Geetha Ramkumar

Mean winds and tidal components during counter electrojet events

The first observations of mean winds and the amplitude and phases of the tidal components measured with a meteor wind radar located at Trivandrum (8.5°N, 77°E) during five consecutive days of counter electrojet events identified in the horizontal component of the earths magnetic field from January 27–31, 1987 are described. The mean zonal winds in the altitude region of 90–105 km are in general westward during counter electrojet days and eastward during no counter electrojet days. The amplitudes and phases of the tidal components on the counter electrojet days are found to be substantially different from those on the no counter electrojet days.

A comparative study of daytime mesopause temperatures obtained using unique ground based optical and meteor wind radar techniques over the magnetic equator

[1] This paper presents the first ever comparison of daytime Mesopause OH rotational temperatures as determined using Meinel (8-3) dayglow emissions with those obtained using the recently established and collocated, meteor radar (SKiYMET) over the magnetic equator in India. The measurements were made over Thiruvananthapuram (8.5°N, 76.5°E, 0.5°N diplat.) for January-May 2005 period. Overall agreement had been good (±20 K) with all the gross features coming out fairly well. Further, the temperatures estimated using these two techniques, exhibit significant wavelike modulations with periods spanning ∼3-65 days with varying phase differences, presumably due to their interaction with the longer period waves like the semiannual oscillation (SAO). The consistency of the temperature variabilities at two different altitudes viz. 87 and 94 km enables investigation of the mesosphere lower thermosphere dynamic coupling.

Inertia‐gravity waves associated with the tropical easterly jet over the Indian subcontinent during the South West Monsoon Period

Using the MST radar located at Gadanki (13.5°N, 79.2°E) winds were measured every four hours in the troposphere and lower stratosphere during a 72-hour period from 23 to 26 September 1997 when the tropical tropospheric easterly jet (TEJ) was present south of Gadanki with its core at a height of ∼16 km. Analysis of the radar data shows the presence of ∼56 hour oscillations (close to local inertial period of ∼51 hours) in the wind components with significant amplitudes. Examination of the corresponding horizontal wind vector shows clockwise rotation with altitude (upward energy propagation) in an altitude region (17.7–20.8 km) above the TEJ and anticlockwise rotation (downward energy propagation) in another altitude region (8.3–10.5 km) below. This indicates the generation of Inertia Gravity Waves (IGW) in the TEJ and their propagation both upwards and downwards from the source region. The inferred vertical and horizontal wavelengths of the IGW are ∼0.7 and ∼75 km respectively in the stratosphere and ∼1 and ∼143 km respectively in the troposphere.

Nonmigrating diurnal tides in the troposphere and lower stratosphere over Gadanki (13.5°N, 79.2°E)

A study of diurnal tidal oscillations in horizontal winds using Indian mesosphere-stratosphere-troposphere radar located at Gadanki (13.5°N, 79.2°E) shows that during autumnal equinox season diurnal amplitudes of 1-2 m s -1 of horizontal winds prevail in the troposphere. The vertical structure of amplitudes and phases indicate a vertical wavelength of ∼3 km in the lower troposphere and ∼6 km in the upper troposphere. A comparison of these amplitudes and vertical wavelengths with results from theoretical tidal models suggests that these oscillations in the lower troposphere are manifestation of nonmigrating diurnal tidal oscillations excited by eddy heat flux in the planetary boundary layer and those in the upper troposphere excited by latent heat release in deep convective clouds. However, nonmigrating diurnal tidal modes generated by solar radiation absorption by water vapor and clouds in the troposphere cannot be ruled out.

Equatorial waves in temperature in the altitude range 4 to 70 km

Using altitude profiles of temperature in the range 4 to 70 km derived from Mesosphere–Stratosphere–Troposphere radar and lidar observations at Gadanki (13.5°N,79.2°E) from 18 January 1999 to 5 March 1999, characteristics of equatorial waves are studied. Two-dimensional Fourier-transform analysis of the temperature profiles is carried out to identify the periodicities and their vertical wave numbers. From the characteristics obtained, equatorial slow Kelvin waves with periodicities 15.7 d, 9.4 d, 7.8 d and 6.7 d are identified in the troposphere and stratosphere regions and among these 7.8 d and 6.7 d periodicities are found to penetrate into the mesosphere. Equatorial waves with smaller periodicities in the range 5.2 d to 3.6 d are also observed. The vertical flux of horizontal momentum (zonal) of the identified slow Kelvin-wave periodicities in the altitude region 4–25 km is estimated. It is found that equatorial waves modulate tropical tropopause temperature and altitude. Copyright

Climatologies of tidal winds in the radio-meteor region over Trivandrum (8°N)

Abstract The climatologies of tidal wind fields in the 80–105 km region using Meteor Wind Radar (MWR) observations during June 1984–1988 at Trivandrum (8.5°N) are presented. The most significant feature revealed by the amplitude and phase profiles of the tidal winds is the simultaneous presence of two modes, differing in vertical wavelengths, for each oscillation. In contrast to the expected (1,1) mode, the (1,−2) diurnal mode is found to be stronger. In addition to the stronger (2,2) semi-diurnal mode, higher order modes are also found to be significant. The 24h oscillation is found to be the strongest component and 12 h and 8 h oscillations are equally significant. The tidal wind amplitudes over Trivandrum are found to be larger during the equinoctial months. The amplitudes and vertical wavelengths of the two interfering modes of the 24 h, 12 h, 8 h oscillations are deduced and compared with our current theoretical understanding on tidal winds and observations from other low latitude stations.

A study of equatorial wave characteristics using rockets, balloons, lidar and radar

Abstract A co-ordmated experimental campaign was conducted for 40 consecutive days from 21 February to 01 April 2000 using RH-200 rockets, balloons, Rayleigh lidar and MST radar, with the objective of delineating the equatorial waves and estimating momentum fluxes associated with them. Winds and temperatures in the troposphere, stratosphere and mesosphere over two low latitude stations Gadanki (13.5°N, 79.2°E) and SHAR (13.7°N, 80.2°E) were measured and were used for the study of equatorial waves and their interactions with the background mean flow in various atmospheric regions. The study shows the occurrence of a strong stratospheric cooling (∼25 K) anomaly along with a zonal wind anomaly and this low-latitude event appears to be linked to high-latitude stratospheric warming event and followed by subsequent generation of short period (∼5 days) oscillations lasting for a few cycles in the stratosphere. Slow and fast Kelvin waves and RG wave (∼-17-day and ∼7.2-day and ∼4.2-day periods respectively) have been identified. The mean flow acceleration produced by the divergence of the momentum flux due to the observed Kelvin waves in the 35–60 km height region were compared with the zonal flow accelerations computed from the observed zonal winds. Contribution by the slow and fast Kelvin waves was found to be only ∼25 % of the observed acceleration during the evolution of the westerly phase of the semi-annual oscillation.

The annual and semi-annual wind fields in low latitudes

Abstract The annual oscillations (AO) and semi-annual oscillations (SAO) of the monthly mean zonal and meridional winds in the troposphere, stratosphere and mesosphere have been studied using simultaneous data from balloonsondes, rocketsondes and Meteor Wind Radar (MWR) acquired at Trivandrum. A study is also made of the AO and SAO of the diurnal, semi-diurnal and ter-diurnal wind oscillations in the 80–102 km altitudes using MWR observations. The same observations have been used to study the altitude profiles of the annual mean winds, and the annual mean tidal winds. It is found that the AO phase in the NS wind leads with increasing altitude in the stratosphere and mesosphere while SAO phase lags with altitude. The SAO of EW tidal wind amplitudes are slightly stronger than AO. For the NS wind, the AO of the diurnal tide has the largest amplitude and the SAO of the semi-diurnal tide also has a relatively large amplitude. The results are compared with earlier reports from other low latitude stations and with the CIRA 1986 model for 8.5 °N latitude. Further, they are discussed in the light of tidal dissipation in and above the mesopause ( > 70 km).

Gravity wave signatures in the dip equatorial ionosphere‐thermosphere system during the annular solar eclipse of 15 January 2010

The present work pertains to the eclipse-induced gravity wave modulations in the ionosphere-thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at E region altitudes, and ionosonde-derived foF1 and foF2 parameters are used to analyze the characteristics of the eclipse-induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30–100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave-induced winds in generating blanketing Es over the equator is also examined.

Variability of mesopause temperature derived from two independent methods using meteor radar and its comparison with SABER and EOS MLS and a collocated multi-wavelength dayglow photometer over an equatorial station, Thumba (8.5° N, 76.5° E)

Two independent methods for deriving mesopause temperature using meteor radar installed at an equatorial station, Thumba (8.5° N, 76.5° E), are discussed in this article. This meteor radar-derived mesopause temperature is then compared with two different types of spaceborne measurement, namely (i) Sounding the Atmosphere using Broadband Emission Radiometry (SABER) and (ii) the Earth Observing System Microwave Limb Sounder (EOS MLS), and a collocated multi-wavelength dayglow photometer (DGPM). The meteor radar-derived temperature is in fairly good agreement with all the three measurement techniques, with an uncertainty of ±10°. This study focuses on a detailed evaluation and inter-comparison of mesopause temperature derived from different measurement techniques. An attempt is also made to compare the suitability of these observations to study planetary waves and other oscillation activities in the mesospheric region.