Electromagnetic waves and atmospheric gravity waves (AGWs) attributed to thunderstorm/lightning events canalter the ionospheric conductivity and produce total electron content (TEC) fluctuations at acoustic and gravity wave (GW)frequencies. This study investigates the thunderstorm-induced E- and F-region irregularities over the near-equatorial Indian sector on October 16, 2019 and April 9, 2022, based on the observations from a 205 MHz VHF radar at Cochin (10.04 degrees N,76.33 degrees E), India and global navigation satellite system (GNSS)receiver at a nearby station, Changanacherry (9.44 degrees N, 76.55 degrees E).Concurrent with the thunderstorm activity, E-region irregular-ities were observed on both the days following the lightning strikes. Lightning-induced sporadicE(Es) with a vertical width of 2 km was observed on April 9, 2022 and lasted for 2 h longer than the case observed on October 16, 2019. Subsequent to the disappearance ofEs, quasi-periodic irregularities spanning from 60 to 550 km altitude were observed. Lightningstrikes induced wave-like fluctuations in TEC with subsequent enhancements in amplitude scintillations. Vertically propagating AGW-induced ionospheric acoustic waves (AWs) of similar to 4 min periodicity on October 16, 2019 and similar to 3 min periodicity onApril 9, 2022 were observed. GW periodicity of 12-13 min be came prominent at ionospheric heights after 1-2 h of the lightning strike. The mesoscale convective system with multiple cells developed over the Indian-near equatorial sector could have caused the early AWs and delayed GWs. This observation showcases the potential application of ground-based systems in exploring the lower atmosphere-ionosphere coupling process during thunderstorms

Aspect sensitivity in VHF radar refers to the extent to which the power and spectrum width of echoes vary with changes in the zenith angle, which is related to the backscattering process. The scattering of clear air signals is primarily caused by Fresnel reflection, anisotropic scattering, and isotropic scattering. The aspect sensitivity and accuracy of moment and wind estimation in clear air radars are influenced by the beam width, beam pointing angle from zenith, and atmospheric conditions. This study proposes a method to determine the sensitivity of the recently developed Stratosphere-Troposphere (ST) wind profile Radar in Cochin, India (10.04 degrees N, 76.33 degrees E). The radar operates at a distinct frequency of 205 MHz in the far VHF band. The experiment is conducted at an altitude of 5 to 20 km above the ground by adjusting the beam's orientation with a resolution of 2 degrees in both the east-west and north-south directions. The estimation of wind components is subject to uncertainty due to the varying aspect angles caused by the distinct dispersion properties in this height range. The study found that power variance is lowest between 6 and 12 km in both north-south and east-west directions, while daily fluctuations in aspect-sensitive echoes complicate wind component estimation. Correlation length (zeta) ranges from 0.5 to 15 m, indicating various air scattering processes. Notably, theta(s) is smaller near the zenith and increases with tilt angles, exceeding 20 degrees up to 14 km before declining at higher altitudes, indicating significant anisotropy at elevated levels. The wide range of R factor values (0.1 to 0.9) across different heights causes significant ambiguity in wind estimation. In this study, the impact of various aspect sensitivity parameters on wind estimation on days with clear air has been analyzed.

Diurnal variation of the low-level jet (LLJ) and tropical easterly jet stream (TEJ) over Cochin - the gateway of Indian summer monsoon - was investigated during the monsoon onset over Kerala (MOK), active-break, and withdrawal phases in the year 2021. Horizontal wind profiles obtained from the 205 MHz wind profiling radar at Cochin University of Science and Technology, India, were analyzed to delineate the core speed and core heights of the LLJ and TEJ. Distinct diurnal variability in wind patterns was observed during various monsoon phases. The LLJ and TEJ core speed and heights exhibited significant diurnal variations, with the LLJ reaching maximum diurnal amplitudes between 2.5 m s-1 (onset phase) and 0.5 m s-1 (break phase). Notably, the LLJ core speed during the break phase was approximately half of that during the active phase. The ratios of the diurnal range to the time mean LLJ core speed varied between 0.19 (active phase) and 1.33 (onset phase). Moreover, the LLJ core height is higher during active compared to break. Similarly, the TEJ core height at Cochin is marginally higher during active compared to that during break. Interestingly, an enhanced TEJ core speed and weakened LLJ were observed during the break phases. The variation of moisture flux exhibited positive correlation with the core speed with 2-3 h lag, significantly influencing the diurnal variability of rainfall. The causative factors behind the diurnal oscillations during various phases of the monsoon are discussed.

Atmospheric radars, otherwise known as clear air radars, are advanced remote sensing system to investigate the atmospheric features through observation of the winds. They primarily depend on the back-scattered signals due to radio refractive index gradients associated with turbulent eddies to determine the velocity and range. Detecting the weak clear-air signal dictates the use of long coherent dwell times, low-noise system, low antenna side lobes, and careful attention to siting and potential interference [1]. In recent years, atmospheric radars have emerged as powerful tools for atmospheric research around the globe due to its capability of accurately measuring both the horizontal and vertical components of the wind in the vertical. The state-of-the-art VHF radar operating at 205 MHz at the Advanced Centre for Atmospheric Radar Research, Cochin University of Science and Technology (CUSAT) has played a pivotal role in exploring the physical processes occurring not only in the troposphere and lower stratosphere, but also in the earth's ionosphere reaching up to the F-region. In addition to estimating the 3-dimensional wind components ranging from 315 m to beyond 20 km above the earth's surface, it has the potential of detecting sub-metre scale field-aligned irregularities (FAI) in the E-and F-regions. The unexploited 200 MHz frequency band has the advantage of high signal to noise ratio, less cosmic noise, and good vertical coverage and in addition to high resolution especially in the lower troposphere. This paper unravels the potential of this particular Radar in exploring diverse atmospheric/ionospheric processes in the southern peninsular region.

Madden Julian Oscillation (MJO), one of the dominant intra-seasonal oscillations over the tropics, plays a major role in preconditioning the atmosphere and ocean prior to the monsoon onset over Kerala (MOK). However, the onset process exhibits large variability even when MJO exists in the same phase. This study attempts to un-derstand the characteristics of MOK during different MJO phases and the cyclogenesis process in the north Indian Ocean in concurrence with MOK. During the study period from 1980 to 2020, almost 80% of MOK have occurred in MJO phases I, II and III. The remaining 20% MOK occurred in all other phases of MJO except in phase VIII. The onset days are categorized as early (before 30th May), normal (30th May to 03rd June) and delayed (after 03rd June), and their occurrences under different MJO phases are examined. Various environmental parameters associated with the MJO phases were analysed to understand the cyclogenesis process close to the time of the MOK. Phase I is conducive to cyclogenesis over the Arabian Sea, whereas phase III favours that in the Arabian Sea and the Bay of Bengal. MJO phase II does not support cyclogenesis during the MOK. The decrease in the positive anomaly of lower-level westerlies towards the north of the equator gives rise to cyclonic shear vorticity, which supports the formation of cyclones during phases I and III. Associated with MOK, the total precipitable water shows a negative anomaly in phases I and II of MJO and a positive anomaly in phase III. The convective activity in the north Indian Ocean remains enhanced in phase III, where the amplitude of MJO is also found to be maximum.

Stratosphere–troposphere (ST) wind profiler radar (WPR) installed at Cochin, India, is a clear air VHF Doppler radar system designed for continuous wind velocity measurements under all-weather conditions. Clear air WPRs detect backscattered signals due to Bragg scattering from small irregularities in the refractive index caused by turbulence. The 205-MHz frequency band is able to detect both clear air and rain echos, simultaneously. Due to the large spectral width and merging of clear and rain echoes during rainy periods, the conventional signal processing techniques become inept at distinguishing the two echoes. An innovative hybrid approach is applied in this study to mitigate the ambiguity in identifying clear air and rain echoes separately. A combination of an exponentially modified Gaussian model and a pure Gaussian model is used to fit the raw spectrum in the hybrid approach adaptively. The data adaptive fitting procedure adopted can identify two distinct peaks during the rainy period, and the fitting process reduces to a single peak during non-rainy periods. Furthermore, the number of peaks is confirmed by locating the number of zero-crossings (NZCs) of the power gradient of the fit data. This unique approach is applied in the 205-MHz ST radar framework for the first time to solve the issue of multiple peaking in the ST radar spectrum.

During recent decades, the Arabian Sea witnessed frequent occurrences of cyclones during the onset phase of the Indian Summer Monsoon. This study examined the impact of these cyclones on the establishment and northward propagation of the monsoon system. The main dynamical components of the monsoon semi-permanent systems, including wind at 850 hPa and 150 hPa, are analysed separately for Arabian Sea Cyclone Years (ACY) and Non-Cyclone Years (NCY). Pentad (5-day average) composites for ACY and NCY are prepared with respect to pentads centred on monsoon onset date over Kerala (MOK). Six pentad composites (2 before; 1 during and 3 after the onset pentad) are prepared to examine the evolution of the monsoon system over the Indian region. The analysis shows that LLJ and TEJ have been altered by the cyclones formed within +/- 8 days of monsoon onset over Kerala. For ACY, the LLJ and TEJ patterns require 10 days to stabilise into (acquire) its normal pattern (spatial and intensity) as NCY. Pentad (similar) analysis of SST also shows a consistent pattern to that of LLJ. As the speed of LLJ increases, the SST decreases along the path of LLJ. Total Precipitable Water shows a significant change over the Arabian Sea rather than the Bay of Bengal on the day of onset and thereafter. The changes in the upper tropospheric meridional temperature gradient are responsible for TEJ alteration. Cyclone-induced perturbations are found to disturb the thermodynamical and dynamical state of the background monsoon circulation. The high value of vertical wind shear of zonal wind over the Indian Monsoon area during NCY is ideal for monsoon and arises due to strong LLJ and TEJ. The north-south temperature difference is less for ACY, which leads to a slight delay in establishing TEJ after the onset of monsoon. Kinetic energy at 925 hPa level is higher during NCY than ACY and is supporting the rainfall variations.

The rapid response of the atmosphere to the reduction in solar radiation during the annular solar eclipse on 26th December 2019 with maximum obscurity of 91.16% over Cochin, located in the southwest peninsular India, has been investigated. A suite of instruments encompassing an advanced VHF wind profiling Radar operating at 205 MHz, GPS radiosonde, and automatic weather station was employed to record the temporal and vertical variations in various meteorological parameters. Compared to the corresponding values averaged for two control days neighboring the eclipse day, the temperature, relative humidity, net radiation, pressure and wind speed at the surface were altered by -4.0 degrees C, +39.6%,-383.0 Wm(-2), +0.62 mb, and-2.6 ms(-1), respectively. The corresponding percentage vari-ations are-13.6%, +71.0%,-67.0%, +0.06%, and-99.0% during the maximum phase of the eclipse, and approached close to the control-day values within about 2 h. Multiple temperature inversions as large as 1 degrees C were observed up to a height of about 7.3 km. The heights of mean planetary boundary layer (PBL) and tropopause were decreased by about 355 m (-50%) and 900 m (-5.5%), respec-tively during the peak of obscuration. Meanwhile, the vertical profiles of both the horizontal and vertical wind velocities demonstrate substantial reduction. The vertical wind dispersion, which characterizes the fluctuations in vertical air motion, was reduced by 34% in the PBL compared to its corresponding values on the control days. Subsequently, wavelet spectral analysis of the vertical wind yields a quasi-periodic oscillation indicating the presence of internal gravity waves with periodicities of 40-60 min in the lower altitudes up to 3 km during the eclipse day. Besides reporting the characteristic variations in various atmospheric parameters, the present study reveals the height-time variations in the vertical winds, a vital parameter of significance in the studies of convection, turbulence, and atmospheric waves. (C) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

The thermodynamic characteristics of the atmosphere during the pre-monsoon thunderstorm period and the transition phase from pre-monsoon to monsoon period over the gateway of Indian summer monsoon is investigated in this study. The study utilizes Stratosphere-Troposphere (ST) wind profiling Radar, Microwave Radiometer, and GPS radiosonde observations. A sharp increase in the boundary-layer and middle-layer moisture content, abrupt changes in the mixing height, lifting condensation level (LCL), convective available potential energy (CAPE), and other thermodynamic variables are noted. The pre-monsoon period is characterized by intermittent deep convection that moistens the middle layers, while stratiform clouds are observed in the monsoon period and deep moist layer is observed. The dry middle-layers observed before the monsoon onset got moistened consistently as the low-level jet strengthened. The LCL height was lower than the mixing height after the onset of monsoon. The wet-bulb zero level (WBZ) height was reduced to 2 km during the pre-monsoon and shifted gradually to higher altitudes as the onset of monsoon occurs. Variations in WBZ height may help in the predictability of monsoon onset. The study has also investigated the characteristics of a thunderstorm event before the onset and suitable indices for early indication of the storm.

In this paper we examine the epochal changes in the frequency of cyclones over the North Indian Ocean during the pre-onset and onset phases of the monsoon. We consider three epochs; namely, the early (1955-74), middle (1975-94) and recent (1995-2014) epochs. It is found that the number of cyclones in the Bay of Bengal (BOB) decreases throughout the three epochs. Over the Arabian Sea (ARB), however, there is a decrease in the early epoch, before then reaching a minimum in the middle epoch followed by an increase in the recent epoch, thus exhibiting epochal variability. Dynamic and thermodynamic parameters along with Genesis Potential Index (GPI) are examined to understand the frequency variation in cyclogenesis over the ARB and BOB. Over the ARB, thermodynamic factors such as mid-level moisture, surface latent heat flux and sensible heat flux, and dynamic parameters such as lower-level convergence and upper-level divergence, are favorable during the early and recent epochs but unfavorable during the middle epoch, and these results are found to be consistent with the observed epochal variability in the frequency of cyclogenesis. However, all these influential parameters are found to have decreased over the BOB during the entire 60-year period.

Altitude structure of turbulence in the troposphere and lower stratosphere (TLs) over the Indian peninsula is delineated using radiosonde observations carried out from six stations (August 2013 to December 2017) as part of the Tropical Tropopause Dynamics (TTD) Campaign under the GPS Aided Radiosonde Network Experiment for Troposphere-stratosphere Studies (GARNETS) program. Thorpe analysis applied to the potential temperature profiles, taking into account the impact of atmospheric moisture and instrumental noise, is used to estimate the turbulence parameters. This study shows that while the occurrence of turbulence is high in the lower (0-2 km) and upper (10-15 km) troposphere, the region 3-8 km is relatively devoid of turbulence at all the six stations. In general, similar to 60% of the Thorpe scale (L-T) values in the troposphere are less than 250 m at all the stations, except at Cochin where it extends up to 400 m. Though the altitudinal structure of turbulence shows large variability from station-to-station, it does not show any systematic latitudinal pattern. This study also shows that the Thorpe method appears to be good in identifying convectively induced turbulence than the dynamically (shear) induced turbulence. In the lower troposphere, the turbulence is due to both convective and dynamic instability at all the stations. While turbulence in the altitude region between 10 and 15 km is mainly due to convective instability at Coimbatore, Gadanki and Goa and due to dynamic instability at Cochin, both these instabilities contribute significantly to the generation of turbulence at Trivandrum and Hyderabad. Occurrence of convective instability (turbulence) under clear-air conditions in the upper troposphere, suggests wave breaking as a mechanism.

A state-of-the-art 205-MHz wind profiling radar designed for measuring both the horizontal and vertical wind components from 315 m to beyond 20 km has been installed at the Cochin University of Science and Technology (CUSAT), India (10.04°N, 76.33°E, dip angle ~7.1°N). Subsequently, the radar was operated with special configuration to probe the ionosphere. After a series of experiments, the radar was successfully configured to receive ionospheric reflections from about 90 to 500 km range covering the E and F layers, with high resolution (45 m for the 90–110 km region). The received echoes are identified as signatures of field-aligned irregularities (FAIs) of the E and F regions of the ionosphere. The E region echoes were observed at an altitude range of 90–110 km. Both continuous and quasi-periodic structures were identified. Further analysis from the spectrum shows that the E region FAIs are Type 2 in nature. The night time spread-F observed so far is of either bottom type or bottom side in nature. This letter portrays the scope of employing 200 MHz range of very high frequency (VHF) band for ionospheric observations, and the technical details and the initial results of the experiment conducted with this stratosphere–troposphere (ST) Radar.

The processes in Stratosphere and Troposphere are strongly correlated with each other and exchange of water vapor, momentum and energy between these two layers have much significance on climate. Recent studies indicate that stratospheric water vapor and its variability play an important role in changing climate. High-resolution data, even though available only at few locations, are highly useful for the analysis of stratosphere troposphere exchange. A stratosphere-troposphere (ST) radar (205 MHz) is operational at the Advanced Centre for Atmospheric Radar Research (10.04N; 76.33E), Cochin University of Science and Technology, India. This radar provides accurate measurements of upper troposphere-lower stratosphere (UTLS) region. Observations made during May 16 - 19, 2017, few days prior to the onset of Indian summer monsoon show that strong convection reached the tropopause height and disturbed the tropopause.

Several studies indicate that the El Nino-Southern Oscillation (ENSO) has a profound influence on the Asian Summer Monsoon. It is explained that the warm/cold phases of ENSO perturb the Walker circulation, thereby modifies the Asian summer monsoon circulation as well as the rainfall pattern. The influence of ENSO on the winds in the upper troposphere and lower stratosphere over the Asian summer monsoon region associated with ENSO is not much attempted. In this study, the role of ENSO on the circulation patterns of the upper troposphere and lower stratosphere has been investigated during the boreal summer over the Asian Summer Monsoon region. ECMWF reanalysis (ERA) Interim data has been used for a period of 37 years (1980-2016) for the study. During the summer monsoon season, strong easterly winds in the upper troposphere, known as the Tropical Easterly Jetstream (TEJ), appear from late May to September. The lower stratospheric winds are dominated by the quasibiennial oscillation (QBO), which changes the speed and direction in every alternate year. The TEJ exhibits unique characteristics in El Nino and La Nina phases. The intensity of TEJ enhances in both horizontal and vertical direction in La Nina years, whereas it shrinks during El Nino years. In the lower stratosphere, the influence of easterly phase dominates during the La Nina period, whereas it becomes relatively weak during El Nino events. In La Nina composite, the core of TEJ extends vertically to the lower stratosphere. An intense upward flow from the upper troposphere to lower stratosphere is found in the longitude belt of 70°-80° E in the La Nina period. Convergence in the lower level and divergence in the upper level is generally weak in El Nino years. On the other hand, a westward shift of strong upper level divergence is seen in La Nina conditions. Appearance of strong/weak anticyclonic flow near to the tropopause height over the Tibetan Plateau is prominent during La Nina/El Nino year results in the significant inter-annual variability of the circulation in the upper troposphere lower stratosphere region.

A preliminary study on the first‐time observation of the submeter‐scale disturbances in the ionospheric E layer above the near‐equatorial site of Cochin (10.04°N, 76.33°E; Declination: −1°37′, Inclination: 7°2′), India, using an atmospheric radar in the high very high frequency range at 205 MHz has been carried out. Selected cases of E layer disturbances observed in the 90‐ to 105‐km altitude region, during summer, equinox, and winter periods are reported. It is noted that the submeter‐scale disturbances exist in the near‐equatorial E region with a life span of few minutes to several hours. Both continuous and intermittent echoes were seen in the 90‐ to 105‐km region. The continuous irregularities remain at constant altitude initially, descend and vanish in the later stage. The E layer irregularities observed in the morning and noon were continuous, whereas the echoes after the late afternoon were quasiperiodic in nature with a lifetime of 10–15 min. However, no apparent mean Doppler is observed during this period and is expected to be associated with a turbulent cascade from long wavelengths to shorter ones.

India experienced a heavy rainfall event in the year 2013 over Uttarakhand and its adjoining areas, which was exceptional as it witnessed the fastest monsoon progression. This study aims to explore the causative factors of this heavy rainfall event leading to flood and landslides which claimed huge loss of lives and property. The catastrophic event occurred from 14th to 17th June, 2013 during which the state received 375% more rainfall than the highest rainfall recorded during a normal monsoon season. Using the high resolution precipitation data and complementary parameters, we found that the mid-latitude westerlies shifted southward from its normal position during the intense flooding event. The southward extension of subtropical jet (STJ) over the northern part of India was observed only during the event days and its intensity was found to be increasing from 14th to 16th June. The classical theory of westward tilt of mid-latitude trough with height, which acts to intensify the system through the transfer of potential energy of the mean flow, is evident from analysis of relative vorticity at multiple pressure levels. On analysing the North Atlantic Oscillation (NAO), negative values were observed during the event days. Thus, the decrease in pressure gradient resulted in decrease of the intensity of westerlies which caused the cold air to move southward. During the event, as the cold air moved south, it pushed the mid-latitude westerlies south of its normal position during summer monsoon and created a conducive atmosphere for the intensification of the system.

ABSTRACTThe influence of El Niño/La Niña on the upper‐tropospheric circulation pattern, especially on tropical easterly jet stream (TEJ) over the Asian summer monsoon region, has been studied using ECMWF Reanalysis (ERA) Interim Data for a period of 34 years (1980–2013). The TEJ shows distinctive features during El Niño and La Niña events. The intensity of the TEJ increases and it spreads horizontally and vertically during La Niña years, whereas it weakens and shrinks spatially in El Niño years. Walker circulation over the summer monsoon region shows strong easterlies in La Niña events accompanied with vertically enhanced zonal flow. Lower level (∼850 hPa) convergence as well as upper level (∼150 hPa) divergence is strong in La Niña events and a westward shift of divergence centre is also observed in these years. A strong (weak) anticyclonic flow at upper levels at about 150 hPa is prominent during La Niña (El Niño) years over the Tibetan region causing significant inter‐annual variability of the TEJ.

Wind profiler radars (WPRs) are the state-of-the-art technology used for continuous remote sensing of 3-D wind structures from near surfaces to a height of about 20 km. Depending on the magnitude of atmospheric turbulence, the size, power, and frequency of the radar can vary. In this letter, the technical aspects of the design and the development of a 205-MHz very high frequency WPR installed at Cochin (10.04° N, 76.33° E), India, for troposphere and lower stratospheric studies in the tropical monsoon region are presented. The stratosphere-troposphere (ST) radar at Cochin is unique because it is the first WPR in the world operating at 205 MHz with a bandwidth of 5 MHz. The mini WPR with 49 three-element Yagi-Uda antennas at 205 MHz is working successfully and providing accurate wind measurements from 1 to 3.5-8 km, depending on atmospheric conditions. WPR data are comparable with collocated high-resolution Global Positioning System radiosonde observations. The main ST radar with 619 antenna elements is in the installation stage and is expected to be operational within a period of six months.

The estimation of the power spectrum of discrete-time signals is one of the most fundamental and useful tools in signal processing. However, there are practical situations where one needs to look beyond the power spectrum, especially to extract information regarding the phase relations and deviations from Gaussianity. This has created considerable interest in the use of higher order spectra such as bispectrum, for the analysis of signals, particularly in the presence of additive Gaussian noise. This paper examines the use of Gammatone Cepstral Coefficients computed from the spectrum reconstructed from the bispectrum of the signal as a feature set for underwater target classification. A prototype Neural Network classifier with back propagation algorithm has been trained with the proposed feature set and the performance has been evaluated, which has yielded acceptable classification results.

The thermal inversion characteristics during active and break cycles of two consecutive and contrasting monsoon years were studied using GPS radiosonde profiles in Goa (15°46′ N; 73°08′ E), located on the west coast of India. The lower tropospheric inversions were associated with the active phase and it dissipated during the break phase due to weakening of the low level jet. A strong secondary inversion also appeared at 5–6km in the break period. The genesis of both the inversions is linked with advection of warm and dry air from the Arabian and Persian high lands. During the active phase, convection overrides the stability shield provided by the advection induced low level inversion. Associated with the active phase, the temperature difference within the inversion invariably reaches above a threshold of 1°C, whereas it is low in break conditions.