward by molecular conduction to match the atmospheric flux (i.e., the sum of the evaporative, radiant, and sensible heat transfers). This requires the surface temperature to be typically a few tenths of a degree less than that of the bulk water lying beneath. In areas where surface currents diverge, the cool skin is horizontally stretched and so it thins, resulting in a slight increase in surface temperature to maintain the heat flux; likewise, where surface currents converge, the cool skin is compressed and so it thickens, resulting in a slight decrease in surface temperature. Details of this conceptual model are being tested through comparisons of computer simulations and laboratory measurements.1 For an upward heat flux of approximately 100 W/m2 and low levels of ambient turbulence, the predicted temperature fluctuations due to horizontal straining are of the order of 0.1°C.

Aircraft Sampling: To see if these predicted fluctuations in surface temperature could be visualized, measurements were made using a digital, midrange infrared camera having a resolution of 0.02°C. The camera was deployed aboard a small, manned aircraft and oriented to view at nadir. Sampling was done in December 2002, over the inner West Florida continental shelf and near the mouth of Tampa Bay. Altitude was about 200 m, giving a surface resolution of 0.4 m. A variety of hydrodynamic phenomena were observed, including surface-penetrating turbulence, breaking waves, wakes, Langmuir circulation, and internal waves. The latter two, having been analyzed in detail, are described below.

Langmuir Cells: Under moderate winds, the imagery shows long, dark (cool) streaks that reveal a set of counter-rotating cells called Langmuir circulation (Fig. 9). Surface convergences form between successive pairs of cells and are responsible for the cool streaks in the imagery. The spacing of the streaks in Fig. 9 is 10 to 20 m, but the water depth was only 3 m. This gives each cell an unusually large width-to-height aspect of about 2.5. Theoretical work2 suggests Langmuir cells can become unstable through "pairing" of adjacent cells, resulting in adjacent streaks merging and individual streaks terminating. Evidence for this predicted behavior is clear in the infrared imagery (two circled areas). Such detailed structures persisted over several minutes and drifted with the downwind surface current.

FIGURE 9
Nighttime infrared image showing long dark streaks caused by Langmuir cells. The streaks are about 0.2°C cooler than the ambient surface water. A wind of about 5 m/s was blowing from top to bottom, approximately parallel to the streaks. Upper circle highlights where one streak terminates; lower, where two other streaks merge. Such details persisted between successive aircraft passes and drifted downwind. Bright (warm) spots occurring throughout the image represent small-scale breaking waves.

FIGURE 10
Daytime infrared image showing a group of about six internal waves—the dark and bright bands oriented top to bottom. The left-most dark band is the leading edge of the group, which is propagating toward the left-hand edge of the image. The distance between the more prominent dark bands is 15 m. Winds were light (about 1 m/s). The temperature variations in the image are about ±0.15°C.

Internal Waves: Under winds too low to generate Langmuir cells and surface waves, the imagery shows a pattern of both dark and bright bands (Fig. 10). These arise from the alternately converging and diverging surface currents of a group of about six internal waves. The left-most dark band is the leading edge of the group, which propagated seaward (toward the left) during late flood tide. In this case, the spacing of the dark bands (about 15 m) gives the wavelength of the internal waves. The temperature fluctuations induced by the internal waves are about 0.15°C, which is consistent with expectations. Fine structure within some bright bands may be caused by instabilities generated by vertical current shear within the waves themselves. Temperature variability on such scales would degrade acoustic signal coherence, for example.

Summary: High-resolution infrared imagery of the ocean surface has been obtained under low to moderate winds. The examples of Langmuir circulation and internal waves illustrate the effects of hydrodynamic straining of the cool skin. The infrared images are able to provide new and detailed views of such phenomena. Such views are not easily obtained with other imaging sensors, such as microwave radars that rely on backscatter from a wind-roughened sea surface. The imagery has raised some intriguing questions: Do the wide Langmuir cells result from interaction with the bottom? Can in-water measurements be made to confirm that internal wave instabilities are being imaged? Under what conditions does the imagery begin to reveal structures within the atmospheric boundary layer? These questions will be explored in follow-up studies.

Acknowledgments: The aircraft sampling was made possible through an interagency agreement with the National Oceanic and Atmospheric Administration.

[Sponsored by ONR]