On the distinction between SAR signatures of oceanic and atmospheric internal waves

J.C.B. da Silva, A.C.S. Sutcliffe, C. Camara
Department of Physics, Fac. Science, University of Lisbon
Lisbon, 1749-016, Portugal
jdasilva@fc.ul.pt

ABSTRACT

Radar signatures of atmospheric and oceanic internal waves observed on spaceborne synthetic aperture radar (SAR) images of the sea surface are analysed. They are interpreted in terms of models describing the radar imaging and the dynamics of these two types of waves. Criteria for discrimination between atmospheric and oceanic signatures are investigated. We show that in some instances it is very difficult to determine unambiguously whether the modulation pattern observed in the SAR images is of atmospheric or oceanic origin. For investigating internal wave features of atmospheric origin it was found that the Scorer parameter is very valuable. Scorer (1949) showed that energy can be trapped at low atmospheric levels when the so-called Scorer parameter decreases with height. Atmospheric vertical profiles (radio soundings) of wind speed and stability are used to study the vertical trapping criteria described by the Scorer parameter. Once the Scorer parameter profile l(z) is calculated, and compared with the wave number k of the observed internal waves, one can establish if there are favourable conditions for atmospheric internal wave propagation. If l² > k² , waves are oscillatory in the vertical, and therefore waves with wave numbers whose magnitudes are less than the Scorer parameter can propagate vertically. If however l² < k², wave amplitude decays exponentially with height, and thus waves with wave numbers whose magnitude is greater than the Scorer parameter will be trapped below such regions. The latter condition is used to classify a SAR internal wave signature as atmospheric, in case no evidence of oceanic origin is found. In addition, we have investigated some mechanisms for atmospheric internal wave generation, such as the lee wave mechanism and resonant over reflection.

Last Update: 2005-03-16