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Search Results: 1 - 10 of 325217 matches for " S. Gurubaran "
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Seasonal variabilities of low-latitude mesospheric winds
R. Rajaram,S. Gurubaran
Annales Geophysicae (ANGEO) , 2003,
Abstract: Observations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70–98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70–80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80–98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation. Key words. Meteorological and atmospheric dynamics · General circulation · Middle atmosphere dynamics · waves and tides.
Variabilities of mesospheric tides and equatorial electrojet strength during major stratospheric warming events
S. Sridharan, S. Sathishkumar,S. Gurubaran
Annales Geophysicae (ANGEO) , 2009,
Abstract: The present study demonstrates the relationship between the high latitude northern hemispheric major sudden stratospheric warming (SSW) events and the reversal in the afternoon equatorial electrojet (EEJ), often called the counter-electrojet (CEJ), during the winter months of 1998–1999, 2001–2002, 2003–2004 and 2005–2006. As the EEJ current system is driven by tidal winds, an investigation of tidal variabilities in the MF radar observed zonal winds during the winters of 1998–1999 and 2005–2006 at 88 km over Tirunelveli, a site close to the magnetic equator, shows that there is an enhancement of semi-diurnal tidal amplitude during the days of a major SSW event and a suppression of the same immediately after the event. The significance of the present results lies in demonstrating the latitudinal coupling between the high latitude SSW phenomenon and the equatorial ionospheric current system with clear evidence for major SSW events influencing the day-to-day variability of the CEJ.
Influence of gravity waves and tides on mesospheric temperature inversion layers: simultaneous Rayleigh lidar and MF radar observations
S. Sridharan, S. Sathishkumar,S. Gurubaran
Annales Geophysicae (ANGEO) , 2008,
Abstract: Three nights of simultaneous Rayleigh lidar temperature measurements over Gadanki (13.5° N, 79.2° E) and medium frequency (MF) radar wind measurements over Tirunelveli (8.7° N, 77.8° E) have been analyzed to illustrate the possible effects due to tidal-gravity wave interactions on upper mesospheric inversion layers. The occurrence of tidal gravity wave interaction is investigated using MF radar wind measurements in the altitude region 86–90 km. Of the three nights, it is found that tidal gravity wave interaction occurred in two nights. In the third night, diurnal tidal amplitude is found to be significantly larger. As suggested in Sica et al. (2007), mesospheric temperature inversion seems to be a signature of wave saturation in the mesosphere, since the temperature inversion occurs at heights, when the lapse rate is less than half the dry adiabatic lapse rate.
Lower E-region echoes over the magnetic equator as observed by the MF radar at Tirunelveli (8.7° N, 77.8° E) and their relationship to Esq and Esb
R. Dhanya, S. Gurubaran,K. Emperumal
Annales Geophysicae (ANGEO) , 2008,
Abstract: The spaced antenna medium frequency (MF) radar at Tirunelveli (8.7° N, 77.8° E, geographic; 1.7° N, magnetic dip), the only one of its kind currently operating close to the magnetic equator, has provided an opportunity to investigate the electrodynamical processes related to the equatorial electrojet (EEJ) and their influence on the radar scatterers at medium frequencies in the lower E-region heights (90–98 km). Making use of the full correlation analysis that enables determination of useful geometrical parameters from the ground diffraction pattern, the present work delineates for the first time the characteristics of the radar scatterers during the occurrences of equatorial sporadic E (Esq) and blanketing sporadic E (Esb) noticed in simultaneous ionospheric sounding records at Tirunelveli. The ground magnetometer data provide indirect information on the strength of the EEJ and afternoon reverse EEJ or counterelectrojet (CEJ). The results presented in this work also reveal the height dependence of the radar echo intensity and some of the geometrical parameters at certain times, thus clearly bringing out the complex interplay of various physical processes in the probing region.
Observations of OI 557.7 nm nightglow at Kolhapur (17° N), India
N. Parihar, S. Gurubaran,G. K. Mukherjee
Annales Geophysicae (ANGEO) , 2011,
Abstract: Ground-based nightglow observations of the atomic oxygen green line at 557.7 nm have been carried out at a low latitude station Kolhapur (17° N), India, during November 2003–April 2004 and December 2004–May 2005. The nocturnal behaviour of OI 557.7 nm intensity and a comparative study with simultaneous OH Meinel band temperature measurements has been presented. OI 557.7 nm intensity and OH temperature variations covary on many occasions. It was found that an 8 h tide characterizes the variation of intensity and temperature on most nights, and especially during the month of January. This is the first report of prolonged measurements of OI 557.7 nm emission from India.
Simultaneous measurement of OI 557.7 nm, O2 (0, 1) Atmospheric Band and OH (6, 2) Meinel Band nightglow at Kolhapur (17° N), India
N. Parihar, A. Taori, S. Gurubaran,G. K. Mukherjee
Annales Geophysicae (ANGEO) , 2013,
Abstract: Near-simultaneous measurements of OI 557.7 nm, O2 (0, 1) Atmospheric Band and OH (6, 2) Meinel Band nightglow were carried out at Kolhapur (17° N), India during February–March 2007. Atmospheric temperatures around 87 and 94 km were derived from the knowledge of intensity measurements of spectral features OH (6, 2) Meinel Band and O2 Atmospheric Band, respectively. An account of the behaviour of derived temperatures has been presented. The nocturnal behaviour of OH and O2 temperatures is governed by the waves of tidal origin, whereas the signatures of planetary wave-like oscillations is noted in the night-to-night variation of two temperatures. This is probably the first report of planetary waves observed in nightglow temperatures from the Indian subcontinent.
First results from ground-based daytime optical investigation of the development of the equatorial ionization anomaly
D. Pallam Raju,R. Sridharan,S. Gurubaran,R. Raghavarao
Annales Geophysicae (ANGEO) , 2003,
Abstract: A meridional scanning OI 630.0-nm dayglow photometer was operated from Ahmedabad (17.2°N dip lat.) scanning a region towards the south in the upper atmosphere extending over ~5° in latitude from 10.2°N to 15.2°N dip latitude. From the spatial and temporal variabilities of the dayglow intensity in the scanning region we show for the first time, evidence for the passage of the crest of the equatorial ionization anomaly (EIA) in the daytime by means of a ground-based optical technique. The relationship between the daytime eastward electric field over the dip equator in the same longitude zone as inferred from the equatorial electrojet strength and the evolutionary pattern of EIA is clearly demonstrated. The latter as inferred from the dayglow measurements is shown to be consistent with our present understanding of the electrodynamical processes in the equatorial region. The present results reveal the potential of this ground-based optical technique for the investigation of ionospheric/thermospheric phenomena with unprecedented spatial and temporal resolution.
First results from the CAWSES-India Tidal Campaign
S. Gurubaran, D. Narayana Rao, G. Ramkumar, T. K. Ramkumar, G. Dutta,B. V. Krishna Murthy
Annales Geophysicae (ANGEO) , 2008,
Abstract: The first CAWSES-India Tidal Campaign was conducted by the Indian scientific community during March–April 2006. The objectives of this campaign were: (1) To determine the characteristics of tides in the troposphere and lower stratosphere (0–20 km) and mesosphere and lower thermosphere (MLT) region (80–100 km), (2) to explore and identify what lower atmospheric processes drive middle atmospheric tides in the Indian continental region and (3) to provide information on those short-term variabilities of MLT tides that are likely to have an impact on the ionospheric variabilities and contribute to the upper atmospheric weather. Data sets from experiments conducted at the three low latitude radar sites, namely, Trivandrum (8.5° N, 76.9° E), Tirunelveli (8.7° N, 77.8° E) and Gadanki (13.5° N, 79.2° E) and fortnightly rocket launches from Thumba were made use of in this study. An important observational finding reported in this work is that the radar observations at Tirunelveli/Trivandrum indicate the presence of 15–20 day modulation of diurnal tide activity at MLT heights during the February–March period. A similar variation in the OLR fields in the western Pacific (120–160° longitude region) suggests a possible link between the observed tidal variabilities and the variations in the deep tropical convection through the nonmigrating tides it generates.
Common volume coherent and incoherent scatter radar observations of mid-latitude sporadic E-layers and QP echoes
T. K. Pant, D. Tiwari, S. Sridharan, R. Sridharan, S. Gurubaran, K. S. V. Subbarao,R. Sekar
Annales Geophysicae (ANGEO) , 2004,
Abstract: The day-to-day measurements of the daytime intensities of hydroxyl (OH) Meinel (8-3) band airglow emissions at 731.6 and 740.2nm carried out from the equatorial station Thiruvananthapuram (8.5° N, 76.5° E, 0.5° dip) during the period of January-March 2001 have been investigated. This investigation provides evidence for the presence of a long period (≈16 days) wave modulating these intensities at the mesopause altitudes. Simultaneous radar measurements of zonal wind at ~87km, i.e. mesopause from Tirunelveli (8.7° N, 77.8° E, 0.33° dip), a location nearby, also reveal the presence of these long period oscillations. The daytime airglow and zonal wind undergo changes simultaneously. Similar modulations are seen in the solar 10.7cm flux also preceding dayglow and wind variabilities by 4-5 days. It is inferred in the present case that the changes in the solar flux are the cause of the generation of this long period wave in the atmosphere below the mesosphere. The oscillations in the measured dayglow intensities in the mesopause region and the winds at ~87km are resulting from the modulation caused by this wave in this region after a delay of 4-5 days.
Intraseasonal oscillation (ISO) in the MLT zonal wind over Kolhapur (16.8° N) and Tirunelveli (8.7° N)
M. V. Rokade, R. Kondala Rao, S. S. Nikte, R. N. Ghodpage, P. T. Patil, A. K. Sharma,S. Gurubaran
Annales Geophysicae (ANGEO) , 2012,
Abstract: Simultaneous observations of the mean zonal winds at 88 km obtained by the medium-frequency (MF) radars at Kolhapur (16.8° N, 74.2° E) and Tirunelveli (8.7° N, 77.8° E) have been used to study the intraseasonal oscillation (ISO) in the MLT region. The influences of the intraseasonal variations in the lower tropospheric convective activity associated with the Madden-Julian oscillations on the latitudinal behavior of intraseasonal oscillations (ISO) of the zonal winds in the equatorial mesosphere and lower thermosphere (MLT) have been studied. The ISO activity in the lower tropospheric convective activity is examined by employing outgoing long wave radiation (OLR) as a proxy for deep convective activity occurring in the tropical lower atmosphere. The ISO activity in the zonal wind over TIR is more correlated with that in the convective activity compared to the ISO over KOL. The latitudinal and temporal variabilities of the ISO in MLT zonal winds are explained in terms of the intraseasonal variabilities in the convective activity.
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