Relationships between strong internal waves in the coastal zone and their radar and radiometric signatures

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Abstract
Strong internal waves (IW) in the form of soliton groups were observed off the Oregon coast with in situ and remote sensors, including shore‐based X band and Kaband Doppler radars and an airborne microwave radiometer operating at 37 GHz. Here we analyze the relationships between oceanic isotherm vertical displacements, internal currents, and radar backscatter cross sections, along with Doppler velocity signals at horizontal (HH) and vertical (VV) polarizations, and 37‐GHz brightness temperatures measured at intermediate incidence angles from an airship. Analysis of these observations shows that (1) the horizontal spatial structure of the IW field depends on whether it is forced by a strong (spring) tide or weak (neap) tide; (2) while both HH and VV are strongly modulated by IWs, the modulation depth of HH always exceeds that of VV, and they both increase with the IW amplitude; (3) 37‐GHz brightness temperature modulations were in phase with radar signal modulations; and (4) the phase of radar signal modulation with respect to the IW is such that the minimum radar signal intensity and lowest microwave brightness temperature lie close to the maximum of the IW thermocline depression or, equivalently, the horizontal current excursion near the surface. This last observation conflicts with the expectation that the highest backscattered signal would be found near the region of greatest surface strain rate (the surface current gradient). Existing theoretical models are briefly reviewed for interactions between the IW field and the intensity of short gravity‐capillary waves that might be responsible for this behavior.