Indian Mesosphere-stratosphere-troposphere Radar Research Articles

Middle atmospheric structure, dynamics, and coupling from three decades of Indian MST radar and complimentary observations: An overview

The genesis for Indian Mesosphere-Stratosphere-Troposphere (MST) radar came from the Indian Middle Atmospheric Programme (IMAP) to study the middle atmospheric dynamics and its underlying processes. During the early 1990s VHF radar operating at 53 MHz has been set up at Gadanki (13.5°N, 79.2°E), India, which started operating initially in the stratospheric and tropospheric (ST) mode. It is upgraded to MST mode in 1993. Since then, it has served various scientific experiments not only from India but under international collaboration. Numerous focused campaigns that have national and international importance were conducted. Using this unique data in the last 30 years along with complementary measurements, several stimulating results has been reported covering a wide range of topics that includes gravity wave characteristics, tides, planetary waves, oscillations, tropopause dynamics and stratosphere-troposphere exchange processes, monsoon dynamics and circulations, mesospheric structure and dynamics, vertical and lateral coupling and long-term trends. The present communication will highlight the unique results obtained using long-term observations of Indian MST radar and complementary techniques covering the complete middle atmosphere. Unique results include quantification of the momentum carried by the high-frequency waves generated in the troposphere up to the mesosphere, demarcating the complete tropical tropopause layer using MST radar observations alone, indication for the strengthening of the tropical easterly jet in the recent decade, unusual behavior of the Hadley circulation during drought and El Nino years, the revelation of the mechanisms for the occurrence of mesospheric echoes at the tropical latitudes, quantification of the turbulence, evidence for the role of anthropogenic changes on the dynamics of the mesosphere and generation of the empirical model by combing multi-instrument data sets. These data set also served in validating numerous ground-based and space-borne measurements including several re-analysis products over a tropical latitude. Issues remained unresolved and ways to tackle in future experiments are highlighted.

Volume imaging of aspect sensitivity in VHF radar backscatters: first results inferred from the Advanced Indian MST radar (AIR)

ABSTRACT Very-high frequency (VHF) radar echoes are known to be aspect sensitive in general, which could be either the manifestation of a thin stable layer providing sharp refractive index gradient or shear-driven steep layer structures. The present study for the first time shows the volume imaging of aspect sensitivity in the recently upgraded Advanced Indian Mesosphere Stratosphere Troposphere (MST) radar (AIR). The present configuration of radar can provide a better understanding of three-dimensional structures of turbulence and instabilities, which was not possible when the radar was operated in two orthogonal planes. Both isotropic and anisotropic layers/turbulence are observed at various height levels over the radar volume. Enhanced aspect sensitivity is observed between 6 and 9 km, which is attributed to the presence of dynamic instability arising due to strong wind shear and atmospheric stability. All the scatterers tend to become isotropic when both the square of wind shear and stability parameters are above 0.25 × 10−3 s−2. Due to the high aspect sensitivity, there may be an underestimate of 5–10% in wind velocity measurements, and it is higher for the low off-zenith beam angle.

Long‐Term Observations of Stratosphere‐Troposphere Exchange Using MST Radar and Aura MLS Measurements Over a Tropical Station Gadanki

AbstractThis paper presents the climatology of stratospheric intrusion into the troposphere during the Indian Summer Monsoon (ISM) observed using the mesosphere‐stratosphere‐troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E). The most significant and new observation is the presence of enhanced signal‐to‐noise ratio (SNR) of 3 dB and half‐power full spectral width of >0.2 m s−1 in the vicinity of tropopause during the ISM. Downdrafts (updrafts) are dominated in the vicinity of tropopause from April to June (July to March). The result of stratospheric air intrusion is also supported with 10 years of ozone measurements obtained from Aura‐Microwave Limb Sounder (MLS) and 5 years of in situ ozonesonde observations. Detailed analysis shows that the horizontal advection along with turbulence in the vicinity of tropopause caused by the tropical easterly jet and tropopause altitude variability is the prime candidate for stratosphere to troposphere transport of ozone. In contrast, the tropical tropopause temperature plays a significant role in the troposphere to stratosphere transport of water vapor during the ISM. The significance of the present study is to qualitatively constitute the climatology of these exchange processes over Gadanki using Indian MST radar.

The Advanced Indian MST Radar (AIR): System Description and Sample Observations

AbstractAn upgraded version of the Indian MST radar, called Advanced Indian Mesosphere‐Stratosphere‐Troposphere (MST) radar (AIR), has recently been commissioned. The prime objective of constructing the AIR is to make it a more useful instrument for conducting new experiments, which enable us to broaden our knowledge on the dynamical and electrodynamical processes including coupling from the boundary layer to the ionosphere. The AIR works at 53 MHz, and the system has been rebuilt, keeping the original square antenna array consisting of 1,024 Yagi antennae. The AIR is a high‐power, fully active phased array system with dedicated 1‐kW solid‐state transmit‐receive units, providing a total peak power of 1,024 kW, and control systems that allow steering the beam with 360° azimuth and 24° zenith agility. Also, the antenna system has been configured with modularity and scalability, which facilitates selection of antenna subarray and aperture for carrying out multireceiver experiments. The antenna radiation pattern tests reveal that the beam width and first side lobe level are close to the designed values of 2.2° and −13.2 dB, respectively. Currently, the system is equipped with 18 receivers to carry out spaced receiver experiments. The transmitters have been designed to transmit very short and long pulses for studying boundary layer to ionosphere. This paper presents the system description of the AIR and sample observations made using the system in Doppler beam swinging mode demonstrating its capabilities for studying neutral atmosphere and ionosphere.

Anomalous Behavior of Vertical Wavenumber Spectra Over a Tropical Station of India

AbstractThe vertical wavenumber (VWN) spectra of height profiles of winds over a tropical station of Gadanki (13.5°N, 79.2°E; obtained using Indian Mesosphere Stratosphere Troposphere radar) show distinct nature of VWN spectra, which do not match with the so‐called universal wavenumber spectra. The universal wavenumber spectra have three distinct regions: unsaturated (having log‐log slope of 2), saturated (having log‐log slope of −3), and turbulence region (having log‐log slope of −5/3). In present work, we report the presence of positive slope at the higher end of the VWN spectra indicating the presence of unsaturated gravity waves, a kind of which is not reported so far. Many negative slopes of the order of ~−6, ~−9, and ~−12 are noticed at the end of the spectra beyond the turbulence region of the wavenumber spectra, which is also not reported in any previous studies up to now.

Sparse iterative covariance-based estimation approach for processing atmospheric radar data

The Doppler estimation is an important problem for Mesosphere–Stratosphere–Troposphere (MST) Radar data for detection and estimation of the weather parameters like turbulence intensity, mean radial velocity, humidity, temperature, wind speed. For Doppler estimation, one has to compute the Power Spectral Density (PSD). Various parametric and nonparametric methods have been developed. Recently, a new category of spectrum estimation method called Sparse Iterative Covariance Based Estimation (SPICE) is also developed. SPICE is a robust, user parameter-free, high resolution, iterative and globally convergent estimation algorithm. In this paper, the simple gradient approach is used for minimization of the weighted covariance estimation analyzing the data collected from the Indian MST radar at Gadanki (13.5°N, 79.2°E). The same method is applied for radar data to estimate the power spectrum and Doppler frequency. The zonal (U), meridional (V), wind speed (W) are calculated and the results have been validated using simultaneous Global Positioning System (GPS) Sonde data.

Atmospheric radar signal processing using principal component analysis

Principal Component Analysis (PCA) is a simple non-parametric method for extracting relevant information from high-dimensional data sets. In this paper, we analyze the data collected from the Indian MST (Mesosphere, Stratosphere, Troposphere) radar at Gadanki (13.5°N, 79.2°E) using PCA. We tested the PCA for various simulated signals like narrowband, wideband and exponential signals which may contain more than one frequency both in absence and presence of noise. For the simulated data, it is observed that PCA works for low SNR, i.e. it successfully detects the frequency in the highly noise-corrupted signal also. Finally, we applied PCA to the radar data for estimating the power spectrum and thus in turn estimating the Doppler frequency components. We estimate the zonal (U), meridional (V), wind speed (W) etc. from the Doppler frequencies. Compared with existing algorithms, PCA works well at higher altitudes and results have been validated using the GPS sonde data.

Characteristics of the Tropical Easterly Jet: Long‐term trends and their features during active and break monsoon phases

Using high–vertical resolution Indian mesosphere‐stratosphere‐troposphere (MST) radar and GPS sonde observations from Gadanki (13.5°N, 79.2°E), an attempt has been made for the first time to delineate the characteristics of Indian summer monsoon winds with special emphasis on the Tropical Easterly Jet stream (TEJ), its day‐to‐day, monthly, and interannual variability using observations available for more than a decade (1996–2008). For comparison, data available during 1996–2008 from routine radiosonde observations from nearby India Meteorological Department (IMD) observatory located at Chennai (13.0°N, 80.1°E) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP‐NCAR) reanalysis data sets have been considered. Relatively poor vertical resolution data from IMD lead to underestimation of the jet characteristics like jet width, vertical shear above and below the jet, and the wind reversal height. Although reanalysis data sets represented well the spatial variability, they underestimate the peak zonal winds by about 5 m/s when compared to Indian MST radar and GPS sonde observations. Large day‐to‐day variability of TEJ characteristics (peak jet speed, jet height, jet width, zonal wind reversal height, and vertical shear of zonal wind) over Gadanki is noticed. No significant trend is observed in peak jet speed using reanalysis data sets either over Gadanki grid or over jet core region in Indian summer monsoon months. However, MST radar and GPS sonde observations show increasing trend (∼1 m/s/yr) in peak jet speed particularly from the year 2000. Characteristics of the TEJ during active and break phases showed quite different features. Thus, statistics generated using the long‐term database from Indian MST radar and GPS sonde provide valuable information for studying the TEJ characteristics and will be useful to incorporate in the future reanalysis data sets.

Multipeak Raindrop Size Distribution Observed by UHF/VHF Wind Profilers during the Passage of a Mesoscale Convective System

Abstract The Indian Mesosphere–Stratosphere–Troposphere radar (IMSTR), Lower Atmospheric Wind Profiler (LAWP), and Joss–Waldvogel (JW) disdrometer measurements during the passage of two distinctly different (in terms of total rain and rainfall rate) convective storms are utilized to understand the nature and origin of the multipeak raindrop size distribution (MRDSD). Important issues, such as the preferential stage and height at which bi- or multimodal rain distribution occurs in a mesoscale convective system (MCS) are addressed. For both of the storms, the MRDSD is observed during the transition period from convection to stratiform rain. The pattern and variation of the MRDSD during this period is strikingly similar in both of the storms. The MRDSD is first observed above the freezing level in the presence of heavy riming. The subsequent spectra have shown bimodal distribution below the freezing level, and the bimodality is attributed to the coexistence of ice and supercooled droplets. Interestingly, the bimodal distribution has not varied much with altitude when it is produced because of the coexistence of ice and supercooled droplets. The MRDSD is also observed at few range gates and for a short duration. Such a type of MRDSD is seen during the transition period between decaying and intensifying rain.

Low‐latitude mesospheric mean winds observed by Gadanki mesosphere‐stratosphere‐troposphere (MST) radar and comparison with rocket, High Resolution Doppler Imager (HRDI), and MF radar measurements and HWM93

Low‐latitude mesospheric winds are investigated using data collected from 1995 to 2006 (11 years) by Indian MST radar located at Gadanki (13.5°N, 79.2°E). Clear eastward and westward flow in zonal wind is noticed during solstices and equinoxes, respectively. The meridional wind shows equatorward flow below 75 km and poleward flow above 75 km quite consistent with that observed with other techniques. The winds show a clear semiannual oscillation (SAO) with maxima during equinoxes. The strength of the SAO during spring is larger than that of fall equinox, and the first peak occurs at higher altitudes than the second peak. The observed features are compared with other techniques, namely, rocket, High Resolution Doppler Imager (HRDI), and medium frequency (MF) radar and also with horizontal wind model HWM93. In general, good comparison is seen among various techniques, with some discrepancy observed in amplitudes. Interestingly, decrease in eastward wind with time during winter months is noticed. The significance of the present results lies in showing the consistency/inconsistency of various experimental techniques to measure the middle atmospheric winds, which are very important to assess the climate variability.

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

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...

Study of mean vertical motions over Gadanki (13.5°N, 79.2°E), a tropical station, using Indian MST radar

Long‐term mean vertical velocities observed by the Mesosphere‐Stratosphere‐Troposphere (MST) radar over a tropical station, Gadanki, India (13.5°N, 79.2°E), are presented for the first time in this paper. Profiles of mean vertical velocities show wave like structure with a vertical wavelength of ∼6 km, and the observed values range from 3 to 20 cm s−1 on average. During monsoon and postmonsoon seasons, larger magnitudes are observed up to 20 cm s−1. From the present study, an interesting feature, reversal in vertical velocities from downward to upward between 5 and 10 km during the monsoon season, is observed. This upward motion in long term averaged vertical velocity is thought to be due to reversal in wind direction around this height range and also due to horizontal velocity convergence, which is frequently observed over this region. One more reversal is observed near the Tropical Easterly Jet (TEJ) (around 16 km), which is from downward to upward and always showing minimum vertical velocities close to zero. This reversal can be attributed to instabilities associated with the jet streams. Large negative values are observed in the lower troposphere below 6 km, which are attributed to large vertical wind variances observed in this region indicating gravity wave activity.

Tropical precipitating systems observed with Indian MST radar

Three campaigns are conducted with the Indian mesosphere‐stratosphere‐troposphere (MST) radar, located at Gadanki (13.5°N, 79.2°E), India, to study the precipitating systems in the tropics. This study mainly deals with (1) classification of precipitating clouds and the spectral characteristics of echoes associated with these cloud systems and (2) characteristics of the radar bright band. The radar gets echoes scattered both from refractive index fluctuations and precipitation particles in moderate to heavy precipitation conditions. These echoes are separated in the spectral domain to determine the vertical air motion and the Doppler velocity of hydrometeors simultaneously. The tropical precipitating systems are classified as stratiform and convective using the reflectivity and vertical velocity distribution. The echo power, spectral width, and vertical velocities of the ambient air and hydrometeors in both the cloud systems have been compared. Aspect sensitivity of the echoes from the hydrometeors and refractive index fluctuations in both stratiform and convective atmosphere is studied. A transition stage, where the stratiform precipitation is associated with the convection, is also reported. Backscattered power from precipitation particles is used to estimate the reflectivity factor (dBZ), and these values along with spectral width and vertical velocity values are used to identify the bright band structure. The reflectivity at the bright band, up to 42 dBZ, is found to be 10–12 dB more than the average value of reflectivity below the bright band. Discussion on the factors contributing to this enhancement is also included. A clear layered structure around the 0°C isotherm in the reflectivity profile of the precipitation echo confirms the presence of the bright band. The thickness of the bright band is estimated and is correlated with the peak reflectivity at the bright band. Comparison of the average terminal velocity of hydrometeors with their average Doppler velocities below the bright band shows the presence of gentle updrafts of a few cm s−1 in stratiform precipitation. These studies are made for the first time with the Indian MST radar and also demonstrate the capability of a VHF radar in studying precipitating systems in addition to the turbulence to which these radars are highly sensitive.

Signal and noise level estimation for narrow spectral width returns observed by the Indian MST radar

Use is made of five sets of multibeam observations of the lower atmosphere made by the Indian mesosphere‐stratosphere‐troposphere (MST) radar. Two aspects of signal processing which can lead to serious underestimates of the signal‐to‐noise ratio are considered. First, a comparison is made of the effects of different data weighting windows applied to the inphase and quadrature components of the radar return samples prior to Fourier transformation. The relatively high degree of spectral leakage associated with the rectangular and Hamming windows can give rise to overestimates of the noise levels by up to 28 dB for the strongest signals. Use of the Hanning window is found to be the most appropriate for these particular data. Second, a technique for removing systematic dc biases from the data in the time domain is compared with the more well‐known practice of correction in the frequency domain. The latter technique, which is often used to remove the effects of ground clutter, is shown to be particularly inappropriate for the characteristically narrow spectral width signals observed by the Indian MST radar. For cases of near‐zero Doppler shift it can remove up to 30 dB of signal information. The consequences of noise and signal level discrepancies for studies of refractivity structures are discussed. It is shown that neither problem has a significant effect on Doppler shift or spectral width estimates.

Stratosphere‐troposphere ozone exchange observed with the Indian MST radar and a simultaneous balloon‐borne ozonesonde

The stratosphere‐troposphere (ST) mode operation of the Indian mesosphere‐stratosphere‐troposphere (MST) radar provided evidence of multiple stable layer structures near the tropopause and also of its “weakening” during late night hours on several occasions. Mass exchange can take place between the troposphere and the stratosphere during periods of such weakening. To examine whether there is any transport of ozone from the stratosphere to the troposphere at the time of tropopause weakening at tropical latitudes, simultaneous observations were carried out using the Indian MST radar located at Gadanki (13.5°N, 79.2°E), now fully operational in MST mode, and ozonesonde flights from Trivandrum (8.9°N, 76.6°E). Four campaigns of simultaneous observations were conducted during January 3–8, November 9–16, and December 5–10, 1994, and June 7–24, 1995. The results show occurrence of the tropopause weakening several times during January 3–8, November 9–16, and December 5–10, 1994. Simultaneous observations of ozone profiles, particularly on January 7–8, November 10 and 16, and December 7–8, 1994, the days of tropopause weakening, show evidence of a decrease in stratospheric ozone and a corresponding increase in tropospheric ozone after this event, the total ozone remaining essentially constant on these days with respect to other days. Tropopause weakening on January 4–5, 1994, however, does not show any considerable change in the ozone profiles. The results are presented here relating the degree of tropopause weakening to the extent of vertical mass exchange between the troposphere and the stratosphere. An effort has been made to examine the role of horizontal advection of ozone in increasing tropospheric ozone at the region of interest; the geographical distribution of tropospheric ozone residual (TOR) has been analyzed by the two data sets obtained independently from Meteor 3 total ozone mapping spectrometers (Meteor 3 TOMS) and the Upper Air Research Satellite (UARS). Simultaneously, the possibility of horizontal transport of ozone from the areas surrounding Tirupati and Trivandrum in increasing tropospheric ozone due to synoptic scale circulation has also been verified by close inspection of synoptic weather charts at 1000, 700, 500, and 300 hPa obtained from the National Center for Medium‐Range Weather Forecasting general circulation model at 0000 and 1200 UTC on January 7 and 0000 and 1200 UTC on January 8, 1994.

Preliminary observations using ST mode of Indian MST Radar: detection of the signature of the tropopause

An MST (mesosphere/stratosphere/troposphere) radar is currently under installation at Gadanki, a low latitude station in India. As an intermediate step, a low power-aperture version of the system was operated in the ST mode. The paper gives an account on the salient features of the system and presents preliminary observations conducted on the detection of the tropopause. The observations clearly demonstrate the potential of the radar to carry out high resolution studies on the tropopause above this tropical station.