Y. Bhavani Kumar

Coordinated MST radar and lidar observations for the study of mesospheric structures over a tropical station

Abstract VHF radar observations at a frequency of 53 MHz at Gadanki (13.5°N,79.2°E), India, during the period from September 1995 to August 1999 are used to study the tropical mesospheric structures. MST radar echoes have shown an enhancement in echo power of about 6– 8 dB above the average noise level. These echoes are intermittent in time and can be observed at an altitude between 70 and 76 km . The seasonal variation of these echoes shows the maximum percentage occurrence during summer, closely followed by equinoxes and minimum during winter. This seasonal variation is also accompanied by shifting of height to lower heights during winter. Coordinated MST radar and Nd:YAG lidar observations were conducted to study the mesospheric structures during March 1998–June 1999. In the Lidar temperature profiles, inversion of about 20– 30 K is observed at the heights of the MST radar-enhanced echoes. Enhanced radar reflectivity is observed on both the preceding and succeeding days of lidar observations of strong temperature inversion. Percentage occurrence of MST radar echoes and temperature inversions show one-to-one correspondence for most of the cases. The effect of temperature inversion on the radar reflectivity is also studied. The presence of temperature inversion and enhanced radar reflectivity are discussed in the light of gravity-wave breaking processes at that height region.

The first lidar observations of the nighttime sodium layer at low latitudes Gadanki (13.5 ◦ N, 79.2 ◦ E), India

We report on the first lidar observations of the nighttime mesospheric sodium layer from Gadanki (13.5°N, 79.2°E) site in India. The lidar measurements of upper atmospheric sodium made on 6 nights between the 10 and 16 January 2005 are presented in this paper. The Gadanki lidar uses a Nd:YAG pumped dye laser, tuned to the sodium D2 line (589.0 nm), as a transmitter. Using the system, sodium number density profiles have been obtained with a vertical resolution of 300 m, a time sampling of 120 s. During the initial six nights of observation, the peak sodium concentration is found at a height of 95 km, and the top side scale height is usually about 2 km. On three occasions, a secondary peak was observed at heights between 87 and 92 km. Measurements at Gadanki site indicate that the mean sodium abundances appear to decrease after sunset and increase before sunrise. The average nocturnal columnar abundances were in the range 2–8.9 × 109 cm2. The nightly mean centroid heights range between 92.9 and 95.2 km and the rms widths vary between 4.3 and 4.9 km. On some nights, wave like structures in the sodium layer were observed with wavelength of about 3 km and downward phase velocities of about 1 km/hr. Four sporadic layers were observed during the initial 54 h of observation. The formation and decay of an intense sporadic sodium layer was observed on the night of 11 January 2005. The layer was found to develop between 93 and 90 km altitude and appear between 0230 and 0430 LT.

Radiative effects of elevated aerosol layer in Central Himalayas

Systematic observations of light detection and ranging (LIDAR) to detect elevated aerosol layer were carried out at Manora Peak (29.4° N, 79.5° E, ∼1960 m a.s.l), Nainital, in the Central Himalayas during January–May 2008. In spite of being a remote, high-altitude site, an elevated aerosol layer is observed quite frequently in the altitude range of 2460–4460 m a.s.l with a width of ∼2 km during the observation period. We compare these profiles with the vertical profiles observed over Gadanki (13.5° N, 79.2° E, ∼370 m a.s.l), a tropical station, where no such elevated aerosol layer was found. Further, there is a steady increase in aerosol optical depth (AOD) from January (winter) to May (summer) from 0.043 to 0.742, respectively, at Manora Peak, indicating aerosol loading in the atmosphere. Our observations show north-westerly winds indicating the convective lifting of aerosols from far-off regions followed by horizontal long-range transport. The presence of strongly absorbing and scattering aerosols in the elevated layer resulted in a relatively large diurnal mean aerosol surface radiative forcing efficiency (forcing per unit optical depth) of about −65 and −63 W m−2 and the corresponding mean reduction in the observed net solar flux at the surface (cooling effect) is as high as −22 and −30 W m−2. The reduction of radiation will heat the lower atmosphere by redistributing the radiation with heating rate of 1.13 and 1.31 K day−1 for April and May 2008, respectively, in the lower atmosphere.

Comparison of mixing layer heights determined using LiDAR, radiosonde, and numerical weather prediction model at a rural site in southern India

ABSTRACT There is no agreed reference method for accurately determination of mixing layer height (MLH) in the existing literature. In part, this is due to different definitions of the atmospheric boundary layer exist, depending on the quantities and the physical processes invoked. In addition, MLH during late afternoon transition period is highly challenging to determine and perform model simulations because of the rapid variations in turbulent kinetic energy. For the first time, MLH has been determined at remote tropical site of Gadanki, India (13.45°N, 79.17°E, 360 masl) using ground-based elastic backscatter LiDAR (EBL). This article focuses on the late afternoon transition period and compares it with MLH obtained from the EBL to concurrent radiosonde (RS) observations [MLH (RS)] and numerical models. Five different techniques have been applied to the EBL backscatter profiles for the determination of MLH. The mean of the five methods agreed to within 15% with the RS-derived MLH under various synoptic conditions at the site. This indicates the potential capability of continuous monitoring of MLH by our EBL system. However, MLH determined by Weather Research and Forecasting model and European Center for Medium-Range Weather Forecasts Re Analaysis (ERA)-interim reanalysis systematically underestimated of the MLH (LiDAR) by about 62% and 48%, respectively. The mean growth rate of diurnal evolution of MLH was found to about 120 and 200 m h−1 during winter and spring seasons, respectively.

Lidar measurements of aerosol at Varanasi (25.28° N, 82.96° E), India during CAIPEEX scientific campaign

A compact dual polarization lidar (DPL) was designed and developed at National Atmospheric Research Laboratory (NARL) for daytime measurements of the boundary layer aerosol distribution and depolarization properties with very high vertical and temporal resolution. The lidar employs a compact flashlamp pumped Q-switched Nd:YAG laser and operates at 532 nm wavelength. The lidar system uses a stable biaxial configuration between transmitter and receiver units. The receiver utilizes a 150 mm Schmidt Cassegranin telescope for collecting laser returns from the atmosphere. The collected backscattered light is separated into co and cross-polarization signals using a polarization beam splitter cube. A set of mini-PMTs have been used for detection of light from atmosphere during daylight period. A two channel transient recorder system with built-in ADC has been employed for recording the detected light. The entire lidar system is housed in a compact cabinet which can be easily transported for field measurements. During 2014, the lidar system was installed at the Banaras Hindu University (BHU) campus, Varanasi (25.28° N, 82.96° E, 82 m AMSL) and operated for a period of three months in to support the cloud aerosol interaction and precipitation enhancement experiment (CAIPEEX) conducted by Indian Institute of tropical meteorology (IITM). During this campaign period, the lidar measurements were carried out in the vertical direction with spatial resolution of 7.5 m and time sampling of 30s. The lidar measurements revealed the occurrence of boundary layer growth during convective periods and also detected the long-range transport dust layers with significant depolarization. In the present paper, we present the lidar measurements obtained during the campaign period and discuss the observation of transport of dust layer over the experimental site with support of back trajectory analysis and satellite data. The Lidar observations were compared with the available satellite observations also presented here.

Measurements of long range transport using two wavelength and polarization lidar over tropical rural site Gadanki (13.45° N, 79.17° E)

This paper describes the measurements carried out on shape and size information of boundary layer aerosol particles using a lidar system developed at NARL, Gadanki. The lidar system profiles the boundary layer at 1064 and 532 nm wavelengths, which are fundamental and second harmonic components of Nd:YAG laser. However, the polarization measurements are conducted at 532 nm only. Using an external dichroic mirror in the laser path, the Nd:YAG laser output is separated into its harmonics. The fundamental harmonic of Nd:YAG laser is steered into atmosphere using a hard coated mirror and the atmospheric returns at 1064 and 532 nm are collected using two independent telescopes. The laser backscatter corresponding to 1064 nm is detected using an Avalanche photodiode; whereas the co and cross polarized signals returns corresponding to 532 nm laser are detected using a set of mini-PMT units. A three channel transient recorder unit is employed for recording the signals utilizing the analog and photon counting electronics. The lidar system is possible to operate in daylight period and can provide information on scattering properties of boundary layer aerosols. In the present study, measurements during long range transport events were performed at NARL, Gadanki during the year 2012. We proposed to present two case studies on long range transport that occurred during the year 2012. We present the results in terms of aerosol backscattering coefficient, depolarization ratio and color ratio with support of back trajectory analysis.This paper describes the measurements carried out on shape and size information of boundary layer aerosol particles using a lidar system developed at NARL, Gadanki. The lidar system profiles the boundary layer at 1064 and 532 nm wavelengths, which are fundamental and second harmonic components of Nd:YAG laser. However, the polarization measurements are conducted at 532 nm only. Using an external dichroic mirror in the laser path, the Nd:YAG laser output is separated into its harmonics. The fundamental harmonic of Nd:YAG laser is steered into atmosphere using a hard coated mirror and the atmospheric returns at 1064 and 532 nm are collected using two independent telescopes. The laser backscatter corresponding to 1064 nm is detected using an Avalanche photodiode; whereas the co and cross polarized signals returns corresponding to 532 nm laser are detected using a set of mini-PMT units. A three channel transient recorder unit is employed for recording the signals utilizing the analog and photon counting electronics. The lidar system is possible to operate in daylight period and can provide information on scattering properties of boundary layer aerosols. In the present study, measurements during long range transport events were performed at NARL, Gadanki during the year 2012. We proposed to present two case studies on long range transport that occurred during the year 2012. We present the results in terms of aerosol backscattering coefficient, depolarization ratio and color ratio with support of back trajectory analysis.

Retrieval of mixed layer height (MLH) from lidar using analytical methods and estimation of MLH growth rates over a tropical site Gadanki

A single channel elastic-backscatter lidar system was developed in-house at National Atmospheric Research Laboratory, an institution under Department of Space and Government of India, for studies on boundary layer dynamics during convective periods. The developed lidar system operates at the second harmonic wavelength of Nd: YAG laser and uses biaxial configuration. The lidar system utilizes a mini PMT for detecting laser returns from the atmosphere and operates in the analogue mode of data acquisition. The analogue recorder operates at 20 MHz sampling and uses a 12 bit A to D converter. The lidar system capable to operate at a maximum vertical resolution 7.5 m and 1-sec time sampling. However, in the present study, the lidar was operated with 30 m vertical resolution and 30-sec time sampling to understand the boundary layer dynamics during convective periods. The lidar measurements conducted between January and March 2014 were used in the present study. The laser backscatters obtained at 532 nm wavelength were corrected for noise and range before application of above mentioned analytical methods. This study presents evaluation of mixed layer height (MLH) from lidar using different analytical methods such as gradient, variance and wavelet techniques and presentation of inter-comparison between methods to achieve suitable method for assessment of MLH. The estimated MLH is then compared with the simultaneous radiosonde observations and empirical model values. We computed the MLH growth rates and observed that a significant enhancement was seen during the transition from winter to pre-monsoon period which could be attributed to increased convective activity over the tropical site. We present the lidar measurements and discuss the MLH retrieval and growth rates over Gadanki using lidar measurements.

Cirrus observations using ground-based LIDAR and Terra MODIS instrument

A climatological study of cirrus occurrence has been carried out using the ground-based lidar observations over Gadanki (13.5°N, 79.2° E) during 1998-2004. The Moderate Resolution Imaging Spectroradiometer (MODIS) measurements on the Terra spacecraft are also used for remote sensing of high clouds cirrus from space during 2001-2004. The interannual study using LIDAR and MODIS shows an enhancement in the cirrus occurrence during 2001 and fewer amount during 2002. The interseasonal variation of cirrus occurrence frequencies shows much of the occurrences during the monsoon season. Further lidar observations shows that the cirrus cloud tops typically extended to near the 16.53 km, the average tropical tropopause height. The distribution of maximum cloud base height frequency is confined to 10-12km. Frequency of occurrence of cloud physical thickness with respect to cloud base height (Zb) gives a higher occurrence between 11- 15 km and typically the thickness of 2-4 km. At the cloud base height Zb>15km, which is in the vicinity of tropopause, the cirrus is found to have lesser thickness. A significant observation from this statistical study over this latitude shows appearance of cirrus at two different altitudes because of different formation mechanisms. We will also discuss the formation mechanisms for the occurrence of tropical cirrus at this latitude.