Low‐latitude mesospheric echo characteristics are investigated using data collected during June 1994 to July 2005 (11 years) by the Indian mesosphere, stratosphere, and troposphere radar located at Gadanki (13.5°N, 79.2°E). Mesospheric echoes are frequently observed during 1000–1530 hrs (local time) in the height range of 68–78 km and are found to be highly intermittent in both space and time, consistent with those reported elsewhere. Although echoes are observed throughout the year, strong seasonal dependence has been observed in both echo occurrence and signal‐to‐noise ratio (SNR). Percentage occurrence (PO) of mesospheric echoes shows two maxima, one during late March equinox and early summer, and another during September. However, corresponding SNR suggests that strong echoes occur in both equinoxes with a minimum during winter. A clear semiannual variation is observed in PO of echoes with a peak occurring during the months of May and October. Similar variation is observed in SNR with peaks in March and September–November. These features are quite different from those observed at midlatitudes and high latitudes. Annual oscillation seems to fit well above 78 km and below 68 km, although on many occasions, occurrence of echo is poor at these heights. The ratio of vertical to off‐vertical beam SNR (which could be taken as a measure of aspect sensitivity) was close to unity at these heights, indicating that scattering is due to turbulence‐generated refractive index fluctuations. A positive correlation (R = 0.37) between PO and solar activity is observed, whereas a negative correlation (R = −0.55) is found between SNR and solar activity. The echo characteristics observed have been compared in detail with those reported from midlatitudes and high latitudes. The mechanisms behind the observed features are discussed in the light of mesospheric temperature inversions (MTIs), which are often noticed at this location, and wave breaking at these altitudes.

Rainfall over south peninsular India during the northeast (NE) monsoon season (Oct-Dec) shows significant interannual variation. In the present study, we relate the northeast monsoon rainfall (NEMR) over south peninsular India with the major oscillations like El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Equatorial Indian Ocean Oscillation (EQUINOO) in the Indian and Pacific Oceans. For establishing the teleconnections, sea surface temperature, outgoing long wave radiation, and circulation data have been used. The present study reveals that the positive phase of ENSO, IOD, and EQUINOO favor the NEMR to be normal or above normal over southern peninsular India. The study reveals that the variability of NEMR over south peninsula can be well explained by its relationship with positive phase of ENSO, IOD, and EQUINOO.

[1] The present study delineates the low-latitude thermal structure in the altitude range of 30 to 110 km using Gadanki (13.5°N, 79.2°E) Rayleigh lidar (1998Rayleigh lidar ( -2007, Thumba (8.5°N,77°E) rocketsondes (1970Thumba (8.5°N,77°E) rocketsondes ( -1991, and SABER/TIMED satellite (2002)(2003)(2004)(2005)(2006)(2007) observations. This study particularly addresses whether (1) the lidar data available only during nighttime is sufficient to study the background mean thermal structure in 30-80 km altitude region, (2) the nonavailability of the lidar data during cloudy seasons (monsoon) will affect the derived background mean thermal structure, and (3) any alternate satellite observations can be used for getting the thermal structure of the middle atmosphere. The comparisons between temperatures measured by Rayleigh lidar and SABER show good agreement, suggesting that SABER data can be used effectively to study the mean thermal structure. The nocturnal average and diurnal average of temperature from SABER show similar features, suggesting that data available from lidar only during nighttime can be effectively used to study the mean background thermal structure between 30 and 80 km. Large difference between SABER and lidar observations during monsoon suggests that low data rate available from the lidar is not sufficient to obtain the mean thermal structure during cloudy seasons. Beside this, variations in stratopause (mesopause) height and temperature are also studied. The stratopause and mesopause lie in the height region of 47-49 km and 97-99 km, with peak temperature of 265 K and 170 K, respectively. Stratopause height and temperature show clear semiannual oscillation. No significant seasonal variation is observed either in mesopause height or in temperature at this low latitude.

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An advanced meteor radar, viz, Sri Venkateswara University (SVU) meteor radar (SVU MR) operating at 35.25 MHz, was installed at Sri Venkateswara University (SVU), Tirupati (13.63°N, 79.4°E), India, in August 2013 for continuous observations of horizontal winds in the mesosphere and lower thermosphere (MLT). This manuscript describes the purpose of the meteor radar, system configuration, measurement techniques, its data products, and operating parameters, as well as a comparison of measured mean winds in the MLT with contemporary radars over the Indian region. It is installed close to the Gadanki (13.5°N, 79.2°E) mesosphere-stratosphere-troposphere (MST) radar to fill the region between 85 and 100 km where this radar does not measure winds. The present radar provides additional information due to its high meteor detection rate, which results in accurate wind information from 70 to 110 km. As a first step, we made a comparison of SVU MR-derived horizontal winds in the MLT region with those measured by similar and different (MST and MF radars) techniques over the Indian region, as well as model (horizontal wind model 2007) data sets. The comparison showed an exquisite agreement between the overlapping altitudes (82-98 km) of different radars. Zonal winds compared very well, as did the meridional winds. The observed discrepancies and limitations in the wind measurement are discussed in the light of different measuring techniques and the effects of small-scale processes like gravity waves. This new radar is expected to play an important role in our understanding of the vertical and lateral coupling of different regions of the atmosphere that will be possible when measurements from nearby locations are combined.