Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 241–250 www.elsevier.com/locate/jastp MF radar observations of terdiurnal tide in the mesosphere and lower thermosphere at Wakkanai (45:4 ◦ N; 141:7 ◦ E), Japan S.P. Namboothiri ∗ , P. Kishore, Y. Murayama, K. Igarashi Communications Research Laboratory, 4-2-1 Nukui Kita, Koganei, Tokyo 184-8795, Japan Received 2 October 2002; received in revised form 3 September 2003; accepted 25 September 2003 Abstract MF radar observations of winds in the mesosphere and lower thermosphere (MLT) at Wakkanai (45:4 ◦ N; 141:7 ◦ E) have been signicantly contributing to the studies of MLT dynamics. The present paper is an attempt to describe the characteristics of the terdiurnal tide at Wakkanai. We have used the continuous data gathered during the 5-year period 1997–2001. Mainly, the analysis focuses upon the seasonal aspects of the terdiurnal tide characteristics. The observed results have been compared with the results reported at London, Ontario, another station located at a similar midlatitude (43 ◦ N) sector. It is observed that the terdiurnal tide is a regular feature in the MLT region. There exists strong day-to-day variability in the terdiurnal tide amplitudes. Short-time variability conrms that the terdiurnal oscillation can have an amplitude comparable to that of the diurnal and semidiurnal tides on occasions. Presence of dominant 8-h oscillation is observed in all seasons. However, a strong seasonal dependence is not observed in their amplitude variations. Amplitudes show slightly larger values in the winter season than the corresponding values in the summer season. The terdiurnal amplitudes at Wakkanai are more comparable to the diurnal tide amplitudes than the semidiurnal values. Comparison of the observed characteristics at Wakkanai and London reveals some similarities as well as dierences. c  2003 Elsevier Ltd. All rights reserved. Keywords: MF radar; Middle atmosphere dynamics; Waves and tides; Climatology 1. Introduction Tidal oscillations of winds/temperature are a dominant feature of the mesosphere and lower thermosphere (MLT). Characteristics of the diurnal and semidiurnal tidal com- ponents have been studied experimentally/theoretically for decades. On the experimental front, radar observations played a key role in such investigations focussing the sea- sonal and geographic characteristics of these global scale waves. While the diurnal and semidiurnal tides have been investigated extensively, studies on terdiurnal tide in the MLT region are seldom reported, probably because of being the third harmonic in the wind decomposition. The ∗ Corresponding author. Tel.: +81-42-327-6929; fax: +81-42- 327-6678. E-mail address: nambooth@crl.go.jp (S.P. Namboothiri). terdiurnal tide was rst reported by Revah (1967) using me- teor radar observations at Garchy (47 ◦ N; 3 ◦ E). Later on, reports based on both meteor and medium frequency (MF) radar observations at a few stations gave more information on the general characteristics of the terdiurnal tide (e.g., Glass and Spizzichino, 1974; Cevolani and Bonelli, 1985; Manson and Meek, 1986; Cevolani, 1987; Teitelbaum et al., 1989; Reddi et al., 1993). Seasonal patterns for the northern midlatitudes suggest that the terdiurnal tide reaches its max- imum amplitude during the winter. Also it has been shown that there are more irregular phase variations and that the wavelengths are shorter in summer than in winter. It is gen- erally expected that the amplitude of the terdiurnal tide will be smaller than the semidiurnal or diurnal tides. However, observations reported from a few locations have indicated that the amplitude of the terdiurnal tide is comparable to that of the diurnal tide (Cevolani, 1987; Teitelbaum et al., 1989; 1364-6826/$ - see front matter c  2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2003.09.010