Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 349–358 www.elsevier.com/locate/jastp Coordinated MST radar and lidar observations for the study of mesospheric structures over a tropical station M. Venkat Ratnam a , D. Narayana Rao a ; ∗ , T. Narayana Rao a , M. Krishnaiah a , Y. Bhavani Kumar b , V. Siva Kumar b , P.B. Rao b a Department of Physics, Sri Venkateswara University, Tirupati - 517 502, India b National MST Radar Facility, Gadanki, Post Box No.123, Tirupati - 517 502, India Received 12 March 2001; received in revised form 25 September 2001; accepted 17 October 2001 Abstract VHF radar observations at a frequency of 53 MHz at Gadanki (13:5 ◦ N; 79:2 ◦ E), India, during the period from September 1995 to August 1999 are used to study the tropical mesospheric structures. MST radar echoes have shown an enhancement in echo power of about 6–8 dB above the average noise level. These echoes are intermittent in time and can be observed at an altitude between 70 and 76 km. The seasonal variation of these echoes shows the maximum percentage occurrence during summer, closely followed by equinoxes and minimum during winter. This seasonal variation is also accompanied by shifting of height to lower heights during winter. Coordinated MST radar and Nd:YAG lidar observations were conducted to study the mesospheric structures during March 1998–June 1999. In the Lidar temperature proles, inversion of about 20–30 K is observed at the heights of the MST radar-enhanced echoes. Enhanced radar reectivity is observed on both the preceding and succeeding days of lidar observations of strong temperature inversion. Percentage occurrence of MST radar echoes and temperature inversions show one-to-one correspondence for most of the cases. The eect of temperature inversion on the radar reectivity is also studied. The presence of temperature inversion and enhanced radar reectivity are discussed in the light of gravity-wave breaking processes at that height region. c  2002 Elsevier Science Ltd. All rights reserved. Keywords: Turbulence; Gravity waves; Reectivity; Temperature inversions 1. Introduction The VHF radar probing of the mesosphere was started long back and it is generally accepted that echo power above thenoiselevelisdetectedonlyinthedaytime,anddisappears during nighttime (Balsley and Gage, 1980; Roettger et al., 1979; Muraoka et al., 1998). Since the backscattered echoes at mesospheric heights are mainly due to irregularities in electron density gradient, which is a daytime phenomenon, associated with the neutral turbulence, the radar observations are conned only to daytime. These echoes at mesospheric heights occur in the form of turbulent layers as reported ∗ Fax: +91-8574-48485. E-mail address: profdnrao@hotmail.com (D. Narayana Rao). by several investigators (Roettger, 1987; Yamamoto et al., 1987; Muraoka et al., 1988, Tsuda et al., 1990). In these studies, the observed echo layers have been discussed on the basis of turbulent scattering, and the enhanced echoes have been related to the turbulence generated by the breaking of tides and gravity waves. This aspect is well documented by Fritts (1989) and references therein. In this context, the mesospheric thermal structure in which a secondary minimum is often observed is worth noting. The presence of secondary minimum of tempera- ture at mesospheric heights has been detected long back by rocket observations and was rst reported by Schmidlin (1976). Later Chandra (1980) explained this by using a one-dimensional model to demonstrate such features, which might be a direct result of turbulent mixing and dissipation. Using mid-latitude lidar data, Hauchecorne et al. (1987) 1364-6826/02/$ - see front matter c  2002 Elsevier Science Ltd. All rights reserved. PII:S1364-6826(01)00101-8