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M. S. V. Rao

Abstract

A series of 23 successful meteorological rocket experiments were conducted at Thumba Equatorial Rocket Launching Station from July 1964 through July 1966.

An analysis of the data collected shows the following pattern of winds in the equatorial upper atmosphere. In the stratosphere, the wind flow is predominantly easterly. However, westerlies are observed above 25–30 km in April and October-November. In the mesosphere westerlies become more frequent and are noticeable not only in the above months, but also in February through April. The transition to this pattern from the middle latitude wind regime seems to take place gradually in the subtropics.

From a study of the wind shears over Thumba, as well as from radar observations of the spread of chaff, it is noticed that during certain periods a region of high shears and pronounced turbulence manifests itself above 50 km.

Further, from chaff dispersion studies, it has been possible to make a preliminary estimate of the diffusion coefficient in the stratosphere and mesosphere.

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M. S. V. Rao and J. S. Theon

Analysis of satellite-derived oceanic rainfall maps reveals certain distinctive characteristics of global patterns for the years 1973–74. The main ones are: 1) the forking of the Intertropical Convergence Zone in the Pacific, 2) a previously unrecognized rain area in the South Atlantic, 3) the bimodal behavior of rain belts in the Indian Ocean, and 4) the large interannual variability in oceanic rainfall. These interesting features are discussed.

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V. V. M. Jagannadha Rao, A. Narendra Babu, S. Vijaya Bhaskara Rao, and D. Narayana Rao

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 Niño event, however, an anomalous pattern of winds is seen and Hadley circulation is observed to be weakened.

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V. V. M. Jagannadha Rao, D. Narayana Rao, M. Venkat Ratnam, K. Mohan, and S. Vijaya Bhaskar Rao

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 velocities computed using the kinematic method tend to approach the shape of radar vertical velocity profiles as the separation of the radiosonde network decreases. The vertical velocities computed using the adiabatic method are found to be small and are attributed to a small tilt in isentropic surfaces caused by small latitudinal temperature gradients commonly observed in the Tropics; they also may be partly due to neglect of diabatic heating. The bias between radar and radiosonde vertical velocities tends to decrease when the time used for averaging of MST radar data is approximately 6 h (before and after 3-h average).

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V. V. M. Jagannadha Rao, M. Venkat Ratnam, Y. Durga Santhi, M. Roja Raman, M. Rajeevan, and S. Vijaya Bhaskara Rao

Abstract

Global 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 refractivity on the day of monsoon arrival might give an indication of clear transition of atmospheric conditions and the detection of monsoon onset. Further, a good relation is also found between the activity of monsoon and variability in the refractivity.

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M. Roja Raman, M. Venkat Ratnam, M. Rajeevan, V. V. M. Jagannadha Rao, and S. Vijaya Bhaskara Rao

Abstract

The 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.

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S. K. Mishra, V. B. Rao, and M. A. Gan

Abstract

Horizontal structure and evolution of large-scale flow and an embedded synoptic-scale cyclonic vortex over northeast Brazil as separate systems and dynamical interaction between them are studied at 200 hPa. A quasi-stationary cyclonic vortex with its average position at 10°S and 35°W that formed and remained active during 5–10 January 1993 is selected for the investigation. The evolution of large-scale flow in the prevortex period 1–4 January is also explored. An efficient and effective scale separation technique is developed and used to separate the large-scale flow and embedded synoptic-scale vortex.

It is shown that a strong positive shear zone developed in the latitude domain 17.5°–7.5°S, within the South Atlantic trough region before the vortex formation. The shear zone has a characteristic meridional (zonal) scale of 1000 km (3000 km) and satisfies strongly the necessary condition for barotropic instability. It is identified that the development of a strong shear zone is associated with the intensification of a Bolivian anticyclone and associated ridge and their eastward shift, and intensification of the South Atlantic trough, east–west orientation of the Atlantic trough, and the presence of a transient trough over the equatorial Atlantic Ocean.

The average structure of vortex including zonal and meridional characteristic scales is computed from the synoptic bandpass flow. The vortex is identified as a nonlinear wave packet with an average zonal wavelength of 2750 km and it is confined to a latitude belt of about 17.5°. The vortex shows a strong westward tilt with latitude; the convergence zone is located to its southwest and it is a weak cold cored system. Maximum cyclonic vorticity of the vortex is −3.24 × 10−5 s−1, which is comparable to the value for embedding flow.

The momentum transports due to the vortex, large-scale eddy, and the vortex–large-scale eddy interaction are computed. It is found that the vortex and vortex–large-scale eddy westerly momentum transports are southward, down the gradient of embedding zonal flow, and their divergence (convergence) is located over the latitudes of large scale westerlies (easterlies). The sensible heat transports are weak. It is noted that the vortex–large-scale flow interaction leads to the weakening of the shear zone and restoration of the large circulation features to their January 1993 mean configuration, which have undergone significant deviation during the prevortex period. The signature of vortex–large-scale interaction is also seen in the evolution of dynamical parameters q y and n 2 (square of refractive index parameter).

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K. Nisha, Suryachandra A. Rao, V. V. Gopalakrishna, R. R. Rao, M. S. Girishkumar, T. Pankajakshan, M. Ravichandran, S. Rajesh, K. Girish, Z. Johnson, M. Anuradha, S. S. M. Gavaskar, V. Suneel, and S. M. Krishna

Abstract

Repeat XBT transects made at near-fortnightly intervals in the Lakshadweep Sea (southeastern Arabian Sea) and ocean data assimilation products are examined to describe the year-to-year variability in the observed near-surface thermal inversions during the winter seasons of 2002–06. Despite the existence of a large low-salinity water intrusion into the Lakshadweep Sea, there was an unusually lower number of near-surface thermal inversions during the winter 2005/06 compared to the other winters. The possible causative mechanisms are examined. During the summer monsoon of 2005 and the following winter season, unusually heavy rainfall occurred over the southwestern Bay of Bengal and the Lakshadweep Sea compared to other years in the study. Furthermore, during the winter of 2005, both the East India Coastal Current and the Winter Monsoon Current were stronger compared to the other years, transporting larger quantities of low salinity waters from the Bay of Bengal into the Lakshadweep Sea where a relatively cooler near-surface thermal regime persisted owing to prolonged upwelling until November 2005. In addition, the observed local surface wind field was relatively stronger, and the net surface heat gain to the ocean was weaker over the Lakshadweep Sea during the postmonsoon season of 2005. Thus, in winter 2005/06, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the Lakshadweep Sea. This led to a weaker horizontal sea surface temperature (SST) gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters and, hence, a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification.

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G. S. Bhat, S. Gadgil, P. V. Hareesh Kumar, S. R. Kalsi, P. Madhusoodanan, V. S. N. Murty, C. V. K. Prasada Rao, V. Ramesh Babu, L. V. G. Rao, R. R. Rao, M. Ravichandran, K. G. Reddy, P. Sanjeeva Rao, D. Sengupta, D. R. Sikka, J. Swain, and P. N. Vinayachandran

The first observational experiment under the Indian Climate Research Programme, called the Bay of Bengal Monsoon Experiment (BOBMEX), was carried out during July–August 1999. BOBMEX was aimed at measurements of important variables of the atmosphere, ocean, and their interface to gain deeper insight into some of the processes that govern the variability of organized convection over the bay. Simultaneous time series observations were carried out in the northern and southern Bay of Bengal from ships and moored buoys. About 80 scientists from 15 different institutions in India collaborated during BOBMEX to make observations in most-hostile conditions of the raging monsoon. In this paper, the objectives and the design of BOBMEX are described and some initial results presented.

During the BOBMEX field phase there were several active spells of convection over the bay, separated by weak spells. Observation with high-resolution radiosondes, launched for the first time over the northern bay, showed that the magnitudes of the convective available potential energy (CAPE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 2–3 kg−1 following convection, and recovered in a time period of 1–2 days. The surface wind speed was generally higher than 8 m s−1.

The thermohaline structure as well as its time evolution during the BOBMEX field phase were found to be different in the northern bay than in the southern bay. Over both the regions, the SST decreased during rain events and increased in cloud-free conditions. Over the season as a whole, the upper-layer salinity decreased for the north bay and increased for the south bay. The variation in SST during 1999 was found to be of smaller amplitude than in 1998. Further analysis of the surface fluxes and currents is expected to give insight into the nature of coupling.

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T. T. Wilheit, A. T. C. Chang, M. S. V. Rao, E. B. Rodgers, and J. S. Theon

Abstract

A theoretical model for calculating microwave radiative transfer in raining atmospheres is developed. These calculations are compared with microwave brightness temperatures at a wavelength of 1.55 cm measured by the Electrically Scanning Microwave Radiometer (ESMR) on the Nimbus 5 satellite and rain rates derived from WSR-57 meteorological radar measurements. A specially designed ground-based verification experiment was also performed, wherein upward viewing microwave brightness temperature measurements at wavelengths of 1.55 and 0.81 cm were compared with directly measured rain rates. It is shown that over ocean areas, brightness temperature measurements from ESMR may be interpreted in terms of rain rate with about an accuracy of a factor of 2 over the range 1–25 mm h−1 rain rate.

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