V. V. M. Jagannadha 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.

Mean Vertical Velocities Measured by Indian MST Radar and Comparison with Indirectly Computed Values

Abstract Mean vertical velocities and their variations observed with Indian mesosphere–stratosphere–troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E), a tropical station in India, are presented. In this study, a comparison has been made between Indian MST radar–measured vertical velocities and those computed by radiosonde data using kinematic and adiabatic methods. From this study, it is observed that the signs of vertical motion estimated by the kinematic method agree well with MST-radar values, although the magnitudes differ, except in a small region where radar vertical velocity changes in sign from negative to positive in the lower troposphere during monsoon months. This upward motion in this season is attributed to horizontal convergence due to change in wind direction that is not observed in radiosonde data when averaged, because of poor height resolution of the radiosonde (500 m or more varying with height) as compared with the radar range resolution (150 m). Profiles of vertical velociti...

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.

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.

On the Detection of Onset and Activity of the Indian Summer Monsoon Using GPS RO Refractivity Profiles

AbstractGlobal positioning system (GPS) radio occultation (RO) data available during 2001–10 have been used to examine the variations in the refractivity during the onset of Indian summer monsoon (ISM) over the east Arabian Sea (5°–15°N, 65°–75°E). An enhancement of 5–10 N-units in the refractivity is observed around 4.8 km (~600 hPa) a few days (9.23 ± 3.6 days) before onset of the monsoon over Kerala, India. This is attributed to moisture buildup over the Arabian Sea during the monsoon onset phase. A sudden increase (1.5–2 K) in mean upper-tropospheric temperature at the time of onset and during the active phase of the monsoon is attributed to convective activity and the release of latent heat. On the day of monsoon onset over Kerala, an appreciable dip in the refractivity is observed that persisted for 1–3 days followed by an enhancement in refractivity with the active phase of the monsoon. An arbitrary value of 128 N-units difference between 4.8 km (~600 hPa) and 16 km (~100 hPa) coupled with a dip in...