National Atmospheric Research Laboratory Research Articles

Radon metrology at NARL, Gadanki

It is unavoidable to be exposed to radon and its progeny, and numerous researchers in India have conducted substantial research to understand any potential health implications. It's interesting to observe that the time scale of several atmospheric processes and the half-life of radon, which is 3.82 days, are comparable. As a result, the Radon measurements can be used to understand various research problems in the area of atmospheric physics. The assessment of three years' data of radon activity concentration, as well as its daughter, ambient gamma dose, and climate data, was carried out at the National Atmospheric Research Laboratory, Gadanki, India (13.46° N, 79.17° E). It is possible to observe the breakdown of periodicity, a strong daily and seasonal trend, and the relationship between the measured and dependent variables while looking at a time series in general. Radon activity was shown to exhibit a clearly defined diurnal pattern, and FFT analysis revealed the Radon time series' latent memory of sub-diurnal fluctuations. Radon and its progeny were found to have mean levels of 12.2 ± 2.1 Bq/m3 and 4.2 ± 2.3 mWL, respectively. At NARL, the ambient gamma radiation dose rate was 186 ± 9.3 nSv/hour, which is within UNSCEAR's allowed limits.

Airglow-imager based observation of possible influences of subtropical mesospheric gravity waves on F-region ionosphere over Jammu & Kashmir, India

As a joint research collaboration between the National Atmospheric Research Laboratory (NARL), and the University of Kashmir (KU), NARL installed an all-sky airglow CCD imager (with centre wavelengths of 630 nm, 557.7 nm [2 nm band widths] and 840 nm [150 nm wide band with blocking notch at 866 nm to avoid the contamination of molecular oxygen emissions]) in the University campus in Srinagar (75°E, 34°N, geographic), Jammu and Kashmir, India (western Himalayan region). To understand the upper atmospheric dynamics and ionospheric electrodynamics and their associated physical coupling mechanisms, the imager observes airglow emissions of OH molecules (~ 85 km height; 840 nm) and atomic oxygen occurring at the heights of ~ 97 km (557.7 nm) and ~ 250 km (630 nm). Airglow observations in Kashmir commenced in the night of August 11, 2017 and the present work reports on the characteristics of first-time observation of Medium Scale Travelling Ionospheric Disturbances (MSTIDs with horizontal wavelengths of ~ 100–300 km) over Kashmir region during 20:30—22:30 IST (Indian standard time) on August 15, 2017 (India independence day). Initially, the phase front of MSTIDs was aligned along the north-west and south-east direction and moved at ~ 57 m/s towards the south-west direction and finally the westward direction by aligning along the meridian before they disappeared. Along with SAMI-3 ionospheric model simulations, simultaneous multiwavelength airglow observations indicate that secondary gravity waves generated due to dissipation of upward propagating mesospheric gravity waves in the heights of ~ 85–95 km would have contributed to the generation of MSTIDs in the F region ionospheric plasma through electrodynamical coupling between the E and F region (Perkins instability) ionosphere.

One year of 222Rn concentration at a typical rural site in South India

The simultaneous measurements of atmospheric radon, ambient gamma radiations dose, and relevant meteorological parameters were carried out at the National Atmospheric Research Laboratory (NARL), Gadanki, India (13.459° N, 79.175° E) during June 2013–May 2014 are analyzed and presented. The results show that radon strongly correlates with temperature, relative humidity, and a weak correlation with air pressure, ambient gamma dose during fair weather days. Radon's well-defined monthly variability is observed, with the highest during winter and lowest during monsoon season. The fast Fourier transform analysis revealed a hidden memory in variations in radon activity with prominent peaks at 24 h and 12 h, indicating the influence of atmospheric stability on the abundance of radon in air. About 99% of radon activity lies below 70 Bq/m3 with a mean value of 11.81 ± 4.83 Bq/m3, and about 99% ambient gamma dose levels range from 140 to 240 nSv/h at NARL with a mean value of 192.17 ± 17.43 nSv/h. The ambient gamma dose levels are well within limits prescribed by the UNSCEAR.

Study of ambient gamma dose levels at national atmospheric research laboratory, Gadanki, India

Simultaneous observations of ambient gamma dose levels, temperature, relative humidity, and atmospheric pressure were carried out at National Atmospheric Research Laboratory (NARL), Gadanki, India (13.459°N and 79.175°E), from November 2011 to May 2014. The results show that about 92% of ambient gamma dose values lie between 150 and 200 nSv/h and best possible fit resulted in Gaussian fit with adjusted R2 of 0.99. A weak Pearson's correlation coefficient was found between ambient gamma dose levels and selected meteorological parameters measured over the location. No seasonal trend was observed in ambient gamma dose levels, but pronounced seasonal variations in temperature, relative humidity, and air pressure were found. A sudden increase in ambient gamma levels was observed during precipitation event (Nilam cyclone) and may be attributed to an additional contribution of precipitation washed 222Rn progeny aerosols within the atmosphere. The mean ambient gamma dose over NARL was 186 ± 4.3 nSv/h and is within world average given by UNSCEAR.

Dictionary learning technique and penalized maximum likelihood for extending measurement range of a Rayleigh lidar

Extending the temperature measurement range and sustaining the system performance are important requirements in Rayleigh lidars. While replacing hardware and increasing the power aperture product are ways of achieving these objectives, the data analysis approach is another means for achieving the same. A method to retrieve atmospheric temperatures using the penalized maximum likelihood method after denoising the backscattered signal using the dictionary learning technique is presented and compared with the conventional method. The proposed combination has the advantage of improving the measurement range and reducing the standard error (SE) in temperatures. The penalized maximum likelihood function is solved using the method of successive approximations, and the SE in temperature is calculated using Monte Carlo simulations. Observations from the Rayleigh lidar at the National Atmospheric Research Laboratory, India, are used for testing the approach. When compared with the conventional method, the SE in temperatures improved by 5K at 84 km, and the average height improvement was about 6 km.

Lidar Observed Optical Properties of Tropical Cirrus Clouds Over Gadanki Region

The presence of cirrus cloud has its impact on the earth’s radiation budget. In order to study the effect of cirrus clouds in the tropical regions, it is essential to understand and characterize their optical properties. The optical properties of high altitude cirrus clouds are obtained using the polarization diversity ground based Mie lidar instrument at a tropical latitude station in the Indian subcontinent. Lidar measurements are taken for one year (2013) at National Atmospheric Research Laboratory (NARL), located at Gadanki (13.5° North, 79.2° East; 375m AMSL), India and are used for the present investigation. Altitude variations of optical depth and depolarization ratio are discussed. In the altitude range of 10–17 km, the range of the optical depth and depolarization ratio of cirrus cloud was found to be 0.01–0.4 and 0.1–0.4 respectively. The interdependence of optical depth as a function of depolarization ratio is analyzed and a positive correlation is observed (0.3950). From the measured optical depth values, it is categorized that 8%, 77% and 14% of the cirrus clouds are sub-visual, thin and thick clouds. The monthly and seasonal variations of optical properties of cirrus clouds were analyzed. Summary of cirrus cloud layer statistics and the statistical variation (seasonal) of the optical properties of cirrus clouds is presented for the period of study.

Assessment of INSAT-3D/INSAT-3DR Derived Atmospheric Motion Vectors Using Wind Profiler Observations Located at Gadanki, India

The accurate estimation of atmospheric motion vectors (AMVs) derived using sequences of images from geostationary satellites is very important because of their extensive use in improving initial conditions of the numerical weather prediction (NWP) models. To assess the quality of retrieved AMVs, validation of these winds with ground-based observation is very important. In the present study, the evaluation of AMVs retrieved using the data from geostationary satellites, viz. INSAT-3D/INSAT-3DR, has been carried out with respect to wind profiler data and radiosonde data. The wind profiler used in the present study is an L-band radar lower atmosphere wind profiler and is installed at the National Atmospheric Research Laboratory, Gadanki, located at 13.27° N, 79.10° E. This instrument measures continuous high-resolution wind measurements in the lower atmosphere. The radiosonde data available near Gadanki are used for validation. In this study, normal infrared (IR) AMV derived using 10.5–11.5 μm channel of INSAT-3D and INSAT-3DR satellites and staggering-mode IR AMV retrieved using INSAT-3D/INSAT-3DR combination are used. AMV accuracy assessment was performed for the years 2017 and 2018 by calculating statistical scores, viz. root mean square vector difference, wind speed bias and a number of matching points. In addition, the INSAT-3D/INSAT-3DR AMV and profiler wind data are also used to study the characteristics of low-level jet during active and break period of the Indian summer monsoon of 2017 and 2018. The accuracy assessment through this study shows the usefulness of AMVs for improving the initial conditions in NWP models in future.

Comprehensive Study on Tropical (10°N-15°N) Mesospheric Inversion Layers Using Lidar and Satellite (Timed-Saber) Observations

One of the interesting and poorly understood features of mesosphere and lower thermosphere (MLT) region is the phenomenon of Mesospheric Inversion Layers (MILs). The poor understanding of MILs is due to limited access of their occurrence height region, however the lidars are more efficient tools which provide stratosphere and mesosphere nocturnal temperatures with high temporal and vertical resolutions. The state-of-the-art lidar system comprising Mie, Rayleigh lidars installed at National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E), India has provided an excellent opportunity to undertake this study. The Nd:YAG laser source with lower power (11W) has been replaced by the one with higher power (30W) in January 2007. As the laser power has been increased, the molecular back scatter signal is also increased and consequently the top height level of the temperature retrieval has been increased to ~90-95 km. In the present study, the role of dominant causative mechanisms for the occurrence of MILs has been discussed using mainly the lidar and satellite (TIMED-SABER) observations over Gadanki region.

Spectral Analysis of MST Radar Signal using Maximum Likelihood Estimation Algorithm

In this work, we propose Maximum likelihood estimation of low- rank Toeplitz covariance matrix (MELT) with reduced complexity algorithm for computing the power spectral density of mesosphere-stratosphere-troposphere (MST) radar data. MELT is designed based on the method of majorization-minimization and it is an iterative algorithm to update the powers in each successive step. We tested MELT algorithm for complex signal, which contain multiple frequency components in existence of different noise conditions. For simulated complex data, it can be seen that MELT works much better for low Signal to Noise Ratio (SNR) conditions and also effectively detects the frequency components with a fine resolution in the existence with high noise impact. At last, MELT algorithm is applied to the radar data received from MST radar established at National Atmospheric Research laboratory (NARL), Gadhanki. MELT algorithm estimates the accurate Doppler spectra and thus in turn, estimate the wind parameters using Doppler profiles. For the purpose of validation, the obtained radar results through MELT are compared with the Global Positioning System (GPS) radiosonde.

Development of modular and robust high power solid-state transmit–receive module for Gadanki Ionospheric Radar Interferometer

The study of ionospheric effects1 on the satellite-based communication signals has been a continued topic of interest both for academic interest and global positioning system aided navigation applications. These communication signals are affected by the field-aligned irregularities, suffers from fading and range spreading when they pass through the ionosphere. 2 To study the low latitude ionospheric plasma irregularities, dedicated radar with wide beam steering capabilities is required. Recently Gadanki Ionospheric Radar Interferometer, which operates at 30 MHz frequency, has been realized at National Atmospheric Research Laboratory, Gadanki, to scan a larger part of the sky up to ±50° in East-West direction. The radar system employs a 160 element phased antenna array, antenna system, state-of-the-art 8 kW high power solid-state transmit–receive modules, and direct digital receiver. In order to carryout round-the-clock scientific observations, highly reliable, robust designs are required to realize the high power transmit–receive modules. Detailed design philosophies are described with modular concept and better thermal designs in this paper to achieve the high power requirements. Performance results of the transmit–receive module and the sample scientific results obtained by employing these transmit–receive modules are presented in this paper.

Automated Algorithm for Accurate Doppler Profile Detection of MST Radar Signals Considering Removal of Interference

Identifying the accurate radar echo returns from the MST (Mesosphere-Stratosphere–Troposphere) region poses serious challenges, as the radar echo signals are of very low amplitude and hidden in noise. The process of extraction of the relevant signal information becomes intricate when the spectra sometimes are infected with interference signals of non-atmospheric origin. An automated algorithm is suggested for eliminating the interference bands in the Doppler spectra of MST radar signals. The process of removal of interference and de-noising of the radar echoes using soft thresholding techniques was achieved simultaneously. The algorithm was practically applied to interference contaminated MST radar data taken from National Atmospheric Research Laboratory (NARL), Gadanki, Tirupati and studied for removal of interference bands. It was found that the projected algorithm effectively removes multiple interference bands and also improves signal detectability. The power and Doppler values (moments) are estimated using FFT and HHT (Hilbert Huang Transform) and median Doppler is plotted on Doppler power spectrum and compared.

Analysis and Evaluation of Data-Adaptive Spectral Estimation Algorithms for Processing MST Radar Data

The National Atmospheric Research Laboratory (NARL) situated at Gadanki, Andhra Pradesh, constitutes the primary source for Indian MST radar data (Mesosphere-Stratosphere-Troposphere). The primary purpose of the MST radar is to analyze and explore the atmospheric dynamics. The MST radar is developed with an array antenna in the active phase and consists of 1024 Yagi-Uda antennas being operated at a frequency of 53 MHz. The NARL provides atmospheric wind data by using MST radar. The wind speed parameters are calculated from the radar echo signals by using spectral estimation. In this paper, the analysis and evaluation of data-adaptive spectral estimation algorithms namely nonparametric iterative adaptive approach (IAA) and semiparametric methods—sparse learning via iterative minimization (SLIM) and sparse iterative covariance-based estimation (SPICE)—have been presented. These algorithms exhibit high resolution and low sidelobe levels. The detectability of the radar signals is improved at low signal-to-noise (SNR) conditions. The zonal speed (U), meridional speed (V) and wind speed (W) are computed and are validated using the Global Positioning System (GPS) radiosonde data. From the results, the SPICE algorithm outperforms the other two data-adaptive algorithms and the existing periodogram method.

The relation between climate change in the Mediterranean region and global warming

The recent (twentieth century) and future (twenty-first century) climate evolution in the Mediterranean region is analyzed in relation to annual mean global surface temperature change. The CMIP5 (Coupled Model Intercomparison Project, Phase 5) simulations, the CRU (Climate Research Unit) observational gridded dataset, and two twentieth century reanalyzes (ECMWF, European Center for Medium range Weather Forecasts) and NOAA ESRL (National Oceanic and Atmospheric Administration-Earth System Research Laboratory) are used. These datasets to large extent agree that in the twentieth century: (a) Mediterranean regional and global temperatures have warmed at a similar rate until the 1980s and (b) decadal variability determines a large uncertainty that prevents to identify long-term links between precipitation in the Mediterranean region and global temperature. However, in the twenty-first century, as mean global temperature increases, in the Mediterranean region, precipitation will decrease at a rate around − 20 mm/K or − 4%/K and temperature will warm 20% more than the global average. Warming will be particularly large in summer (approximately 50% larger than global warming) and for the land areas located north of the basin (locally up to 100% larger than global warming). Reduction of precipitation will affect all seasons in the central and southern Mediterranean areas, with maximum reduction for winter precipitation (− 7 mm/K or − 7%/K for the southern Mediterranean region). For areas along the northern border of the Mediterranean region, reduction will be largest in summer (− 7 mm/K or − 9%/K for the whole northern Mediterranean region), while they will not experience a significant reduction of precipitation in winter.

Multiangle lidar observations of the Atmosphere

Atmospheric Lidars are used extensively to get aerosol parameters like backscatter coefficient, backscatter ratio etc. National Atmospheric Research Laboratory, Gadanki (13°N, 79°E), India has a powerful lidar which has alt-azimuth capability. Inversion method is applied to data from observations of lidar system at different azimuth and elevation angles. Data Analysis is described and Observations in 2D and 3D format are discussed. Presence of Cloud and the variation of backscatter parameters are seen in an interesting manner.

Fast Fourier Transform power spectrum of radon activity

Measurement of outdoor Radon‑222 ( 222Rn) activity at National Atmospheric Research Laboratory (NARL), Gadanki, India, is carried out using Alpha GUARD PQ 2000 PRO from October 2011 to July 2014 and analyzed. The general inspection of time series reveals the decomposition of periodicity, significant diurnal and seasonal trend, and correlation between the measured and dependent variables. Detailed analyses have revealed that the Fast Fourier transform power spectrum of 222Rn has several energy peaks at different time scales that indicate the daily evolution of radon concentration. The composite seasonal plot shows that at least 21% of the average monthly radon concentration can be attributable to nonlocal effects. In a case study, it is observed that the time evolution of precipitation is one of the important governing factors of radon concentration near the surface. The activity of radon varies from 2 to 65 Bq/m3 with a mean of 12.35 Bq/m3 and ambient gamma dose levels varied between 141.8 and 244 nSv/h with a mean of 188.8 nSv/h.

Rayleigh lidar observation of tropical mesospheric inversion layer: a comparison between dynamics and chemistry

The Rayleigh lidar at National Atmospheric Research Laboratory, Gadanki (13.5°N, 79.2°E), India operates at 532 nm green laser with ~600 mJ/pulse since 2007. The vertical temperature profiles are derived above ~30 km by assuming the atmosphere is in hydrostatic equilibrium and obeys ideal gas law. A large mesospheric inversion layer (MIL) is observed at ~77.4-84.6 km on the night of 22 March 2007 over Gadanki. Although dynamics and chemistry play vital role, both the mechanisms are compared for the occurrence of the MIL in the present study.

Study of Atmospheric Instabilities through Radioactivity

Radon and its progeny concentration are measured at 1m height from surface of Earth in the premises of National Atmospheric Research Laboratory, Gadanki to observe the changes in activity concentration of radon particularly during instabilities that are occurring in the atmosphere. The measurements were carried out using AlphaGUARD and Alpha Progeny Meter for the measurement of radon and its progenies, respectively. It has been observed that, the changes in daily and weekly atmospheric radon levels are related to the stability or turbulence of the lower troposphere. The analysis reveals that from sunny windless days indicates growth and dissolution of the inversion layer. The study of radon concentrations during several atmospheric instabilities including period during Nilam cyclone, has shown interesting features, which are correlated with the conditions of stability or turbulence in the atmosphere.

Activity of radon (222Rn) in the lower atmospheric surface layer of a typical rural site in south India

Analysis of one year measurements of in situ radon (222Rn) and its progenies along with surface air temperature, relative humidity and pressure near to the Earth’s surface has been carried out for the first time at the National Atmospheric Research Laboratory (NARL, 13.5∘N and 79.2∘E) located in a rural site in Gadanki, south India. The dataset was analysed to understand the behaviour of radon in relation to the surface air temperature and relative humidity at a rural site. It was observed that over a period of the 24 hours in a day, the activity of radon and its progenies reaches a peak in the morning hours followed by a remarkable decrease in the afternoon hours. Relatively, a higher concentration of radon was observed at NARL during fair weather days, and this can be attributed to the presence of rocky hills and dense vegetation surrounding the site. The high negative correlation between surface air temperature and activity of radon (R = – 0.70, on an annual scale) suggests that dynamical removal of radon due to increased vertical mixing is one of the most important controlling processes of the radon accumulation in the atmospheric surface layer. The annual averaged activity of radon was found to be 12.01±0.66 Bq m−3 and 4.25±0.18 Bq m−3 for its progenies, in the study period.

Variation of radon progeny concentration over a continental location

Background: The variation of the radon progeny concentration in outdoor environment and meteorological parameters at fine resolution were studied for one year at a continental location, National Atmospheric Research Laboratory, Gadanki, India.Materials and Methods: The concentrations were measured using Alpha Progeny Meter by collecting air samples at a height of 1 m above the Earth’s surface at a known flow rate. Results: Radon progeny concentration shows temporal variations on diurnal and monthly scales, and is due to mixing in the atmosphere. Peak in the early morning hours and low values during afternoon compared to nighttime are due to differential heat contrast between earth’s surface and its atmosphere. However, the activity during February shows maximum compared to June/July months.The diurnal variation of radon progeny shows positive correlation with the relative humidity and negative correlation with ambient temperature.The monthly mean activity of radon progeny for the year 2012 was found to be 4.76 ± 0.73 mWL. Conclusion: The mean concentration of radon progeny in the study region is relatively high compared to the other locations in India and may be due to the rocky terrains and trapping of air-masses near the observation site due to its topography.

Atmospheric Instabilities through Radioactivity Study

The radon in the atmosphere is widely used as tracer. A campaign was conducted at National Atmospheric Research Laboratory, Gadanki, Andhra Pradesh to observe the changes in activity concentrations of radon particularly during instabilities that are occurring in the atmosphere. The observations were made at a height of 1m above from the surface of Earth using AlphaGUARD radon monitoring system that has an accuracy of 85%, which measures radon and Alpha Progeny Meter for the measurement of radon progenies. From the studies, it has been observed that the changes in daily and weekly atmospheric radon levels are accounted for some extent to the stability or turbulence of the lower troposphere. The analysis shows that during calm sunny days, the growth and dissolution of the inversion layer is based on the turbulence and is a measure of radon concentration near to the surface. The study of radon concentrations during several atmospheric instabilities are correlated with the conditions of stability or turbulence in the atmosphere.