The Physics of Fine-scale Remote Sensing of the Air-Sea Interface

M.A. Sletten, G.B. Smith, J.V. Toporkov, and R.A. Handler
Remote Sensing Division

X. Liu and J.H. Duncan
University of Maryland

Introduction: For a wide range of applications- from improved weather forecasting to Naval operations -the need exists to extract quantitative information from remotely sensed imagery of the sea. NRL's Remote Sensing Division is pursuing an integrated set of numerical and experimental investigations designed to understand the physics of radar and infrared (IR) remote sensing of the many small-scale surface features that contribute to the remotely sensed signal. These features include breaking waves, parasitic capillary waves, and shear-driven instabilities.

Ultra-Fine-Scale Radar Scattering from Breaking Waves: When viewed with a radar operating at a low grazing (high incidence) angle, breaking waves can produce a significant portion of the total backscatter generated by the sea surface. Thus these surface features are of great interest to the marine remote sensing community. To further our understanding of the role that breaking waves play in radar remote sensing, NRL and University of Maryland researchers have recently conducted a laboratory investigation in which the ultrahigh-resolution radar backscatter produced by evolving breaking waves was collected simultaneously with wave-height profiles provided by a high-speed digital camera. This data set allows both the wave surface and the radar backscatter it produces to be studied in unprecedented detail.

Figure 7 shows an evolving breaking wave and the radar backscatter it produces. The optical image, Fig. 7(a), shows the surface profile of the wave at an instant in time, as collected by a high-speed digital camera. In Fig. 7(b), range scans of the corresponding radar backscatter envelope, collected by an ultrahigh- resolution instrumentation radar developed by NRL, are presented for both vertical polarization (VV, upper panel) and horizontal polarization (HH, lower panel). Both the optical and radar data are single frames extracted from time sequences that document the evolution of the dynamic surface and its radar echo. This combination of dual-polarized, ultrahighresolution radar and simultaneous, high-speed optical imagery allows unambiguous identification of the scattering centers. When used in conjunction with electromagnetic modeling, the associated scattering mechanisms can also be identified.

FIGURE 7
(a) High-speed photograph of the crest of an evolving breaking wave. The curved line separating the upper and lower sections of the photograph is the surface of the water. The wave has just begun to break and produce small-scale roughness on its forward face, just in front of the crest. (b) Magnitude of the radar backscatter produced by the breaking wave plotted vs range (upper panel: vertically polarized backscatter; lower panel: horizontally polarized). The vertical, dotted lines coincide with the left and right boundaries of the photograph field of view. The strong radar echoes are generated by the small-scale roughness just in front of the wave crest.

Radar scattering from breaking waves is also the subject of numerical modeling studies. Figure 8(a) shows the surface profile of an evolving breaker at an instant in time, as calculated using a numerical implementation of the Navier-Stokes equations developed by NRL. Figure 8(b) displays the simulated radar Doppler spectrum produced by this wave (over its entire lifetime) at a grazing angle of 10 deg and a radar center frequency of 30 GHz. This spectrum was computed by NRL researchers using a numerical solution of the electromagnetic scattering equations in conjunction with Monte-Carlo techniques.¹ The large peak near 90 Hz is caused by surface roughness that is bound to and moves with the crest of the breaking wave. The smaller peak near 40 Hz is caused by small-scale roughness that is advected by the underlying orbital-wave currents. The Doppler spectrum is of fundamental importance in synthetic aperture radar-based techniques designed to measure ocean surface currents. These NRL studies promise to improve these techniques and extend them into the lowgrazing angle regime by identifying the breaking wave contribution.

FIGURE 8
(a) Surface profile of an evolving breaking wave at a single instant in time, generated by a numerical solution to the Navier-Stokes equations. Wave propagation is from left to right. (b) Simulated radar Doppler spectrum at a frequency of 30 GHz for the breaking wave shown in (a). The strong peak near 90 Hz is produced by small-scale roughness bound to the wave crest, while the peak near 40 Hz is produced by freely propagating waves that are advected by the underlying orbital wave currents.

High-Resolution IR Imaging of an Air-Water Interface: NRL researchers are also involved in understanding the thermal properties and hydrodynamic structure of the air-sea interface.² As wind blows across the interface, the surface is cooled and a very thin layer, often called the cool skin of the ocean surface, is formed. This layer is on the order of a few millimeters in thickness and gives rise to an ocean surface temperature that is 0.1 to 0.5 K cooler than the subsurface fluid. An understanding of the physics associated with the nature of this cool skin is of considerable importance in the interpretation of satellite sea surface temperature (SST) imagery and in the development of improved heat flux models. Figure 9 shows images of an air-water interface collected at the wind-wave facility at the University of Delaware by using a high-resolution IR camera with a temperature resolution of 0.02 K. The camera was mounted looking straight down at the water surface, which was driven by wind generated by a blower. In Fig. 9(a), where the wind speed was 2 ms^-1, it is evident that the cool skin has a well-defined structure made up of regions of warm fluid (bright regions) and cool dark streaks. These streaks are formed by a complex shear-induced hydrodynamic instability that have been shown to have a characteristic width λ+ = 100 l⁺, where l⁺= v/u^*, v is the kinematic viscosity of water, u^* = (τ/ρ)^1/2, τ is the surface shear stress, and ρ is the water density. At 4 ms^-1, surface waves generate rapidly and sometimes form small-scale breaking events that appear as warm regions of homogeneous turbulence (Fig. 9(b)). By using companion numerical simulations of these phenomena, NRL researchers have recently developed new models that predict the heat flux at the air-sea interface for low wind speeds.

FIGURE 9
(a) IR image of a water surface at a wind speed of 2 ms^-1 blowing from right to left. The image is 33 cm on a side. The light-colored areas are warm, rising plumes of water; the darker areas are cooler, descending sheets. The insert gives details of the temperature structure across a segment of the surface. Also indicated is the length scale l⁺ =100 l⁺. (b) IR image of the same surface at a wind speed of 4 ms^-1. The large, light-colored area is warmer water exposed by a small-scale breaking wave.

Summary: NRL is engaged in a series of experimental and numerical investigations into the remotely sensed signatures of small-scale ocean surface features. In future work, the IR and radar methods described here will be combined to provide new and unique insights into air-sea interfacial physics.

[Sponsored by NRL and ONR]

References
¹ J.V. Toporkov and G.S. Brown, "Numerical Simulations of Scattering from Time-Varying, Randomly Rough Surfaces," IEEE Trans. Geosci. Remote Sensing, 38 (4), 1616-1625 (2000).
² R.A. Hander, G.B. Smith, and R.I. Leighton, "The Thermal Atructure of an Air-Water Interface at Low Wind Speeds," Tellus, 53A, 233-244 (2001).