Sea Surface Echoes Observed with the MU Radar under Intense Sporadic E Conditions. Tadahiko OGAwA1, Mamoru YAMAMOTO2, and Shoichiro FUKA02

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1 Letter J. Geomaq. Geoelectr., 48, , 1996 Sea Surface Echoes Observed with the MU Radar under Intense Sporadic E Conditions Tadahiko OGAwA1, Mamoru YAMAMOTO2, and Shoichiro FUKA02 1Solar-Terrestrial Environment Laboratory 2Radio Atmospheric Science Center, Nagoya University, Toyokawa, Aichi 442, Japan, Kyoto University, Uji, Kyoto 611, Japan (Received August 17, 1995; Revised November 30, 1995; Accepted December 26, 1995) We present for the first time sea surface echoes observed at ranges of km with the 46.5-MHz middle and upper atmosphere (MU) radar at Shigaraki, Japan. Ionospheric data support that the radar wave propagated toward the target by reflection due to intense sporadic E layer. Although the Doppler spectra obtained are modified more or less due to temporal and spatial changes of the sporadic E layer, fundamental characteristics of the spectra are well explained by a first-order theory of Bragg scatter from sea surface. 1. Introduction Crombie (1955) found experimentally Doppler frequency changes of HF radio signals at MHz due to moving sea scatterers and identified the radio wave scattering mechanism responsible for the dominant nature of the observed Doppler spectra: Bragg backscatter from the gravity waves having wavelengths equal to half the radar wavelength. Since this discovery, a variety of HF and VHF radars have been used for remote sensing of sea surface state (e.g., Ward, 1969; Headrick and Skolnik, 1974; Lipa et al., 1981; Dexter and Theodoridis, 1982; Broche et al., 1987; Nadai et al., 1996). Most of these observations have used the HF band (10-30 MHz). Over-the-horizon (OTH) HF radars using sky-wave propagation via the ionosphere are capable of detecting the sea surface at distances of 1,000-3,000 km, whereas fields of view of HF and VHF radars using ground-wave propagation mode are limited within about 300 km. In Japan, the Communications Research Laboratory is developing an HF ocean surface radar (24.5 MHz) for exploring areas close to the sea coast (Nadai et al., 1996). Broche et al. (1987) applied a meteorological ST radar at a frequency of 47.8 MHz as a ground-wave radar to observe the sea near radar site. This paper briefly reports the first detection of sea surface echoes by the middle and upper atmosphere (MU) VHF radar of Kyoto University. These particular echoes were detected by chance when the radar was observing echoes due to 3.2-m scale field-aligned-irregularities (FAI) in the ionospheric F region. On the basis of information on the Doppler spectra, echoing ranges, and ionospheric conditions, we conclude that these echoes were caused by the Bragg backscatter from sea surface and that the radar waves could propagate toward the target by reflection due to intense sporadic E (Es) layer. 2. Results and Discussion The MU radar, located at Shigaraki, Japan (34.9 N, E), is a 46.5-MHz, pulsed monostatic Doppler radar with a two-way half-power antenna beam width of 4.5 /2.3 in the vertical/horizontal plane (Fukao et al., 1985a, 1985b, 1990). By directing the sharp antenna beam toward the north, we can observe echoes caused by the 3.2-m scale E and F region FAI (Fukao et al., 1991; Yamamoto et al., 1991). 447

2 448 T. OCAWA et al. 2.1 Doppler spectra We made continuous F region FAI observations during the night of May 8, 1990, using an antenna beam direction of (azimuth, elevation) =(-5, 32 ) and a peak transmitting power of 1 MW. In the evening hours, the radar detected intermittently the echoes with which we are concerned here. Figure 1 shows an example of the quick-look data displaying Doppler spectra at some selected ranges and a contour map of the spectral intensity in the Doppler velocity - range domain. To depict Fig. 1 we used 256-point FFT (fast Fourier transform) Doppler spectra averaged over the period 1914: :42 LT. Note that positive and negative velocities correspond to flows away from the radar and toward the radar, respectively. The maximum echo intensity is about 13 db, a value being comparable to or less than the F region FAI echo intensity (Fukao et al., 1991), above the noise level. The echoing ranges are limited between 650 and 900 km corresponding to apparent altitudes between 344 and 477 km. The peculiar features to be noted are that each spectrum is rather symmetrical around zero Doppler velocity and that the Doppler spectra have two maxima at extremely low velocities, one between +2 and +3 m/s and the other between -1 and -2 m/s. These spectral features have never been found in the E and F region FAI observations with the MU radar. 2.2 Sea surface scatter Barrick (1972) gives the first-order spectral cross section for the backscatter from sea surface in the absence of ocean current as follows: al(w)=16rrk4(1+sin29)2 > S(-2mk,,sin0)6(w-mwB) m=f1 (1) where m = ±1 denotes the sign of the Doppler shift, k (= 2a/A) is the radar wavenumber, A is the radar wavelength, 9 is the incidence angle of the radar wave vector at the sea surface, wb is the Bragg angular frequency, S(k) is the spatial spectrum of the sea surface height, and 6(x) is the Dirac delta function. LOB is given by (2gk sin0)1/2 where g is the gravitational acceleration. Equation (1) implies that the signal scattered from the sea surface has two spectral peaks ("Bragg lines") at w = ±WB corresponding to the backscatter by radially approaching and receding ocean gravity waves. For the MU radar case (A = 6.45 m), if 9 is 90, fb(= wb/21r) = ±0.695 Hz, and corresponding Doppler velocity VB is equal to ±2.24 m/s: Because 0 is larger than 73 for the radar echoes to be discussed below (Subsection 2.3), we can almost neglect the 9-dependence of fb. When sea surface wind exciting gravity waves blows toward the radar, the spectral intensity S(-2k, sin 0) associated with the approaching waves is larger than S(+2k sin0) associated with the receding waves. If ocean currents exist, two Bragg lines are shifted equally from their predicted positions (±wb) by an amount of w,(= 47rV /A) where V is the radar line-of-sight component of the current velocity vector. Two vertical broken lines in Fig. 1 represent the predicted positions of ±VB. Clearly the positions of the observed spectral peaks are quite close to ±VB, suggesting that we observed the sea scatter echoes. The spectral intensities around -VB are usually stronger than those around +VB, implying that the sea surface winds exciting the gravity waves had components toward the radar. Moreover, close inspection of Fig. 1 indicates that with increasing radar range the spectral peak positions shift gradually toward the right whereas the two peaks are placed at a constant interval of 2VB. We infer from this fact that the ocean current vectors had components (of the order of 1 m/s at 900-km range) away from the radar and that with increasing radar range the away-from-the-radar component increased gradually with a rate of about m/s/km. The Bragg lines in Fig. 1 are not so clearly discernible as those observed by other investigators (e.g., Crombie, 1955; Lipa et al., 1981; Broche et al., 1987; Nadai et al., 1996). Moreover, the ocean current velocity of the order of 1 m/s at 900-km range is faster than velocities (G50 cm/s)

3 Sea Surface Echoes Observed with the MU Radar under Intense Sporadic E Conditions : :42 LT May 8, m/s (-0.695Hz) 2.24m/s (0.695Hz) m -40 T c Mww yy~ Yyu rmr~jm..'mw.nwnw~ 1 "VV~I\v,Y' +hvwm.fyrw..vryrv i0 Doppler Velocity (m/s) 7G0 740 Y ) c 700 c6 6B0 Min.= -62.6dB Max.= -49.7dB I1 E Y 800 rrb a) C) C N 700 I e. I'll k Doppler Velocity Ws) Fig. 1. (Upper) Doppler spectra at some selected ranges and (lower) spectral intensity contour map in the Doppler velocity - range domain averaged over the period 1914: :42 LT on May 8, Positive and negative velocities correspond to flows away from the radar and toward the radar, respectively. Two vertical broken lines represent positions of VB = ±2.24 m/s (fo = ±0.695 Hz). observed with HF radars using ground-wave propagation (e.g., Nadai et al., 1996). We will suggest below that propagation condition for the MU radar wave was changeable with time and space to cause temporal fluctuations of the Bragg line positions. 2.3 Propagation mode of radar wave Since the echoing ranges were between 650 and 900 km, i.e., over the horizon, the radar wave must arrive at the targets via the ionosphere (sky-wave propagation). Normal E and F regions cannot sustain this kind of propagation mode because the electron densities in these regions are usually too low to refract or reflect the 46.5-MHz wave. The most probable candidate is a reflection due to intense Es layer. Assuming an Es altitude of 105 km, we require both foes (or fbes) greater than 12.2 MHz and a radar wave elevation angle of 15.2 in order to detect echoes at 800-km range. This elevation angle, however, is not consistent with the elevation angle (32 )

4 E 450 T. OGAWA et ad. 1,3-JUN :48: r. r +o~ noise level 43.9 db max power 56.2 db T4 Q7 500 f r,«a ylr.' O C Ffd.' V! v-~r~9c Y-~ Doppler 0 Velocity 10 (m/s) 20 Fig. 2. Spectral intensity contour map in the Doppler velocity - range domain at 1848:36 LT on June 13, Positive and negative velocities correspond to flows away from the radar and toward the radar, respectively. Two vertical arrows represent positions of Vg = ±2.24 m/s (fg = ±0.695 Hz). The radar echoes from sea surface are strongly enhanced at velocities near 0 and -4 m/s at ranges between 720 and 850 km. of the MU radar main antenna lobe. Moreover, fes's observed every 15 minutes during the period LT with the ionosondes at Akita (39.7 N, E) and Kokubunji (35.7 N, E) were less than 7.1 MHz, a fact strongly suggesting that the main lobe or side lobes looking toward the north could not detect any sea echoes. Fukao et at. (1990) noted that for the main lobe direction of (azimuth, elevation)=(0.0, 32.2 ), which is very close to our case, two grating lobes that have elevation angels between 0 and 25 appear to the south; one to the southeast (125 azimuth) and the other to the southwest (235 azimuth). The southwest lobe can look the sea around 150 km southwest of Kagoshima at the 800-km radar range. During the period LT, fes's and fbes's at Yamagawa, Kagoshima (31.2 N, E) were between 10.9 and 15.1 MHz and between 10.0 and 14.4 MHz, respectively, with multiple Es echo traces. Thus, the present sea echoes can be explained by the Es reflection, the point of which is located in the Es layer at about 200 km northeast of Yamagawa. Of course, there is a possibility that the southeast lobe instead of the southwest lobe looked the sea around 500 km north of Ogasawara Islands in the Pacific Ocean. This possibility, however, cannot be checked because of lack of nearby ionosonde. Electron density distribution in the Es layer is not stable but changeable with time and space. This may induce temporal and spatial changes of the ray path and phase of the radar wave, resulting in temporal fluctuations of the Bragg line positions, i.e., in the obscure Bragg lines in the Doppler spectra (see Fig. 1). An apparent ocean current velocity of the order of 1 m/s at 900-km range (see Subsection 2.2) may be obtained if a vertical movement of the Es layer was of the order of 0.1 m/s that is comparable to the tidal motions of the layer. Figure 2 shows another example of a contour map of the Doppler spectra (integration time

5 Sea Surface Echoes Observed with the MU Radar under Intense Sporadic E Conditions 451 = 46 s) taken during the F region FAT observations in the evening of June 13, In this case, the sea surface state is completely different from that in Fig. 1: Two spectral peaks shift toward the left, the surface wind blows away from the radar, and the surface current at 850-km range is 2.5 m/s toward the radar. At 1900 LT, fes at Yamagawa was 14.5 MHz with complete disappearance of the F region echoes while fes at Kokubunji was 9.7 MHz. The current velocity of 2.5 m/s is far faster than usual ocean current flows, again indicating that vertical motion of the Es layer largely affects the current velocity estimation. In summary, we have presented for the first time the sea echoes observed with the MU radar under intense Es conditions. Fundamental characteristics of the Doppler spectra axe well explained by the first-order theory of the Bragg backscatter from sea surface. This fact enables us to use the MU radar as a remote sensor of the sea at distances beyond 600 km. To obtain detailed physical parameters of the sea surface state, however, we need development of more sophisticated hardware and software systems and must study the ionospheric effects on the Doppler spectra. The MU radar belongs to, and is operated by the Radio Atmospheric Science Center of Kyoto University. The ionospheric data were supplied through the World Data Center C2 for Ionosphere, Communications Research Laboratory, Tokyo. REFERENCES Barrick, D. E., First-order theory and analysis of MF/HF/VHF scatter from the sea, IEEE Than. Antennas Propag., AP-20, 2-10, Broche, P., P. Forget, J. C. de Maistre, J. L. Devenon, and M. Crochet, VHF radar for ocean surface current and sea state remote sensing, Radio Sci., 22, 69-75, Crombie, D. D., Doppler spectrum of sea echo at Mc/s, Nature, 175, , Dexter, P. E. and S. Theodoridis, Surface wind speed extraction from HF sky wave radar Doppler spectra, Radio Sci., 17, , Fukao, S., T. Sato, T. Tsuda, S. Kato, K. Wakasugi, and T. Makihira, The MU radar with an active phased array system 1. Antenna and power amplifiers, Radio Sci., 20, , 1985a. Fukao, S., T. Tsuda, T. Sato, S. Kato, K. Wakasugi, and T. Makihira, The MU radar with an active phased array system 2. In-house equipment, Radio Sci., 20, , 1985b. Fukao, S., T. Sato, T. Tsuda, M. Yamamoto, M. D. Yamanaka, and S. Kato, MU radar: New capabilities and system calibrations, Radio Sci., 25, , Fukao, S., M. C. Kelley, T. Shirakawa, T. Takami, M. Yamamoto, T. Tsuda, and S. Kato, Turbulent upwelling of the mid-latitude ionosphere 1. Observational results by the MU radar, J. Geophys. Res., 96, , Headrick, J. M. and M. I. Skolnik, Over-the-horizon radar in the HIT band, Proc. IEEE, 62, , Lips, B. J., D. E. Barrick, and J. W. Maresca, Jr., HF radar measurement of long ocean waves, J. Geophys. Res., 86, , Nadai, A., H. Kuroiwa, M. Mizutori, and S. Sakai, Measurement of ocean surface currents using CRL HF ocean surface radar. Part 1. Radial current velocity, J. Oceanography, 1996 (in press). Ward, J. F., Power spectra from ocean movements measured remotely by ionospheric radio backscatter, Nature, 223, , Yamamoto, M., S. Fukao, R. F. Woodman, T. Ogawa, T. Tsuda, and S. Kato, Mid-latitude E region field-alignedirregularities observed with the MU radar, J. Geophys. Res., 96, 15,943-15,949, 1991.