Karanam Kishore Kumar

TIMED/SABER observations of global gravity wave climatology and their interannual variability from stratosphere to mesosphere lower thermosphere

The present study for the first time reports the global gravity wave activity in terms of their potential energy derived from TIMED/SABER observations right from the stratosphere to the mesosphere lower thermosphere (MLT) region. The potential energy profiles obtained from SABER temperature are validated by comparing them with ground based LIDAR observations over a low latitude site, Gadanki (13.5° N, 79.2° E). The stratospheric and mesospheric global maps of gravity wave energy showed pronounced maxima over high and polar latitudes of the winter hemisphere. The interannual variability of the stratospheric gravity wave activity exhibited prominent annual oscillation over mid-latitudes. The equatorial gravity wave activity exhibited quasi-biennial oscillation in the lower stratosphere and semi-annual oscillation in the upper stratosphere. The MLT region maps revealed summer hemispheric maxima over polar latitudes and secondary maxima over the equatorial region. The results are discussed in the light of present understanding of global gravity wave observations. The significance of the present study lies in emphasizing the importance of satellite measurements in elucidating gravity waves, which is envisaged to have profound impact on parameterizing these waves.

Planetary wave-tidal interactions over the equatorial mesosphere-lower thermosphere region and their possible implications for the equatorial electrojet

[1] Optically measured daylight mean mesopause temperatures over a dip equatorial station, Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), have been analyzed in conjunction with simultaneously measured equatorial electrojet (EEJ)-produced magnetic field at the surface. The signature of planetary wave-tidal interactions in the mesosphere-lower thermosphere (MLT) region has been observed for the first time in the day-to-day variability in the EEJ, i.e., the time of its peaking and the duration, as inferred from the EEJ-produced magnetic field on the ground. The present study shows that the planetary wave of quasi 16 day periodicity plays an important role in causing these variabilities, especially during the winter months. The quasi 16 day wave is found to be modulating the mesopause temperature (MT), duration, and time of the maximum EEJ intensity (D EEJ and T EEJ ). During positive excursions of the planetary wave, T EEJ showed a shift toward evening, while the MT showed an increase and D EEJ showed a broadening. Similarly, all these parameters exhibited an opposite trend during negative excursions. The planetary wave-tidal interactions and subsequent modification of the tidal components have been shown to be responsible for the observed variations. This study presents a new perspective addressing the day-to-day variability of the EEJ.

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.

Seasonal and diurnal variation of convective available potential energy (CAPE) using COSMIC/FORMOSAT-3 observations over the tropics

[1] The global pattern of convective available potential energy (CAPE) at seasonal and diurnal time scales is discussed using 1 year of COSMIC/FORMOSAT-3 satellite observations. The calculation of CAPE using temperature and humidity measurements of COSMIC is described. The estimated CAPE is grouped into 5 x 5 grid and is further classified into four seasons, namely, winter, spring, summer, and autumn. The CAPE magnitudes in general have high values over land as compared to oceanic region, which confirmed the consistency of CAPE calculations. The systematic migration of CAPE from Northern Hemisphere to Southern Hemisphere is observed during Northern Hemisphere summer to winter, coinciding with the movement of intertropical convergence zone. Once the seasonal pattern is established, the composite diurnal patterns of CAPE with 2 h resolution are obtained by combing all the observations in one season. Diurnal variation of CAPE has shown domination of semidiurnal variations at some latitudes (12 h) and diurnal variation (24 h) at some other latitudes. The mean removed CAPE is then subjected to Fourier analysis to extract the diurnal variation amplitudes. During the observational period, larger CAPE magnitudes are observed over the Indian Ocean during most of the seasons, comparable in magnitude to that of the land regions. As the CAPE and precipitation patterns have correlation, the present study demonstrated the capability of COSMIC/FORMOSAT-3 to study the diurnal patterns of CAPE, which will have implications in interpreting the tropical diurnal precipitation patterns.

Simultaneous Two-Way Time Transfers between National Physical Laboratory, Space Applications Centre and Madras Earth Station Via the Symphonie Satellite

Simultaneous two-way time transfer experiments via satellite symphonie, in which National Physical Laboratory, New Delhi (NPL), Space Applications Centre (SAC), Ahmedabad and Madras Earth Station, Madras (MES) participated, are reported in this paper. The uncertainties involved in alternate one-way mode of clock synchronization reported earlier were removed and a much improved precision and accuracy was achieved. In addition, the results obtained in a Round Robin method, where one station transmits and the other two receive between NPL, SAC and MES are also presented.

TRMM observations of latent heat distribution over the Indian summer monsoon region and associated dynamics

The latent heat released/absorbed in the Earth’s atmosphere due to phase change of water molecule plays a vital role in various atmospheric processes. It is now well established that the latent heat released in the clouds is the secondary source of energy for driving the atmosphere, the Sun being the primary. In this context, studies on latent heat released in the atmosphere become important to understand the some of the physical processes taking place in the atmosphere. One of the important implications of latent heat release is its role in driving the circulations on various temporal and spatial scales. Realizing the importance of latent heat released in the clouds, a comprehensive study is carried out to understand its role in driving the mesoscale circulation. As Indian summer monsoon (ISM) serves as natural laboratory for studying the clouds and their microphysics, an attempt is made to explore the latent heat distribution over this region using 13 years of Tropical Rainfall Measuring Mission (TRMM) observations. The observed profiles of latent heating over ISM region showed large spatial and temporal variability in the magnitude thus reflecting the presence of organization of convection on mesoscale. The latent profiles in convective and stratiform regions are segregated to study the differences in their interaction with large-scale environment. Various re-analysis dataset were used to examine the role of latent heating distribution on the mesoscale circulation. The significance of the present study lies in establishing the vertical distribution of latent heating and their impact on the background circulation.

Broadcast of ATA Time Signals Via Satellite Symphonie

An experiment, to study the improvements in accuracy and precision in broadcasting standard time ATA format via satellite over ATA high frequency broadcast, is described. The ATA format was transmitted in both, the routine code with ticks and tones and with BCD code carrying information about time and date i.e. Year, Month, Day, Hour, Minute, Second and DUT1 correction. The signals were transmitted from Delhi Earth Station and measurements were made at Delhi Earth Station (DES), Ahmedabad Earth Station (AES), Madras Earth Station (MES) and Transportable Remote Area Communication Terminal (TRACT) stationed at Calcutta: thus covering a large cross-section of India.

On the role of precipitation latent heating in modulating the strength and width of the Hadley circulation

The latent heat released in the clouds over the tropics plays a vital role in driving the Hadley circulation (HC). The present study discusses the influence of latent heating (LH) on the HC parameters viz., centre, strength and total width by using precipitation LH profiles derived from the space-borne observations of the Precipitation Radar (PR) onboard Tropical Rain Measuring Mission (TRMM) and meridional stream function (MSF) derived from ECMWF-Interim reanalysis. The latitude of peak latent heating, width of the latent heating distribution and the total LH released within the ascending limb of the HC are estimated and their influence on the HC centre, strength and width is quantified, for the first time. The present results show that the latitude of peak LH significantly influences the position of the HC centre with correlation coefficient of ~ 0.90. This high correlation between these two quantities seems to be due to their co-variability with the apparent motion of the Sun across the latitudes. The intensity of the HC in the NH as well as SH shows high correlation with the latitude of peak LH with coefficients − 0.85 and − 0.78, respectively. These results indicate that farther the latitude of peak LH from the equator in the summer hemisphere, stronger is the HC intensity in the winter hemisphere. The present analysis also reveals that the total LH released within the ascending limb of HC substantially influence the total width of the HC, with correlation coefficient ~ 0.52, as compared to the other two LH parameters. This observation can be attributed to the fact that the HC is sensitive to the latent heat release in the mid-tropospheric levels in the tropics. An attempt is also made to investigate the degree of variability of these parameters after deseasonalization and results are discussed in the light of present understanding. The significance of the present study lies in providing the observational evidence for the influence of latent heating on the HC strength/width variability, quantitatively, for the first time using TRMM observations of precipitation latent heating.

Impact of Sudden Stratospheric Warming of 2009 on the Equatorial and Low‐Latitude Ionosphere of the Indian Longitudes: A Case Study

Using the Equatorial Electrojet (EEJ) induced surface magnetic field and Total Electron Content (TEC) measurements, we investigated the impact of the Sudden Stratospheric Warming (SSW) of January 2009 on the equatorial electrodynamics and low-latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low-latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behaviour well before the onset of the SSW. This is found to be coincided with the beginning of enhanced planetary wave (PW) activity over high-latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moon. The response of TEC to SSW is found to be latitude dependent as the near-equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low-latitude regions, appearing as an ‘ionization hole’, well after the SSW peak. The investigation revealed the existence of a quasi-16 day wave in the TEC over low-latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favourable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.

VHF radar studies of the migrating and nonmigrating diurnal and semidiurnal tides over a tropical and an equatorial station

VHF radar measurements of winds are utilized to study the diurnal and semi-diurnal tides over Gadanki and Kotatabang. The tidal amplitudes exhibit maxima in UTLS region during June-September over Gadanki and during March and September over Kotatabang. The vertical wavelength is 3–5 km (Gadanki) and 25–30 km (Kotatabang), which reveal the existence of nonmigrating and migrating tides, respectively. Brightness temperature shows that over Gadanki(deep) and Kotatabang(shallow) convective clouds. Release of latent heat due to deep-clouds is found to be the main source mechanism for nonmigrating tides. The present study brings out the differences in tides over equatorial and low latitude.