ST Radar Research Articles

Influence of the Western Disturbance on Winter Precipitation: Observations over the Central Himalayas using VHF Radar

This study investigates the influence of the Western Disturbance (WD) on atmospheric wind patterns and associated precipitation over the Himalayan region utilizing observations carried out using the ARIES ST Radar (ASTRAD) at Nainital [29.36° N, 79.46° E, 1800 m] during March 2024. The observations reveal that during the WD events, the wind direction was predominantly south westerly/westerly with varying speeds, reaching up to 67 m/s. The HYSPLIT model back-trajectory analysis confirmed that the moisture-laden air masses originated from the Atlantic Ocean. Vertical wind measurements indicated significant atmospheric instability and convection, characterized by updraft and downdraft motions in three distinct height regions between 8 km and 10 km, an updraft between 6 km and 7 km, and another downdraft between 4.5 km and 5 km. The core height region of the subtropical westerly jet (SWJ) embedded in WD and shift in height with maximum wind speed are also identified.

Features of Atmospheric Dynamics During Tropical Cyclone AMPHAN Observed Using ST Radar and Doppler Weather Radar

Features of Atmospheric Dynamics During Tropical Cyclone AMPHAN Observed Using ST Radar and Doppler Weather Radar

Technical Aspects of 53 MHz VHF ST Radar Pilot Array at University of Calcutta for Lower Atmospheric Probing

Technical Aspects of 53 MHz VHF ST Radar Pilot Array at University of Calcutta for Lower Atmospheric Probing

Unraveling the dynamics of an intense pre-monsoon hailstorm and summer monsoon deep convective system over the central Himalayas using a VHF radar

Himalayan region being influenced by extratropical and monsoon weather systems coupled with orographically forced convection, are frequently impacted by severe thunderstorms. Negligible ground based observations over this region have restricted the analysis of these storms up to coarse statistical features utilising reanalysis data and space-borne radar missions. High spatiotemporal resolution of the Very High Frequency (VHF) radars makes them well suited for the study of large convective storms. In this context, a 206.5 MHz ARIES ST Radar (ASTRAD) facility at Nainital (29.4oN, 79.5°E, 1793 m amsl) has been used to probe mesoscale dynamics of severe deep convective systems in the pre-monsoon and monsoon season for the first time over the Himalayan region. Pre-monsoon event on May 5, 2020 was observed to have characteristic features of a supercell storm resulting in a nearly two hour long hailstorm (∼1430–1630 IST). Vigorous updrafts with maxima of 15.5 ms-1 were observed extending from the mid-troposphere to up to the lower stratosphere region and downdrafts up to 15.7 ms−1 were detected in the lower troposphere within the hailstorm. Hodographs during peak convective activity revealed strongly ‘veered’ winds up to upper troposphere with Storm Relative Helicity maxima of 484 m2s−2. The strength of convection was moderate (updrafts maxima of 4.5 ms−1) in summer monsoon case on September 2, 2020. In this case, trailing stratiform region is observed with double bright band structure in the radar reflectivity factor. Weakening of the stratified stable layer structures in the UTLS region have been observed for both the case in the regions of strong updraft gradients. Spectral and hodograph analysis of the quasi sinusoidal undulations in the wind components fluctuations revealed the presence of upward propagating short-period gravity waves in the Upper Troposphere and the Lower Stratosphere (UTLS) region for both the cases. The observed period of oscillation was 35.6 min and 27 min with vertical wavelengths of ∼8.4 km and ∼ 5.7 km for the May 5 and September 2 case respectively. Momentum flux enhancement of 30–40 times was observed during convection with extremely large values during the hailstorm indicating enhanced flux exchange between troposphere and stratosphere. The findings of this study will aid the forecasting of these storms and improve their simulation in the mesoscale models, particularly over this Himalayan region having complex topography.

First Study of Optical Turbulence Over an Astronomical Site in the Central Himalayas Using ST Radar Observations

Optical turbulence in the atmosphere is a key factor which governs the sharpness of images produced from ground-based optical telescopes. The intensity of optical turbulence can be conveniently assessed by the estimation of the refractive index structure constant ([Formula: see text]). Here, observations from a stratosphere troposphere radar (206.5[Formula: see text]MHz) that is co-located with the optical telescope facilities in the central Himalayas, are used for the estimation of [Formula: see text]. The integrated profile of [Formula: see text] and winds over about 2–20[Formula: see text]km have been used to estimate seasonal and diurnal variation of “seeing”, wavefront coherence time, isoplanatic angle and scintillation rate. The mean [Formula: see text] over the site varies from 10[Formula: see text] to 10[Formula: see text][Formula: see text]m[Formula: see text] with the largest values in the monsoon and post-monsoon season. The best “seeing” conditions are observed in the winter and post-monsoon season with median “seeing” varying from 0.39[Formula: see text] to 0.81[Formula: see text]. The largest contribution to the diurnal variation of “seeing” comes from turbulences at 2–5[Formula: see text]km height region. A weak correlation of diurnal variation of “seeing” has been observed with surface temperature and relative humidity (RH). Among astroclimatic parameters, the largest isoplanatic angle is associated with winter season with median value of 2.57[Formula: see text] and wavefront coherence time of ∼4.2[Formula: see text]ms. The least scintillation in intensity is observed in the winter season with a rate of ∼10% and most in monsoon season with ∼45% rate. This study is expected to have a positive impact on the optimization of the operation of adaptive optical techniques, observing time and scheduling of scientific programmes for optical telescope facilities in this region of the central Himalayas.

Detection of the impact of a tropical cyclonic system on the dynamics and energetics of the atmosphere using wind profiler radar

Rapid changes in the tropospheric circulation features associated with the overhead passage of the Gaja cyclonic system over the 205 MHz Stratosphere Troposphere wind profiler radar observations at Kochi (10.03° N, 76.33° E), India, have been studied. The severe cyclonic system formed in the southeast Indian Peninsular region weakened into a depression after landfall near the Tamil Nadu coast. On 16th November 2018, the cyclonic system crossed the Western Ghats and travelled westward at 33 knots over the ST radar site at Kochi in the evening. Later it reached the Arabian Sea and intensified again into a severe cyclone. Continuous observations of the vertical structure of the wind pattern at 4-min intervals from the wind profiler radar have been examined. The impact of the transit of the cyclonic system extends up to a height of 13 km in the atmosphere. The vertical distribution of turbulent kinetic energy in the atmosphere indicates a sudden disruption in the tropospheric levels at the time of storm passage. The cyclonic system traversed over the Western Ghats positioned at an altitude of 2500 m. It crossed the radar site at the mean sea level after passing a horizontal distance of 100 km. The abrupt changes in the topographical conditions generate atmospheric gravity waves in the leeward side of the Western Ghats, as observed from the ST radar, are presented. During the period, changes in surface parameters were evaluated using co-located automatic weather station (AWS) data. Satellite information and Doppler weather radar observations from Kochi have also supplemented the investigation.

Wind profiling features and atmospheric dynamics: A few first observations from ST Radar of Gauhati University

Wind profiling features and atmospheric dynamics: A few first observations from ST Radar of Gauhati University

Design of Phase Shift Control For Transmit/ Receive Module of ST Radar

The paper mainly focuses on the beam forming and its analysis for planar phased- array antenna, which is being developed as part of the ST Radar. The beam steering is done electronically by controlling the phase of the T/R module in such a way that the phase difference between each module is equal and it corresponds to the steering angle. The relative phase shift of each T/R module for a specified beam position is worked out using MATLAB and implemented using 6- bit digital phase shifter. The analytical results are practically demonstrated using the STM32F4xx kit of STMicroelectronics. Theoretical values of the elemental phase for each antenna element in the array, are calculated for various positions of the beam (θ & φ). The 6-bit phase shifter results in 5.6250 phase resolution. In the second part of the work, the calculation of the element phase with 6-bit digital phase shifter is discussed. The calculated values of the elemental phase are truncated to an integral multiple of 5.6250.Â

Dissipation rates of turbulence kinetic energy in the free atmosphere: MST radar and radiosondes

Our knowledge of the spatio-temporal variability of the dissipation rates of turbulence kinetic energy (TKE) in the free atmosphere is severely limited because of the difficulty and expense of making these measurements globally. A few MST/ST radar facilities that are still in operation around the globe have provided us with valuable data on temporal variability of the dissipation rate in the atmospheric column above the radars but the data covers an extremely tiny fraction of the global free atmosphere. Moreover, there are limitations to these data also, since restrictive hypotheses are necessary for making these measurements. It appears that simple radiosondes launched from the existing global sonde network might be able to provide a much wider coverage, provided the technique for deducing the dissipation rates from overturns detected by the sondes can be calibrated and validated against existing techniques. An intensive field campaign conducted over the Harrow ST radar site located in western Ontario, Canada, during the summer of 2007 provided precisely such an opportunity. In this paper, we report on the comparison of the TKE dissipation rates derived from the PTU measurements made by ozonesondes launched during the campaign with those obtained from direct ST radar measurements. We find encouraging agreement between the two, which suggests that routine measurements of TKE dissipation rates by radiosondes in the global free atmosphere might indeed be feasible.

Mixing processes in a tropopause folding observed by a network of ST radar and lidar

Abstract. A campaign devoted to stratosphere-troposphere exchange mechanisms studies has been held on 4–5 March 1995, during a tropopause folding passage over western Europe. The observational network included 1 UHF and 3 VHF radar, 1 temperature lidar, and 1 ozone lidar, deployed in the south of France. The fold is associated with a strong quasi meridional jet stream running along the west side of an upper level trough forming a potential vorticity (PV) anomaly. During this campaign, the PV anomaly is advected east-wards without major deformations, with an average velocity of about 30 km per hour. Therefore, a frozen field hypothesis has been used in order to compare the results obtained in the different sites of the network. Under this hypothesis, the same structures associated with the anomaly are observed by each radar or lidar, at the same time relative to this anomaly. The fold is put into evidence by the ozone lidar of the network and by the VHF radars when computing the aspect ratio. The turbulent activity observed by the OHP VHF radar and by the high resolution UHF PROUST radar (30 m altitude resolution) is concentrated in the wind shear regions generated by the jet stream above and below its axis. In the lower level wind shear, turbulent layers are detected within the fold and across its anticyclonic boundary, thus allowing turbulent exchanges with the tropospheric air masses. A large area of aspect ratio greater than 3 is observed in the troposphere during more than ten hours after the passage of the fold by the VHF radars. This particular signature is tentatively analyzed as the progressive dilution of air masses of stratospheric origin extruded from the fold by the turbulent processes. In the upper level wind shears, turbulent layers parallel to the wind isotachs are observed, whose thickness is often smaller than 100 m and are separated by non turbulent regions. A good correlation is found between the spatio-temporal evolution of these turbulent structures observed by the PROUST radar, and the stability structures observed at the same relative time by the temperature lidar. In addition the same turbulent structures are observed to be parallel to the isophase lines of the wind fluctuations, observed at the same relative time by the Lannemezan radar. These characteristics are compatible with the presence of unstable inertia-gravity waves, generated by the jet stream while the turbulent layers could be the signature of their saturation processes.Key words. Atmospheric composition and structure (evolution of the atmosphere) – Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence)

Effect of beam broadening on the VHF Doppler mini-radar simple method for correcting wind velocity errors

A Doppler VHF mini-radar has been developed at LSEET (Laboratoire de Sondages de l'Environnement Terrestre) to permit investigations at low altitudes, where classical large ST-VHF profilers are blind in the first kilometers of the atmosphere, and UHF boundary layer radars are disturbed by precipitations, birds and insects echoes. Due to a small size of the antenna array, beam broadening effects are important and can provide errors in the atmospheric parameter estimation (reflectivity and wind velocity). A simple overlapping correction method based on the decomposition of the power spectrum is employed to retrieve wind velocity profiles. Measurements from a high-resolution ST radar are used as a benchmark which allows data comparisons and evaluation of this new method.

Radar imaging and high-resolution array processing applied to a classical VHF-ST profiler

Among the spaced antenna methods used in the field of atmospheric studies, radar interferometry has been of great interest for many authors. A first approach is to use the phase information contained in the cross-spectra between antenna output signals and to retrieve direction of arrival (DOA) of discrete scatterers. The second one introduces a phase shift between the antenna signals in order to steer the main beam of the antenna towards a desired direction. This paper deals with the later technique and presents a variant of postset beam steering (PBS) which does not require a multi-receiver system. Indeed, the data samples are taken alternately on each antenna by means of high-commutation-rate switches inserted before a unique receiver. This low-cost technique is called “sequential PBS” (SPBS) and has been implemented on two classical VHF–ST radars. The present paper shows that high flexibility of SPBS in angular scanning allows to perform radar imaging. Despite a limited maximum range due to the antennas’ scanning, the collected data give a view of the boundary layer and the lower troposphere over a wide horizontal extent, with characteristic horizontally stratified structures in the lower troposphere. These structures are also detected by application of high-resolution imaging processing such as Capon's beamforming or Multiple Signal Classification algorithm. The proposed method can be a simple way to enhance the versatility of classical DBS radars in order to extend them for multi-sensor applications and local meteorology.

ST radar observations of atmospheric waves over mountainous areas: a review

Abstract. Lee and mountain waves are dominant dynamic processes in the atmosphere above mountain areas. ST VHF radars had been intensively used to investigate these wave processes. These studies are summarized in this work. After discussing features of long-period quasi-stationary lee waves, attention is drawn to the frequent occurrence of freely propagating waves of shorter periods, which seem to be more common and characteristic for wave processes generated over mountainous areas. Characteristics of these waves such as their relation to the topography and background winds, the possibility of trapping by and breaking in the tropopause region and their propagation into the stratosphere is investigated. These orographically produced waves transport energy and momentum into the troposphere and stratosphere, which is considered an important contribution to the kinetic energy of the lower atmosphere. The occurrence of inertia-gravity waves in the stratosphere had been confused with lee waves, which is discussed in conclusion. Finally further questions on mountain and lee waves are drawn up, which remain to be solved and where investigations with ST radars could play a fundamental role.Key words: Meteorology and atmospheric dynamics (Middle atmosphere dynamics; Waves and tides; Instruments and techniques)

ST radar observations of atmospheric waves over mountainous areas: a review

Lee and mountain waves are dominant dynamic processes in the atmosphere above mountain areas. ST VHF radars had been intensively used to investigate these wave processes. These studies are summarized in this work. After discussing features of long-period quasi-stationary lee waves, attention is drawn to the frequent occurrence of freely propagating waves of shorter periods, which seem to be more common and characteristic for wave processes generated over mountainous areas. Characteristics of these waves such as their relation to the topography and background winds, the possibility of trapping by and breaking in the tropopause region and their propagation into the stratosphere is investigated. These orographically produced waves transport energy and momentum into the troposphere and stratosphere, which is considered an important contribution to the kinetic energy of the lower atmosphere. The occurrence of inertia-gravity waves in the stratosphere had been confused with lee waves, which is discussed in conclusion. Finally further questions on mountain and lee waves are drawn up, which remain to be solved and where investigations with ST radars could play a fundamental role.Key words: Meteorology and atmospheric dynamics (Middle atmosphere dynamics; Waves and tides; Instruments and techniques)

Equatorial 150‐km irregularities observed at Pohnpei

First observations of meter‐scale equatorial 150‐km plasma irregularities outside the American longitude sector are reported. The observations conducted at Pohnpei (western Pacific) with a wind profiling ST radar demonstrate the feasibility of making ionospheric electric field measurements with low cost instruments and also reveal a potentially important anisotropy in the angular distribution of 150‐km VHF radar echoes.

Retrieval of ageostrophic wind from a radiosounding network and a single ST radar

Retrieval of ageostrophic wind from a radiosounding network and a single ST radar

Retrieval of ageostrophic wind from a radiosounding network and a single ST radar

AbstractPrevious studies have shown that ageostrophic circulations play an important role in frontogenesis phenomena. They result from the maintenance of the balanced state of the atmosphere which tends to be broken down by the large‐scale forcing of the atmosphere. Their existence explains how a frontal discontinuity can be produced in a few days. From an experimental and forecasting point of view it would be interesting to obtain these circulations in real time. Unfortunately, the ageostrophic wind is not a measurable quantity and its retrieval from experimental data is difficult, particularly at the mesoscale, since instrumental noise leads to a need for significant data correction.In this context, this paper provides answers to some of the main problems related to the retrieval of ageostrophic winds in frontal systems. It focuses on the feasibility of retrieval while retaining, at small enough scale, details of physical interest. The framework of this study is the future implementation of regular meteorological wind‐profiler networks with a sampling time of one hour.This paper proposes to overcome numerous practical problems using an approach based on the variational Analyse du Vent AGéostrophique (AVAG) analysis. Tests of such an approach are done on real data extracted from the Mesoscale Frontal Dynamic Project/FRONTS 87 data base for a typical two‐dimensional frontal case (the IOP 7 case). They confirm that the AVAG analysis is able to provide a non‐noisy and consistent ageostrophic circulation. Comparisons with classical methods demonstrate the robustness of AVAG and its capability to obtain physically meaningful ageostrophic wind fields. This study also estimates the best and most suitable experimental sampling for practical applications. Sensitivity tests on the quality of the retrieved wind field with respect to (i) the sampling time used, (ii) the availability of additional thermodynamic data and (iii) the geometrical pattern of the network are presented. These tests provide evidence of the importance of accounting for thermodynamical‐ and dynamical‐field along‐front variations.

Retrieval of stratified atmospheric reflectivity and wind velocity using inverse methods: application to a VHF ST mini-radar

Abstract The growing interest of measuring wind and turbulence in the lower atmosphere has led to the development of new radar systems. UHF radars, though being an interesting solution from a technical and logistical point of view, have some disadvantages due to their high sensitivity to Rayleigh scattering and interference from precipitations, birds and insects. The standard VHF ST radars were originally designed for high altitude investigations and are consequently not suited for low atmosphere soundings. This context makes it necessary to develop the new concept of a VHF ‘mini-radar’. But the simultaneous use of a small antenna in the VHF band, combined with a beam having a more grazing angle, results in an important mixing of the altitude contributions for each range, a problem which is non existent with previous ST radars. Consequently, the atmospheric reflectivity and wind velocity profiles cannot be directly obtained and have to be treated by other methods. In this context, the aim of the present work consists in the development of an appropriate inverse method. Two different classic methods are considered, the least squares method and the maximum entropy method. The ‘direct problem’ is first addressed, resulting in an integral description of the zeroth and first moments of the Doppler spectra. In order to perform various simulations to test the validity of the two proposed inverse methods in the particular case of the VHF miniradar, a model is built for the radar which includes a set of reference atmospheric profiles. The simulations give evidence for the validity of the inversion processes. The high robustness of the least squares method always leads to significant results. But its over-determined nature results in a poor vertical resolution for the inverted profiles. Consequently this method is not suited to retrieving strong gradients. The maximum entropy method is intrinsically much more appropriate in terms of vertical resolution and consequently leads to valuable results, but its high sensitivity to the data noise requires some additional constraints. The practical efficiency of the methods is tested with real data from the mini-radar, and the resulting retrieved profiles are compared to those obtained simultaneously using a conventional ST radar (the ‘Provence’ radar). As a result of a poor vertical resolution, the least squares method cannot provide a valid retrieval of the atmospheric profiles under real experimental conditions. Nevertheless, a preliminary inversion using the least squares method can be used as a constraint for initializing the maximum entropy inversion process. Although this processing appears to be very efficient for the reflectivity, the retrieved profiles reveal a smoothing effect which seems to be linked to a faulty radar antenna radiation model. In contrast, the retrieval of wind velocity seems to be more difficult and requires additional investigations.

Space–Time Description of Nonstationary Trapped Lee Waves Using ST Radars, Aircraft, and Constant Volume Balloons during the PYREX Experiment

Abstract The third intensive observation period (IOP3) of PYREX was a case of strong lee waves generated by a southerly wind crossing the Pyrenees chain. Upstream radiosounds and measurements obtained by aircraft along the chain transect and by constant volume balloons launched near the crest provided spatial characteristics of the lee wave at different times and heights. Values ranging from 7 to 14 km for the horizontal wavelength, and from 3 to 5 m s−1 for the maximum amplitude of the air vertical velocity, were observed. The lee wave horizontal extent, measured from the crest line, reached 30 to 55 km. In addition, two very high frequency stratosphere-troposphere (VHF ST) radars, one on the mountain mean axis and another downstream in the lee wave field, observed the temporal variations of the vertical profiles of the vertical velocity. The analysis of those observed variations and their vertical distribution allowed the stationarity of the wave to be studied. The lee wave was found to be far from stat...

Energy dissipation rates, eddy diffusivity, and the Prandtl number: An in situ experimental approach and its consequences on radar estimate of turbulent parameters

Different methods have been proposed to derive the energy dissipation rate and eddy diffusion coefficients from ST radar measurements. However, their validity is still questionable because they implicitly assume that the Prandtl number is always equal to one, an assumption which is not verified. An experimental approach to this question, using balloon‐borne experiment results, is proposed in this paper in order to test the validity/invalidity of the methods generally used. In situ observations show that the potential temperature gradient is more efficiently (and probably more rapidly) eroded by the turbulent activity than the wind shear. As a consequence of this observational evidence already mentioned by Browning and Watkins [1970], the structure function constant for temperature fluctuations (CT2) is vanishing within fully developed turbulent layers and exibits maxima on their boundaries, while the structure parameter for wind fluctuations (CV2) presents a broad maximum within the same layer and is decreasing at its boundaries. Consequently, the gradient Richardson number Ri strongly varies within fully developed turbulent layers, from Ri close to zero (near their center) up to Ri >1 (at their boundaries). By contrast, the flux Richardson number Rf, which describes the evolution of the ratio between buoyancy flux and turbulent energy production, remains apparently quasi‐constant and close to its critical value during the erosion processes, so that the Prandtl number is not a constant close to unity but might also strongly vary during the turbulent life cycle. These results are in good agreement with laboratory experiments in statistically stable fluids reviewed by Thorpe [1973] and with experimental results obtained in the boundary layer [Businger et al., 1971; Gossard and Frisch, 1987]. ST radar are generally not able to observe regions where the potential temperature gradient is eroded by the turbulent activity but may obtain strong responses on the boundaries of fully developed turbulent layers. This behavior does not affect the radar capability of estimating eddy dissipation rate ϵ and eddy diffusivity Kθ (or KM) when complementary information on temperature profiles and humidity are available. It is shown that the “nonlocal” mean potential temperature gradient, the wind shear, and the flux Richardson number are the pertinent parameters allowing a correct estimate of the eddy dissipation rates and eddy diffusion coefficients, from Cn2 (CT2) and rms turbulent vertical wind, in regions where the turbulent activity is observable by ST radars.