B. V. Krishna Murthy

Turbulence parameters in the tropical troposphere and lower stratosphere

[1]xa0Using the mesosphere-stratosphere-troposphere (MST) radar observations in the troposphere and lower stratosphere at a tropical station, Gadanki, the rate of turbulent kinetic energy dissipation ϵ, as estimated by the spectral width method, the power method, and the variance method, has been compared. It is found that ϵ from the spectral width method and variance method agree quite well under low background wind conditions, whereas under high background wind conditions, ϵ from the spectral width method appears to be an underestimate. The ϵ from the power method is generally lower than that from the other two methods. Adopting the variance method, the turbulence parameters ϵ and the eddy diffusivity for momentum Km have been estimated for tropospheric and lower stratospheric regions over Gadanki for the period 18 January 1999 to 5 March 1999, making use of the vertical wind observations from MST radar on each night for ∼2 hours. Average altitude profiles of ϵ and Km for the winter season are presented. The turbulence parameter profiles reveal a prominent peak in the upper troposphere. From Richardson number considerations the observed turbulence is attributed mainly to wind shear.

Lidar observations of cirrus cloud near the tropical tropopause: general features

Abstract General features of high-altitude cirrus clouds observed below the tropical tropopause are studied using lidar at Gadanki (13.5°N, 79.2°E). These clouds occur quite frequently over this tropical region. Thin clouds with optical depth 0.3. The preferred altitude of these clouds is between 14 and 16 km, with a vertical extent ranging from 0.4 to 4 km. The backscattered lidar signal from the thin cirrus clouds exhibits significant depolarisation, indicating the presence of randomly oriented nonspherical ice crystals in abundance.

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

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.

First observational evidence of the modulation of gravity wave activity in the low latitude middle atmosphere by equatorial waves

[1]xa0Evidence for the modulation of the gravity wave activity by the equatorial waves in the tropical middle atmosphere is presented using the Rayleigh lidar observations of the temperature over Gadanki (13.5°N, 79.2°E) and the rocketborne measurements of winds over Shriharikota Range (13.7°N, 80.2°E) during the period of 29 February to 31 March, 2000. The potential energy per unit mass associated with the gravity wave activity in the upper stratosphere and mesosphere is found to undergo periodic fluctuations, which is well correlated with the zonal and meridional wind oscillations in the stratosphere produced by the equatorial waves. The most dominant modulation of the gravity wave activity due to equatorial waves is found to occur at periods around 4-days and 12-days. Minimum gravity wave activity in the mesosphere is observed during the southward phase of meridional wind and the westward phase of zonal wind oscillations in the lower stratosphere.

Rayleigh lidar observations of quasi‐sinusoidal waves in the tropical middle atmosphere

[1]xa0Rayleigh lidar observations of temperature for ∼2–7 hours on 38 nights during the period of 18 January–5 March 1999 and for ∼3 hours on 29 nights during 29 February–2 April 2000 in the altitude region of 27–60 km over Gadanki (13.5°N, 85°E) clearly show the dominance of the quasi-sinusoidal waves of periods in the range of ∼6–13 hours in temperature in the tropical middle atmosphere. During 1999, the vertical wavelength of these waves ranges from 7.1 to 17 km with a mean value of 12.0 ± 2.4 km while during 2000 it ranges from 5.5 to 17 km with a mean value of 11.4 ± 3.5 km. The wave periods estimated from the observations during 1999 are generally greater than the observation period on each night and are estimated to be in the range of 6.5–13.1 hours with an average period of 9.7 ± 1.8 hours, which is significantly smaller than the inertial period of 53 hours. The amplitudes of these waves are in the range of 1 to 8 K. Such sinusoidal wave modes are clearly discernible in the temperature profiles for ∼50% nights of observation. Another striking feature is that the quasi-sinusoidal structure is clearly observed in the “equivalent day” analysis also, indicating only small day-to-day variability in phase, a property that is characteristic of atmospheric tides.

Gravity waves in the tropical middle atmosphere: Characteristics and wave-mean flow interaction

Abstract High-resolution Rayleigh lidar observations of temperature in the altitude region of 27–60 km over Gadanki (13.5°N, 79.2°E) during the period of January 18 to March 5 1999 and February 29 to March 31 2000 were used to study the gravity wave characteristics in the tropical middle atmosphere. The vertical wavenumber spectra of gravity waves and the altitude variation of the potential energy per unit mass (Ep) associated with gravity wave activity are found to be similar during both these years. Altitude profile of E p indicates that significant wave damping occurs in the stratosphere while nearly non-dissipating gravity wave growth is observed in the lower mesosphere. The slope of the vertical wave number spectra is observed to be about −2.1, which is less than that expected for saturated spectra (−3). The gravity wave activity in the stratosphere and lower mesosphere is found to undergo temporal variations, which are correlated with the temperature fluctuations in the middle atmosphere. A longer period wave with a mean vertical wavelength of about 12.8 km and wave period of about 8–14 hours is also observed on several nights.