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K. S. Raja Rao and M. Panduranga Rao

Abstract

The geomagnetic crochets recorded at Alibag, Annamalainagar and Trivandrum by the solar flare of 12 July 1961 are examined with reference to the normal Sq, currents prevailing over these stations at the time of the solar flare. The large change in declination at Alibag and changes in horizontal intensity at Annamalainagar and Trivandrum indicate that the solar flare current system is an intensification of the normal Sq current system. Evidence in favor of this conclusion is sought for in the solar flare effects recorded at Alibag by the flares of 19 November 1949, 19 March 1948, and 14 October 1953. A plot of the amplitudes of the solar flare effects in the horizontal intensity at Alibag, Annamalainagar and Trivandrum during the period 1958–59 shows daytime enhancement of the flare effect at the latter two stations only, indicating a close connection between the crochet current system and the electrojet. It is therefore established that the crochet current system is an enhancement of the normal Sq current system.

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Narayan R. Gokhale and K. M. Rao

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No abstract available.

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Song Yang, K-M. Lau, and M. Sankar-Rao

Abstract

In this work, the interannual variability of the Asian summer monsoon is studied by analyzing outputs of the Atmospheric Model Intercomparison Project integration using the Goddard Laboratory for Atmospheres general circulation model. The main effort is devoted to exploring and understanding precursory signals associated with the interannual variability of the Asian monsoon and deciphering possible physical mechanisms responsible for the signals.

It is found that strong precursory signals of highly anomalous Asian summer monsoon appear over the subtropical Asian region during the previous winter-spring seasons. Prior to a strong summer monsoon, the westerlies over subtropical Asia are weaker than normal, and vice versa. Dynamically consistent changes are found in other fields such as atmospheric temperature, geopotential height, and surface temperature. These precursory signals seem to have a barotropic structure in the troposphere. They exist over a broad region and last for two to three seasons. The robustness of the signals is important for improving seasonal prediction of the Asian summer monsoon.

It is hypothesized that the above-described precursory signals of the Asian summer monsoon are linked to land-surface hydrologic processes, such as changes in snow mass and soil moisture in the Asian continent, as well as anomalous sea surface temperature forcing in the Pacific and Indian Oceans. Evidence in support of the above hypothesis can be found in the variability of a variety of parameters from both model simulation and observations.

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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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K. Shankar Rao, Richard M. Eckman, and Rayford P. Hosker Jr.

Abstract

During the 1984 ASCOT field study in Brush Creek Valley, two perfluorocarbon tracers were released into the nocturnal drainage flow at two different heights. The resulting surface concentrations were sampled at 90 sites, and vertical concentration profiles at 11 sites. These detailed tracer measurements provide a valuable dataset for developing and testing models of pollutant transport and dispersion in valleys.

In this paper, we present the results of Gaussian puff model simulations of the tracer releases in Brush Creek Valley. The model was modified to account for the restricted lateral dispersion in the valley, and for the gross elevation differences between the release site and the receptors. The variable wind fields needed to transport the puffs were obtained by interpolation between wind profiles measured using tethered balloons at five along-valley sites. Direct turbulence measurements were used to estimate diffusion. Subsidence in the valley flow was included for elevated releases.

Two test simulations—covering different nights, tracers, and release heights—were performed. The predicted hourly concentrations were compared with observations at 51 ground-level locations. At most sites, the predicted and observed concentrations agree within a factor of 2 to 6. For the elevated release simulation, the observed mean concentration is 40 pL/L, the predicted mean is 21 pL/L, the correlation coefficient between the observed and predicted concentrations is 0.24, and the index of agreement is 0.46. For the surface release simulation, the observed mean is 85 pL/L, and the predicted mean is 73 pL/L. The correlation coefficient is 0.23, and the index of agreement is 0.42. The results suggest that this modified puff model can be used as a practical tool for simulating pollutant transport and dispersion in deep valleys.

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V. K. Anandan, M. Shravan Kumar, and I. Srinivasa Rao

Abstract

A multifrequency phased-array Doppler sodar system has been installed recently at the National Atmospheric Research Laboratory (NARL) for the continuous observation of the lower atmosphere from near ground to the atmospheric boundary layer (ABL). The NARL sodar, developed in technical collaboration with the Society for Applied Microwave Electronics Engineering and Research (SAMEER), was built using piezoceramic tweeters, which are capable of generating 100-W acoustic power. In favorable atmospheric conditions, the sodar gives wind profiles up to 1 km. The performance evaluation is one of the most important aspects for quality assurance of sodar operations. This paper presents the first results of experimental observations of the NARL sodar system and its scientific validation. The NARL sodar has been validated using the simultaneous observation of another sodar system (Scintec model MFAS64). Various physical parameters of the atmosphere are derived using the results obtained from both of the systems. Comparison of simultaneous measurements by both of the sodars, located about 100 m apart, shows good agreement on wind speed, wind direction, and vertical wind variance. The correlation coefficient of more than 0.80 in wind speed and direction between the sodars shows the usefulness of the system for observing the atmosphere and deriving physical parameters below the ABL.

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I. Srinivasa Rao, V. K. Anandan, and M. Shravan Kumar

Abstract

Doppler sodar is being used for studying the lower part of atmospheric boundary layer (ABL) and wind profiling. To obtain maximum altitude coverage multifrequency transmission has been used along with more transmitted power. In this article, the implementation of multifrequency transmission of a Doppler sodar and its decoding to extract the atmospheric parameters are presented. This article also shows the advantage of profiling using multifrequency sodar operation. The range of frequency used for transmission is between 1700 and 2100 Hz. The decoded Doppler spectra have shown significant improvement in signal-to-noise ratio (SNR) as well as higher altitude coverage compared to single-frequency transmission and reception. Wind profiles obtained from sodar have been compared with data obtained from high-resolution GPS sonde balloons, which were launched from a place close to the sodar system. The authors observed that 30% more wind data height coverage in when transmission is in multifrequency mode; the consistency in wind estimate is also improved compared to the single-frequency transmission.

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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. Saikranthi, T. Narayana Rao, M. Rajeevan, and S. Vijaya Bhaskara Rao

Abstract

Daily 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, rain frequency, and rain type. Although the delineation of coherent zones is done using interannual/seasonal rainfall data, these zones exhibit coherency in rainfall variations at intraseasonal scale. Nevertheless, the degree of homogeneity is different for rainfall variations occurring at different temporal scales. Further, the zones show better coherency in excess rainfall years than in deficit rainfall years. Longer-term utility of the delineated zones is studied by examining delineated zones and their coherency in the first and second half of the total data period. Although the regions remain the same in both the periods, the coherency is reduced in the second half, suggesting that the homogeneity of regions may vary in the future.

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K. N. Uma, K. Kishore Kumar, Siddarth Shankar Das, T. N. Rao, and T. M. Satyanarayana

Abstract

The Indian Mesosphere–Stratosphere–Troposphere (MST) radar observations of vertical distribution of mean vertical velocities w in convective regions during the wet and dry spells of the Indian summer monsoon over a tropical station at Gadanki, India (13.5°N, 79.2°E) are discussed. The composite w profile during the wet spell consistently shows a single peak at ~13 km whereas during the dry spell it shows two peaks, one at 5 km and another at 11–13 km. The characteristics of this altitudinal distribution in w are discussed in terms of background wind and thermal structure during both spells of the monsoon. Background w obtained from NCEP–NCAR reanalysis shows subsidence throughout the depth of the troposphere during the dry spell of the monsoon over Gadanki. Analysis of background wind and thermal structure clearly reveal that wind shear and temperature inversion in the midtroposphere are different in the dry spell compared to that of the wet spell, which may be the possible reason for the observed double-peak w structure during the dry spell of the monsoon. The present analysis for the first time brought out the distinct vertical distribution in w and the background meteorological conditions during the wet and dry spells of the monsoon over Gadanki, which may have implications in understanding the monsoon convective systems during the wet and dry spells of the Indian summer monsoon.

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