S. Vijaya Bhaskara Rao

Climatology of low-latitude mesospheric echo characteristics observed by Indian mesosphere, stratosphere, and troposphere radar

[1] 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.

Northeast monsoon rainfall variability over south peninsular India and its teleconnections

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.

VHF radar observations of tropical easterly jet stream over gadanki

The three dimensional wind velocities evaluated using the data obtained from the Indian Mesosphere-Stratosphere-Troposphere (MST) Radar are used to study the tropospheric and lower stratospheric dynamics. Temporal variation of the zonal wind velocities during September 1995 – September 1996 are used to identify and study the tropical easterly jet stream characteristics. The jet is active over the observation site during the months of June, July and August with maximum strength in July. The zonal winds are westerlies upto about 6 km and easterlies start above that height and the maximum winds during jet stream conditions occurred at around 16 km. A reversal in vertical velocities, i.e., descent to ascent, below the jet maximum is observed. Gravity waves generated by the jet induced wind shears are observed with 8 hours periodicity. The wind shears, refractivity turbulence structure constant, Cn2 and the eddy dissipation rate, e were calculated around the jet stream and the results are presented.

Identification and Validation of Homogeneous Rainfall Zones in India Using Correlation Analysis

AbstractDaily rainfall data obtained from 1025 rain gauges spread across the country over 51 years (1951–2001) are subjected to correlation analysis to identify homogeneous rainfall zones over India. In contrast to earlier studies, which were based on seasonal/annual rainfall, the present study identifies homogeneous rainfall regions with the help of seasonal [southwest monsoon (SWM) and northeast monsoon (NEM)] and annual rainfall. India is divided into 26 (20) homogeneous rainfall zones using annual and SWM (NEM) rainfall. The delineated homogeneous regions are compared and contrasted with those defined by earlier studies, employing a variety of schemes. The interseries correlations of rainfall within each zone are found to be better when the zones are identified by the present study than by other studies. The tests that are performed to evaluate coherency of zones reveal that the zones are homogeneous not only at different temporal scales (interannual and intraseasonal) but also in terms of rain amount...

Intriguing Aspects of the Monsoon Low-Level Jet over Peninsular India Revealed by High-Resolution GPS Radiosonde Observations

AbstractThe strong cross-equatorial flow in the lower troposphere, widely known as the monsoon low-level jet (MLLJ), plays an important role in the Indian summer monsoon (ISM) rainfall during June–September. Using high-resolution GPS radiosonde observations over Gadanki (13.5°N, 79.2°E), some new aspects of MLLJ have been reported. In the present study it is found that, on average, the MLLJ exists at 710 hPa over southeastern peninsular India, rather than at 850 hPa as reported by earlier studies. It is observed that the ECMWF Re-Analysis (ERA)-Interim data provide better results on the spatial, temporal, and vertical variation of MLLJ. Further, the characteristics of the MLLJ during the active and break spells of ISM are also investigated; higher MLLJ core height and intensity are found during active phases of the Indian monsoon. This study emphasizes the use of high-resolution measurements for studying monsoon dynamics in detail.

Onset of Indian summer monsoon over Gadanki (13.5°N, 79.2°E): Study using lower atmospheric wind profiler

[1] The onset of Indian summer monsoon (ISM) over Gadanki (13.5°N; 79.2°E) is identified using variations in signal-to-noise ratio (SNR), wind speed, wind direction and vertical velocity at 1.5 km (∼850 hPa) using lower atmospheric wind profiler (LAWP). Strengthening of the low level wind speed attaining 8 ms -1 with directional change from south-easterlies to south-westerlies defines the beginning of the monsoon. Enhancement in SNR few days before with noticeable magnitude of 5-10 dB at the time of onset combined with clear-air vertical velocity reversal from downward to upward few days before onset and persisting during monsoon activity supplements the wind speed criteria in identifying onset. Hydrometeor velocity shows large downward values exceeding more than 1 ms -1 indicating occurrence of rainfall. It is proposed that UHF radar at a location can be used to identify the onset of ISM based on wind speed without considering rainfall separately.

Diurnal variation of ducts observed over a tropical station, Gadanki, using high‐resolution GPS radiosonde observations

A comprehensive study on diurnal variation of ducting using high vertical and temporal resolution radiosonde measurements over the Indian tropical region, Gadanki (13.5oN, 79.2oE) was presented. The diurnal variation of ducts was examined statistically based on both the refractivity and modified refractivity using 3 hourly radiosonde soundings obtained during October 2010 to March 2014 as a part of tropical tropopause dynamics (TTD) campaigns conducted under CAWSES India Phase II program. Strong diurnal variation in the altitude of occurrence of the duct has been found and is at maximum altitude (~2.5 km) during 11-17 LT. Interestingly, it is found that the occurrence of duct altitude closely follows the boundary layer altitude. Diurnal variation of duct altitude is maximum during post-monsoon followed by winter, monsoon and minimum in pre-monsoon. However, duct strength is maximum during winter followed by pre-monsoon, post-monsoon and minimum in monsoon. Duct thickness is found to be varying between 0.4 km and 1 km diurnally with the highest thickness during the winter season. Strong diurnal and seasonal variation in the percentage occurrence of the ducts was found with the highest percentage of ducts observed during winter (77%) followed by post-monsoon (51%), pre-monsoon (44%) and monsoon (10%). All the characteristics of ducts during all the seasons are maximum at 14 LT due to the high solar irradiance over Gadanki from 11-17 LT. The minimum frequency being trapped has been investigated and found that wave trapping occurs for the radars with frequencies 56-438 MHz over this station.

Causative mechanisms for the occurrence of a triple layered mesospheric inversion event over low latitudes

The temperature profile obtained from the space borne instrument “Sounding of Atmosphere by Broadband Emission Radiometry” instrument onboard “Thermosphere Ionosphere Mesosphere Energetics and Dynamics” shows a triple layered mesospheric inversion event on the night of 23 September 2011, when there is an overpass near the low-latitude sites Gadanki (13.5°N, 79.2°E) and Tirunelveli (8.7°N, 77.8°E). The three mesospheric inversion layers (MILs) are formed in the height region around ~70 (lower), ~80 (middle), and ~90 km (upper) with amplitudes ~11, ~44, and ~109 K and thickness of 3.4, 4.9, and 6.6 km, respectively. The formation of the lower and middle MILs can only be observed in the Rayleigh lidar temperature profiles over Gadanki due to upper height limitation of the system. Nearly all the dominant causative mechanisms are examined for the occurrence of the MIL event. The lower MIL at ~70 km is inferred to be due to planetary wave dissipation, as there is a sudden decrease of planetary wave activity above 70 km. Further, it is demonstrated that the middle MIL at ~80 km is due to the turbulence generated by gravity wave breaking which is in turn due to gravity wave-semi-diurnal tidal interaction, though the height of the middle MIL descends at the rate of ~1 km/h, which is nearly the vertical phase speed of diurnal tide, whereas the upper MIL at above 90 km is due to the large chemical heating rate (~45 K/day) generated by the dominant exothermic reaction O + O + M → O2 + M.

Advanced meteor radar installed at Tirupati: System details and comparison with different radars

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.

Anomalous Wind Circulation Observed during 1997/98 El Niño Using Indian MST Radar

Abstract Unique facility of measuring vertical winds using Indian mesosphere–stratosphere–troposphere (MST) radar along with horizontal winds enables the study of the atmospheric circulation over Gadanki, India. Several important features are noted while analyzing the wind field. A tropical easterly jet stream of 35 m s−1 strength is seen around 16 km during monsoon season. Relatively strong jetlike northward motion (southerlies) of 5–7 m s−1 is seen around 14 km during winter months. These two maxima in zonal and meridional wind patterns, even though they differ in strength greatly, occur in two contrasting seasons. Downward motion combined with upper-level northward and lower-level southward motion observed during winter in normal years indicates the signature of tropical Hadley circulation over the study region. During the 1997/98 El Nino event, however, an anomalous pattern of winds is seen and Hadley circulation is observed to be weakened.