This paper illustrates the modulation of meteorological parameters over the tropics in the east and west phases of the quasi-biennial oscillation (QBO) during the sudden stratospheric warming (SSW). Twenty years composite of SSW episodes have been analysed, and the results show a teleconnection between northern polar SSW and changes in meteorological parameters throughout tropical troposphere to stratosphere during both the phases of QBO. The tropical troposphere shows different temperature pattern in the two opposite phases of QBO during polar SSW. The lower tropospheric temperature decreases about five days prior to the onset day, persisting as such as for about five days post-onset day, and increases rapidly during the east phase whereas a sudden dip in temperature is observed in the west phase. The easterly wind at the upper tropospheric region is observed to be coupled to the easterly wind in the QBO core region of stratosphere during the easterly phase of QBO whereas they are decoupled in the westerly phase. Prior to the peak polar warming, a strong convective cell develops over the southern hemispheric tropics, and another convective cell develops over the northern hemisphere during the peak warming period. This study indicates that during the easterly QBO-SSW period, increasing tropopause easterly wind causes tropopause cooling due to easterly wind's coupling with the QBO core region around the onset day of SSW. As a result, the changes in the tropopause strengthen the Hadley cell circulation thus enhancing the convective activity over the tropical region associated with northern polar SSW. Contrary to the prior knowledge of convection being centred mostly along 5 degrees S-10 degrees S, it is found that other convection band located at around 10 degrees N also produces rainfall in the tropical region. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

The remote influence of west Pacific typhoons on the historic Kerala flood in the 2018 Indian summer monsoon (ISM) season is investigated using the weather research and forecasting (WRF-ARW) model. The flood occurred as a result of vigorous monsoon intra-seasonal activity with some districts receiving excess rainfall exceeding 405

Kerala conside red the land of monsoons, located on the southwest coast of India, received contrasting rainfall during the northeast monsoon (NEM) seasons of 2019 (excess) and 2020 (deficit). The reasons for the contrasting NEM rainfall during these two consecutive years were studied by analysing the atmospheric circulation pattern and its interaction with warm ocean waters by using wind data from the stratosphere-troposphere (ST) radar and reanalysis products. The study reveals that weak and variable westerly winds over the southern Indian Peninsula coupled with the location of the Inter-tropical Convergence Zone (ITCZ) north of the peninsula prevented the timely onset of NEM over its core regions during 2019. However, 2019 was an anomalous year with an extreme positive Indian Ocean Dipole (IOD) with unusual cyclonic activity over the Arabian Sea (AS) during the NEM season. These anomalous conditions favoured large amounts of rainfall, especially over the northern parts of Kerala during the latter half of October and compensated for the rainfall deficiency during the early half of October. In 2020, the subdued cyclogenesis over the AS combined with near-normal sea surface temperatures (SST) resulted in reduced convective activity over Kerala during the initial part of the NEM season. After the withdrawal of the southwest monsoon (SWM), the formation of two cyclones in the Bay of Bengal (BoB) and the frequent passage of easterly troughs from the BoB across the southern peninsula brought in anomalous rainfall over the east coast and some parts of Kerala, but the central and northern parts of Kerala largely remained rainfall deficient.

Tropical cyclones do not form easily near the equator but can intensify rapidly, leaving little time for preparation. We investigate the number of near-equatorial (originating between 5°N and 11°N) tropical cyclones over the north Indian Ocean during post-monsoon season (October to December) over the past 60 years. The study reveals a marked 43% decline in the number of such cyclones in recent decades (1981-2010) compared to earlier (1951-1980). Here, we show this decline in tropical cyclone frequency is primarily due to the weakened low-level vorticity modulated by the Pacific Decadal Oscillation (PDO) and increased vertical wind shear. In the presence of low-latitude basin-wide warming and a favorable phase of the PDO, both the intensity and frequency of such cyclones are expected to increase. Such dramatic and unique changes in tropical cyclonic activity due to the interplay between natural variability and climate change call for appropriate planning and mitigation strategies.

This study reveals the geomagnetic storm-induced ionospheric perturbations observed over the near-equatorial Indian stations Cochin and Changanacherry in 2021. The 205 MHz VHF radar at Cochin University of Science and Technology (CUSAT) detected the storm-induced E- and F- region echoes during the moderate (28th February - 6th March 2021) and weak (19th - 25th March 2021) storm events. The fast-fluctuating northward interplanetary magnetic field (IMF BZ) and inhibition of the equatorial ionization anomaly (EIA) observed on 1st March indicate the suppression of daytime eastward electric field by the westward electric field associated with the counter electrojet (CEJ). Concurrent equatorward shift in the EIA crest and minor positive storm enhancement in the vertical total electron content were observed. The CEJ could have helped in the accumulation of photo-generated plasma at the near-equatorial region by pushing the daytime F-layer downward. During the moderate storm phase, post-sunset E-region echoes were detected during 1st - 3rd March, attributed to the disturbance dynamo electric field (DDEF). Nevertheless, during the main phase of the weak storm, a poleward shifting of the EIA crest occurred on 20th March, indicating the presence of a prompt penetration electric field (PPEF), which enhanced the upward E x B drift. A major positive ionospheric storm enhancement that lasted for 7 h occurred on 20th March due to the DDEF. Bottom-type F-region echoes were observed over Cochin during the recovery phase of the weak storm. This work highlights the observation of equatorward and poleward shifting of the EIA using a 205 MHz radar. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

Aeolus is European Space Agency's unique and novel wind measuring satellite mission providing near real-time wind profiles from near surface to an altitude of 30 km. This paper presents the validation of Aeolus wind profiles over Cochin (10.04 degrees N, 76.9 degrees E), India using the 205 MHz wind profile radar. The Aeolus wind profiles (baselines 10 and 11) have been validated for an altitude range of 1 to 18 km during June, 2019 to September, 2021. Aeolus Rayleigh wind for clear (Rayleigh(clear)) and Mie wind for cloudy (Mie(cloudy)) show very good agreement with the radar wind profiles. The Pearson correlation coefficient between radar and Aeolus wind for Rayleigh(clear) and Mie(cloudy) are 0.93 and 0.94, respectively. The systematic and random errors in Rayleigh(clear) are found to be -0.15 m s(-1) and 4.87 m s(-1), respectively, while these values are -0.06 m s(-1) and 3.68 m s(-1) for Mie(cloudy). A detailed error characterization of Aeolus wind profiles with respect to radar is presented in this study. The bias in Aeolus wind is provided in terms of observation altitude, seasons, different windy conditions and ascending/descending orbits.

The VHF radar operating at 205 MHz frequency installed at the Cochin University of Science and Technology (CUSAT) has been configured to probe the nighttime ionospheric field‐aligned irregularities (FAIs) over the near‐equatorial site at Cochin, India during the solar minimum periods. Even though the study period includes the minimum of the 24th and 25th solar cycles, a total of 40 nighttime irregularity events were observed at F‐region heights (150–500 km). Bottom‐type and bottom‐side irregularities were observed in this period, but no large‐scale topside ones at F‐region heights. The nighttime irregularities were most commonly observed at average heights of 200–250 km, accounting for 50% of the total events, and 27.5% of the irregularities preferentially occurred after sunset between 19 and 20 LT. While long‐duration irregularity events were not prominent during this period, irregularities that lasted for less than 50 min were predominant. To check the capability of the 205 MHz VHF radar in observing FAIs, the radar observations were compared with the European Space Agency's ionospheric observation data from the Swarm B satellite. Several derived parameters from the Swarm data set such as electron density ( N e ), background N e (b N e ), electron temperature ( T e ), rate of electron density (ROD), index of ROD (RODI), median vertical total electron content (mVTEC), absolute VTEC (aVTEC), rate of TEC (ROT), ROTI, Bubble Index, and Bubble Probability were utilized for characterizing the ionospheric irregularities. The observation from the Swarm B satellite agrees well with that obtained from the VHF radar.

The low-latitudinal cyclones (LLCs, originating between 5°N–10°N) constitute ≈40% of tropical cyclones (TCs) formed in the Bay of Bengal (BoB). We investigate the interannual variability of post-monsoonal (October to December) BoB LLCs and their teleconnection with El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). It is found that the years with the fewer number of BoB LLCs are associated with anomalous equatorial easterlies that are largely connected with the El Niño and positive IOD. Likewise, equatorial westerly phases, often associated with the La Niña and negative IOD years, favour the LLC formation by providing the initial spin-up required for cyclogenesis. This teleconnection between ENSO/IOD and BoB TC frequency is confined in the low-latitudinal region with little influence for cyclogenesis north of 10°N during ENSO and IOD except during negative IOD. These results may help extend the lead time and improve the seasonal prediction of BoB TCs.

A severe thunderstorm developed on May 18, 2017 over the Kochi ( $$10.03^\circ \hbox {N}$$ , $$76.33^\circ \hbox {E}$$ ), a coastal region located in the southwest Peninsular India, with overshooting tops as high as 16 km. Observations made using stratosphere troposphere (ST) radar located at Kochi during May 17–19, 2017 show that strong convection reached the tropopause height during this event, and considerable mixing has happened. This paper describes the exchange between the stratosphere and troposphere during this thunderstorm event, a few days prior to the onset of Indian Summer Monsoon. The air near the vicinity of the tropopause was characterised by notably high concentrations of humidity and $$\mathrm {CO}$$ . A tongue of air with stratospheric characteristics lay below the tropopause, showing that extensive stratosphere–troposphere exchange had occurred. The effects of such a mechanism on atmospheric budgets of trace species in the stratosphere may alter the lower stratosphere’s chemistry. Detailed estimates of the fluxes are also presented in the paper.

The Indian summer monsoon rainfall shows intraseasonal oscillations manifested as active and break phases, and depending on the phase, the water vapor concentration in the tropical tropopause layer (TTL) varies. Using dedicated Lagrangian backward trajectory analysis and circulation dynamics within the Asian monsoon anticyclone, this study investigates in detail the factors governing water vapor in the TTL during the active and break phases. Backward trajectory simulations have been performed using wind field and heating rate from ERA5 re-analysis data and the convective sources in the TTL are estimated from high resolution satellite measurements. A water vapor maximum in the TTL is found from Iran to the Tibetan Plateau during the active phase, whereas it is concentrated mostly over the Tibetan Plateau during the break phase. This study shows that the differences in the water vapor concentration in the TTL between these phases are primarily caused by the variability in the strength of Asian monsoon anticyclone and convective sources. At TTL, the influence of convective sources from the ocean on water vapor is small and continental regions dominate, mainly from the Indian subcontinent and the Tibetan plateau. The time taken (age) for the vertical transport of air parcels from major convective sources to the 380 K isentropic level, that is close to the thermal tropopause, is estimated to be around 3 weeks.

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.

The response of the Tonga volcanic eruption on the ionospheric F-region over the near-equatorial sector has been studied using the 205 MHz VHF radar at Cochin, Kerala, India. The Tonga eruption, a massive underwater volcanic event that occurred at Hunga Tonga-Hunga Ha'apai in the Pacific Ocean, started on 14th December 2021 and ended with a giant explosion on 15th January 2022. The inter-nal gravity waves (IGWs) generated due to the eruption reverberated over the globe. The effect reached the ionosphere, creating spread-F irregularities. The activity was observed with the help of the 205 MHz VHF radar and other supporting instruments such as automatic weather station (AWS), Global Navigation Satellite System (GNSS) receiver, and Swarm Charlie (Swarm C) satellite. Enhanced surface pressure of 2 hPa was recorded at Cochin and adjoining areas due to the direct (i.e., wave along the shorter great circle path) gravity wave (GW) on 15th January. The shorter great circle path wave required 10 h 50 min to reach the Indian near-equatorial sector whereas, its antipodal counterpart wave (wave along the longer great circle path) reached Kerala on 16th January first as enhanced (0.54 hPa) and subsequently as reduced (0.8 hPa) surface pressure fluctuations. After the transit of IGWs on 15th January, a bottom-side F-region irreg-ularity echo with a plume-like structure lasting for -4.5 h occurred between 276 km and 550 km. Multi-constellation (GPS, GLONASS, and GALILEO) observations and Swarm C satellite data over the Indian equatorial ionization anomaly (EIA) sector showed quasi -periodic oscillations in the total electron content (TEC) and amplitude scintillations resulting from the IGWs of Tonga explosion. (c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

Remote factors have a significant influence on the variability of Indian summer monsoon rainfall. The study investigates atmospheric conditions influenced by the El Niño episodes on the variable summer monsoon over India. Four composites of years were selected based on the occurrence / non-occurrence of El Niño and extreme monsoons over India. This study examines the upper tropospheric and lower tropospheric wind patterns, tropospheric temperature, and velocity potential for 1958 to 2019 using Japanese Reanalysis 55 (JRA 55) data. The regional meridional circulation established by the north-south pressure gradient is more susceptible to the monsoon condition over the Indian Subcontinent than the external factors. The spatial variability and intensity of Australian High in the southern hemisphere play a prominent role in the Indian summer monsoon characteristics. Subtropical circulations in both hemispheres influence monsoon conditions over India irrespective of El Niño condition. The tropical zonal circulation has appeared to be modulated by the El Niño Southern Oscillation (ENSO) condition over the tropical Pacific Ocean, thereby modify the monsoon circulation.

Abstract The study illustrates the evidence of dynamical coupling between the high‐latitude sudden stratospheric warming (SSW) events that occurred in three consecutive winter seasons and the concomitant changes in the wind pattern in the lower stratosphere and troposphere observed with a state‐of‐the‐art 205 MHz stratosphere‐troposphere wind profiling Radar, located at Cochin (10.03°N, 77.33°E), a tropical station in southwest peninsular India. Associated with the occurrence of SSW, the tropical region experiences a change in zonal wind speed and direction at upper tropospheric altitudes after the central day of warming. The zonal wind in the troposphere also reversed its prevailing direction. Downward propagation of zonal wind during the SSW period is noticed, which is also supported by the reanalysis data set. In the lower troposphere, vertical wind experienced sudden fluctuations with varying amplitudes soon after the peak day of the SSW event. The surface meteorological parameters in an area (8°–12°N, 74°–78°E), centered at Cochin, from ERA‐Interim are also examined. An abrupt fall in outgoing longwave radiation, followed by convection and unusual rainfall in the tropical region 5–6 days before the peak of SSW events, is a unique and exciting feature noted in the study. Detailed analysis of the past 20 years of SSW events confirmed the observational evidence of unusual rainfall over the low latitude region related with the onset of SSW. Comprehensive observational and numeric modeling studies are needed to understand the mechanism for the SSW events in high latitudes and the associated convective activity and precipitation in the low latitude region.

Kerala, located at the southwest tip of India, witnessed disastrous floods during the monsoon seasons of two consecutive years, 2018 and 2019. This paper provides a detailed analysis of these two flood events using data from multiple sources. The unusually unstable and convective nature of the 2019 event, as detectable in its higher cloud tops and evidently fuelled by anomalously warm local sea temperatures, raises concerns regarding the changing patterns of rainfall over the southern parts of the west coast of India. Specifically, our analysis reveals that the flood of 2019 in Kerala satisfies the criteria for a mesoscale cloudburst (MsCB) event, more common in the north but a very rare and never before reported phenomenon in the Kerala region. Rainfall exceeding 50 mm in 2 h has been reported from many places between 8.00 and 22.00 UTC on the August 8, 2019. Satellite-derived rainfall and cloud microphysical parameters further reveal the uniqueness of the 2019 MsCB event. If 2019 is a harbinger of how global warming may continue to affect this region, transformations of the cloud structure and the recurrence and character of intense rainfall events could pose a major threat to the highly vulnerable Western Ghats ecosystems.

A stratosphere–troposphere (ST) wind profiler radar operating at a high VHF range (205 MHz) installed at Cochin (10.03°N, 76.33°E), India, provides high‐resolution winds in time and altitude extending from 315 m to 20 km. We evaluated the horizontal and vertical winds obtained from atmospheric global reanalyses (ERA5, ERA‐Interim, MERRA‐2 and NCEP) data and the high‐resolution regional model (WRF) output winds with the ST radar (STR) wind observations at Cochin. The data for one whole year, from 1 August 2018 to 31 July 2019, for 0600 UTC and 1200 UTC were used. Wind data appraisal has been carried out at 24 levels from 315 m to 18.5 km altitude for the reanalyses datasets and 29 levels for model‐simulated winds in different seasons. Vertical profiles of horizontal winds from reanalysis and model‐simulated data somewhat catch the major changes observed from STR. The zonal winds match very well with the STR, whereas meridional winds show moderate similarity. Higher correlations in zonal winds are found during the summer monsoon, whereas meridional winds show apparent likeness during winter. The ERA5, ERA‐Interim, and model‐simulated winds have better agreement with the STR winds. Observed vertical winds from STR displays large fluctuations, while the estimated vertical winds in reanalysis are highly smoothed. Vertical profiles of vertical winds from the reanalysis datasets are not uniform. The reliability of vertical winds in reanalysis and WRF model output values are dubious compared to that of the directly observed winds from STR. The study indicates that improved quality of vertical and meridional winds in reanalysis data is needed by assimilating directly observed data from the available advanced facilities.

The year 2019 experienced an excess monsoon season over the Indian region, with the seasonal rainfall being 110 % of the long period average (LPA). Several zones across the country suffered multiple extreme rainfall events and flood situations resulting in a massive loss of life and property. The first half of 2019 experienced a moderate El Nino Modoki event that lasted till midsummer. Another important feature of 2019 was the strongest recorded positive Indian Ocean Dipole (IOD) that lasted approximately seven months from May to November. This study has examined the reasons for the intra-seasonal variability of rainfall over India during the 2019 monsoon using available remote sensing and reanalysis data. Our analysis has shown that the presence of El Nino and the formation of a very severe cyclonic storm (VSCS) in the Arabian Sea were unfavorable for the monsoon onset and its northward advancement during June. However, the Walker circulation associated with El Nino helped strengthen the IOD developed early in the Indian Ocean, much before the monsoon onset. The anomalously strong IOD strengthened the monsoon circulation during July-September and resulted in excess rainfall over India.

A wind profiler radar at 205 MHz is operational since January 2017, at Cochin ($$10.04^\circ \hbox {N}$$; $$76.33^\circ \hbox {E}$$), a region lying in the west coast of Southern Peninsular India, which also is the entry point of the Indian summer monsoon. Using the radar wind profiles obtained during April to September, the detailed vertical structure of wind during the pre-monsoon and monsoon period was studied for the years 2017 and 2018. The gradual transition from pre-monsoon to monsoon season as manifested by the development of monsoon circulations in the lower and upper troposphere is well captured by the radar observations. Parameters which characterize the strength of monsoon circulations have been derived which are shown to be potential predictors for declaring the monsoon onset over Kerala in an objective manner. The monsoon circulation during the year 2018 was studied in detail in the backdrop of extreme heavy rainfall over Kerala. It is observed that there is an anomalous decrease in the core height, but with high core speed in the Low-level Jet stream (LLJ) during 2018 as compared to year 2017. Owing to this unique placement of LLJ, it can be concluded that intense orographic lifting could have played a role in causing heavy rainfall over Kerala in 2018. The transitions in LLJ prior to heavy rainfall over south-west coast are aptly captured by the radar observations which opens up the possibility of predicting heavy rainfall events through continuous monitoring of monsoon circulation using radar.

The present study analyses the characteristics of remotely sensed wind data from a VHF wind profiler to understand the features of the monsoon low level jet during the historical deluge that occurred in Kerala, India in the monsoon months of 2018. A major flood-induced disaster occurred during the period 09-16 August 2018. The sophisticated radar installed at the Advanced Centre for Atmospheric Radar Research (ACARR), Cochin University of Science and Technology, Kerala, and operating at 205 MHz, provides high spatial (45 m) and temporal (~10 minutes) data of all the three wind components. Deep convective clouds which were advected from the eastern Arabian Sea led to the unprecedented flood situation, which was obvious from the wind profiler data. The radar data was screened for data quality and examined for its daily variation during the flood event. It is observed that the monsoon low level jet showed an increasing tendency during the major event, and crossed a threshold value of around 18 ms on many days. The Radar is capable of acquiring vertical profiles of all the three components of wind from 315 meter to beyond 20 km, and the vertical shear also could be studied. On many occasions, the depth of the westerlies was large enough (> 7 km depth) to bring in ample moisture on to the land area from the adjoining Arabian Sea. It is observed that the high content of moisture due to enhanced speed and depth of westerlies could create havoc in the State. Detailed results with a numerical weather prediction model (WRF) is also presented. The Model could simulate the large-scale features reasonably well over the State. A comparison is made between winds obtained from the WRF and from the Radar.