Microwave brightness temperature variations caused by ocean internal waves - Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International

Oceanic internal waves were observed by microwave radiometers from a blimp during the COPE’99 experiment. Brightness temperature variations caused by sea surface roughness modulation and enhanced wave breaking were compared with model calculations. It is shown that direct wave amplitude modulation by currents cannot explain the observed brightness temperature variations, but inclusion of wave breaking and foam coverage modulation results in a good agreement with experimental data. INTRODUCTION It is well known that oceanic internal waves (IW) strongly modulate sea surface roughness. Due to this mechanism IW are well seen on radar images. By the same reason oceanic IW must modulate microwave radiometric signatures although the observation of such modulation is difficult due to low spatial resolution of radiometers. Nevertheless IW were observed many times from ships by radiometers observing at neargrazing angles. COPE’95 EXPERIMENT A grazing-looking radiometer is sensitive to the slope variance of sea waves while a near-nadir looking radiometer has selective sensitivity to surface ripples [1]. This is why near-nadir radiometric observation of IW would be of special interest. In 1995 Environment Technology Laboratory of NOAA (NOAA/ETL) conducted the Coastal Ocean Probe Experiment (COPE’95) during which a set of microwave radiometers was installed on a blimp [2]. In addition, extensive in situ measurements were made from the research platform FLIP (FLoating Instrument Platform). The blimp proved to be an advantageous platform for radiometric observations of IW because of the high spatial resolution and long averaging time due to low altitude and low air speed. An example of the brightness temperature variations caused by an IW packet is presented in Fig. 1: observed by 37 GHz polarimeter (a) and by dual frequency 23/31 GHz radiometer (b). The curves correspond to vertical (top) and horizontal (bottom) polarizations. Fig. 2 shows the flight pattern of the Blimp, FLIP location, wind direction and IW propagation direction. Triangles in Fig. 2 mark the Blimp positions where the strongest variations of the brightness temperature were observed (see Fig. 1). 24 24.1 24.2 24.3 24.4 24.5 140 145 150 155 160 165 U TC , hours T b , K v .p . h .p. N adir v ie w ing ange 22 ° a) 24 24.1 24.2 24.3 24.4 24.5 135 140 145 150 155 160 165 170 175 U TC , hours T b , K 23 G H z, v .p. 31 G H z, h.p. N adir v ie w ing angle 16 ° b) Figure 1. Brightness temperature variations caused by IW. -124.4 -124.35 -124.3 -124.25 -124.2 45.68 45.7 45.72 45.74 45.76 45.78 45.8 Flight 09/25/95 Long., deg L at ., d eg Flip