The present study investigates the characteristics of elevated aerosol layer (EAL) using high spatio-temporal Boundary Layer Lidar observations over Manora Peak (29.4 degrees N; 79.2 degrees E; 1958m above mean sea level), Nainital located in the Himalayan region. We have chosen four EAL cases on June 17, 2006 (C1); December 12, 2007 (C2); February 06 (C3) and May 07, 2008 (C4) over the observational site. The occurrence of EAL is generally found at an altitude similar to 2-4 km above ground level over the site. It is found that the westerly/northwesterly winds with magnitudes of about 6-9 ms(-1) are conducive for the generation and strengthening of EAL. A 7-days airmass backward trajectory analysis reveals that the lower (higher) altitude source originates from the west coast (northwest) regions. Furthermore, the atmospheric radiative forcing estimated using Santa Barbara DISORT (discrete ordinates radiative transfer) Atmospheric Radiative Transfer (SBDART) model reveals that the atmosphere heats up with 1.35 K day(-1) and 1.32 K day(-1) due to the presence of EAL on C1 and C4, respectively. Our results confirm that the occurrence of EALs causes atmospheric warming, that can impact on the regional radiation budget over the Himalayan region.

Turbulence in the atmosphere plays a vital role in controlling the surface, lower and upper tropospheric dynamics. Here, we have utilized a newly installed Aryabhatta Research Institute of Observational Sciences (ARIES) Stratosphere Troposphere (ST) radar at the high‐altitude subtropical site in the central Himalayan region (Nainital, 29.4°N, 79.5°E, 1793 m above mean sea level) for the first ever estimation of turbulence parameters from this unique location. We have used radar observations made in years 2017 and 2019 with simultaneous and colocated global positioning system (GPS) radiosonde observations. In this context, turbulence parameters like turbulent kinetic energy dissipation rate, and eddy diffusion coefficient due to thermal and momentum fluctuations, have been determined by using (i) wind variance, (ii) Doppler spectral width, and (iii) backscatter signal power methods as well as synergistic radiosonde measurements using Thorpe length scale method. The kinetic energy dissipation rate and eddy diffusivity coefficients were found to be as high as 10−2 m2/s3 and 102.6 m2/s, respectively. Statistical distribution of turbulence parameters derived from radar and radiosonde was found to agree reasonably well in terms of measures of central tendency. The refractive index structure constant (Cn2) shows a decreasing tendency with height, and it is found to vary as large as 10−14 to as small as 10−19 m−2/3. Range and temporal variation of signal‐to‐noise ratio (SNR) indicated the existence of a stable layer around 8 km height. It is also evident from the present study that the turbulence parameters at this central Himalayan region of complex terrain are higher by 1 order of magnitude than those reported from the southern part of India.

The high-altitude regions in the Himalayas are prone to high ozone concentrations frequently resulting from diverse dynamical and transport mechanisms. Here, we report an unusual enhancement in the surface and tropospheric ozone concentrations over the central Himalayan region from ground-based and space-borne measurements in the month of December 2010. The surface ozone levels (similar to 80 ppbv) on 18-19 December 2010 is observed to be two-fold higher relative to the seasonal average (December-January-February) of about 40-50 ppbv in the central Himalayan region. The space-borne measurements from Tropospheric Emission Spectrometer and Ozone Monitoring Instrument onboard Aqua satellite also show higher values in the tropospheric column ozone over this region. The satellite observations indicate an increase in tropopause temperature of about 5 degrees C and decrease in tropopause altitude about 1 km during 18-19 December 2010 resulting in the occurrence of tropopause fold facilitating the stratospheric-tropospheric exchange processes over the study region. The plausible reason for the occurrence of tropopause fold and subsequent enhancement of tropospheric and surface ozone is found to be associated with the breaking Rossby waves in the upper troposphere. The wave breaking leads to the advection of high-PV (potential vorticity) air, with magnitudes of about 3-4 PVU, towards the central Himalayan region from high-latitudes. The vertical component of PV advection also shows a deep stratospheric intrusion of high-PV air into the troposphere. The isentropic transport of ozone across the folding tropopause due to the wave breaking is clearly depicted from the satellite and reanalysis datasets. Therefore, the present study has strong implications of upper tropospheric wave dynamics to the tropospheric and surface ozone over the Himalayan regions having complex topography.

In this report, an effort for the first time has been made to investigate the effect of the total Lunar Eclipse that occurred on 27th July 2018 (the longest lunar eclipse in the 21st century) on the D-region ionosphere. Very Low Frequency (VLF) navigational transmitter signals recorded at Prayagraj (formally Allahabad) station in India is the primary dataset used. The signals from two VLF transmitters: NWC (19.8 kHz) from Australia, and JJI (22.2 kHz) from Japan, have been utilized. When compared to unperturbed night, NWC signal amplitude showed an anomalous decrease of similar to 0.86 dB, while JJI amplitude showed an anomalous increase similar to 0.26 dB during the lunar eclipse. The Wait's lower ionospheric parameters: reflection height (H') and sharpness factor (beta) are estimated by using a rather coarse method of modeling of VLF signals anomalies on eclipse day, which comes out to be H' = 85.25 km & beta = 0.689 km(-1) and H' = 84.59 km & beta = 0.683 km(-1) respectively for NWC and JJI path. Using wait parameters, the electron density (N-e) profile is estimated and this showed a decrease in N-e during eclipse time with reference to the nighttime ambient values. The N-e values for the NWC path, showed relatively higher decrease compared to the JJI path. The plausible reasons for observed N-e decrease on lunar eclipse night could be due to the reflected lunar Lyman-c( and X-ray's radiation which may contribute to the nighttime ionization is blocked during the lunar eclipse, thereby decreasing the nighttime lower ionospheric N-e. (C) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

In order to understand the air quality and plausible sources of atmospheric pollutants over the Emirate of Abu Dhabi (EAD), a detailed analysis is performed by utilizing three years of simultaneous measurements of various gaseous pollutants such as SO2, NO2, CO, and O-3 along with PM2.5 and PM10 concentrations during the period 2011-2013. The diurnal variation of O-3 and SO2 shows noontime maximum in urban and rural regions during all seasons, while NO2 and CO show higher levels in the morning and evening. During winter, maximum (minimum) O-3 (NO2) is observed in the rural region and in contrast, the urban region depicts maximum in spring and summer both during day and night. Large variability is observed during the day and night for all pollutants. The PM(2.5 )concentration shows similar diurnal variation as in the case of CO and NO2, peaking during daytime in winter and spring in the urban region, while the nighttime peak values are prominent irrespective of the season. It is also to be noted that the minimum (maximum) PM10 values are observed in the urban (rural) regions during daytime in all seasons except in winter. The frequency distribution of the hourly mean concentrations of the primary pollutants in the urban and rural regions elucidates that the observed discrepancies in the background concentration levels of different pollutants may be due to different atmospheric conditions. The observations also reveal that the linear relationship between maximum ozone concentration and maximum temperature might not be valid above 313 K. The present observations are discussed in the light of current understanding of the distribution of trace species and plausible sources over EAD region.

It has been known that the atmospheric gravity waves (AGWs) play a major role in shaping up the structure and dynamics of lower, middle and upper atmosphere [1, 2], and relates to various phenomena in the thermosphere. VLF remote sensing is a well-established technique to monitor D-region conditions, including AGWs. We have analyzed various VLF navigation transmitter signals recorded at VLF recording station in North Carolina, USA, called PARI (35.2 N, 82.9 W) for a one-year period from May 2015 to April 2016. The VLF signal (both amplitude and phase) show wave like signature (WLS) in different Transmitter-receiver great circle path (TRGCPs). We have found three event days namely 02 May 2015, 21 January 2016 and 21 April 2016, with periodic variations of 1.2 -3 minutes. One example of observed Wavy signature on VLF transmitter on 21 January 2016 is shown as Figure 1. In general, the period of AGWs is longer than the background oscillations known as Brunt-Vaisala period (BV period) but in the present case observed periods are much lower than the BV periods (~5 minutes). Initial analysis suggests that the probable source of observed WLSs is convective (from lightning discharges/thunderstorm) generated AGWs propagating upward from troposphere. The source regions are identified using lightning location network and satellite data. The more details on the properties and possible source are discussed.

Hight-resolved observations of vertical winds remain nearly non-existing over the Himalayas, despite of anticipated crucial role of vertical motions in transporting pollution across the Himalayan hills. The present study analyze the vertical wind observations from surface to 1 km above ground level over Manora Peak (29.4 degrees N; 79.5 degrees E; 1958 m amsl) in the Himalaya performed using a Doppler Lidar during the Ganges Valley Aerosol Experiment (GVAX). Vertical wind exhibited a pronounced diurnal variability at Manora Peak comprising of upward motions during the daytime (05-10 UT) and downward motions during nighttime typical of a mountain valley system. Mean vertical wind speeds are observed to be varying from -0.8 to + 0.8 ms(-1) during the study period with a variance of 0.1-1.5 m(2)s(-2), which is attributed to the thermally driven turbulence. Mean vertical winds are observed to be stronger in the Doppler Lidar profiles above Manora Peak (-0.8 to 0.8 ms(-1)) as compared to near surface measurements at this station using an ultrasonic anemometer (-0.4 to 0.4 ms(-1)), and low altitude stations in India. Daytime vertical wind speeds are observed to be higher during pre-monsoon (0.81 ms(-1)), as compared to post-monsoon (0.24 ms(-1)) and winter (0.33 ms(-1)). Average Black Carbon (BC) concentrations are significantly higher during strong upward vertical winds, which indicates efficient transport of polluted air mass from low-altitude regions to the Himalaya. Weather Research and Forecasting (WRF) model reproduces the observed diurnal pattern in the vertical wind at the observation site however the model underestimates the variability.

A diagnostic investigation of an extreme rainfall episode that occurred over the central and north Indian region is carried out in this study using data from a suite of observations from space-borne instruments and the reanalysis datasets. This event is unique in the sense that the organized tropical and extratropical forcing stimulated the intense rainfall on 01 Jan 2012. The WindSat (multi-frequency polarimetric microwave radiometer) observations indicate the source of the moisture flux coming from the adjoining tropical Ocean. The dynamical and thereto-dynamical contributions are evaluated based on the atmospheric instability analysis using the Atmospheric Infrared Sounder (AIRS) and reanalysis datasets by computing various stability indices such as total totals (TT) index, Potential Vorticity (PV), static stability and the Convective Available Potential Energy (CAPE). High TT index values ( > 40 K) are observed both in satellite and reanalysis data indicating thermodynamic instability. PV intrusion to low latitudes is also observed with extreme rainfall occurrence ahead of the PV tongue. The vertical structure of PV intrusion shows remarkable features with enhanced upward motions ahead of the intrusion representing the dynamical instability. The reduced static stability, increased CAPE and upper-level cyclonic anomalies together with enhanced moisture in the lower troposphere coming from the adjoining tropical Indian Ocean regulate the amplitude and region of occurrence of the extreme rainfall. Therefore, this study identifies the significant implications of tropical and extra-tropical influences that generate the thermodynamical and dynamical instabilities for the occurrence of the extreme rainfall event over the Central and Northern parts of India.

Turbulence in the atmosphere play vital role in controlling the surface, lower as well as upper tropospheric dynamics both during day and nighttime and also depicting both spatial and vertical inhomogenities over a complex terrains like Himalayan region. Turbulence mostly occurs due to degeneration of mountain induced gravity waves and acts a potential source of vertical mixing in the atmosphere (Dornback, 1998). Inaccurate estimation of turbulence parameters will also have immense effect on the aviation sector in the complex mountainous regions. Another important application of the estimation of turbulence parameters is for astronomical observations since it determines the key parameters of adaptive optics systems and precision of astrometric measurements. Characterisation of turbulence in mountainous region is also vital to understand the dynamics of moxuntain waves and other mesoscale phenomenon which has crucial role in modulating the general circulation wind patterns (Palmer,1987). The flow conditions associated with the turbulence includes high vertical velocities, wind shear and low Richardson number which can be well measured with high vertical and temporal resolutions of wind fields (Horizontal and Vertical) by Stratosphere-Troposphere Radars. In this context, Estimation of Turbulence parameters like Turbulent kinetic energy dissipation rate, ε and eddy diffusion coefficient, K_m will be determined by using standard Wind Variance method. This method utilizes the temporal spectrum of vertical wind in the inertial sub range demarcated by Brunt-Vaisala frequency and Nyquist frequency. Incorporating this method these parameters were earlier reported for tropical site of Gadanki using MST radar observations by Satheesan and Krishna Murthy [1, 2]. The validity of these methods will be tested with the conventional methods using (i) Doppler spectral width and (ii) backscatter signal power.

We present observations of very short period (<5 min) wavy fluctuations (WFs) in the lower ionosphere (75–85 km) on the night of 21 January 2016, using subionospherically propagating very low frequency signals. Four out of six transmitter signals recorded by the very low frequency/low frequency receiver at the Pisgah Astronomical Research Institute showed WFs simultaneously. However, their time of occurrence and intensities were different. Power spectral analysis indicates a period of ~3–4 mins, largely associated with the two regions of strong convective and lightning activity in the lower troposphere. Background wind (at ~15 and 80 km) direction shows westward propagation, suggesting important role of the convective storm located east of Pisgah Astronomical Research Institute. The GPS total electron content analysis also divulges similar WFs at upper ionospheric altitude (~300 km) on the GPS total electron content stations around the two storms. The observational analysis suggests atmospheric gravity waves from the convective regions propagate upward up into the ionospheric altitudes. Hence, the present study reinforces the strong coupling of troposphere and ionosphere through the convectively generated very short period waves.

The subject of pre-earthquake ionospheric signatures has always been contentious and debatable. Some of the previous reports have documented unforeseen and unusual variations in some of the atmospheric and ionospheric parameters well before an earthquake. Here, we analyze the ionospheric response from the Indian Subcontinent to Nepal Gorkha Earthquakes occurred between April and May 2015, which were the most powerful and disastrous natural calamities in past ~80 years over the Himalayan region left ~9000 causalities and more than ~20000 people injured with the property damage of the order of several billion dollars. In view of severe earthquakes occurrences, their prior information on the shorter time scales are warranted for mitigation of associated disasters. Here, we report for the first time, a case which shows a strong link in anomalous variations between VLF sub-ionospheric signal and mesospheric ozone prior to both April 25, 2015 (Mw = 7.8) earthquake and its biggest aftershock on May 12, 2015 (Mw = 7.3). Observations show an unusual variation in VLF signals amplitude /shift in terminator time (TT) strongly linked with positive (negative) mesospheric ozone anomaly in D-region altitudes prior to the Gorkha Nepal earthquakes. It is surmised that simultaneous continuous observations of both VLF waves and mesospheric ozone can be considered as an important tool to identify the prior earthquake signatures in the vicinity of the extremely earthquake-prone zone such as Himalayan region. In this context, the current report opens up a new dimension in lithosphere-atmosphere-ionosphere coupling during the earthquake preparation processes itself.

ABSTRACTA preliminary study was performed to investigate the quality of total column ozone (TCO), water vapour (WV) and aerosol optical thickness (AOT) available from satellite and reanalysis from atmospheric models along with in situ observations over Ahmedabad (23.03 ° N, 72.5 ° E, 55 m above mean sea level), India. Ground‐based measurements from a MICROTOPS II ozonometer as well as space‐based satellite retrieved products from the moderate resolution imaging spectroradiometer (MODIS) and an ozone monitoring instrument were analysed during December 2014 to March 2015. The European Centre for Medium‐Range Weather Forecasts (ECMWF) reanalysis (ERA‐Interim) created TCO and WV were also used to assess the skill of global model reanalysis over the western part of India. An increasing trend was found in the TCO and WV parameters from the winter to summer period. Investigations showed that MODIS satellite retrieved and ERA‐Interim reanalysis WV are able to capture the trends compared with ground‐based hand‐held MICROTOPS II observations. Higher TCO was found in the MODIS data, whereas the ERA‐Interim and ozone monitoring instrument TCO matched well with ground‐based observations. Larger differences were found with the MODIS AOT in comparison to MICROTOPS II AOT observations. Further, analysis showed that ERA‐Interim has sufficient potential to be used for various meteorological applications over the western Indian region. Furthermore, the variations of TCO, WV and AOT were studied over the orographic region of Mount Abu (∼110 km aerial distance from Ahmedabad), located in the Aravali range of mountains. Ground‐based measurements at different altitudes (0.3, 1.1 and 1.67 km above mean sea level) revealed significant variations in WV and AOT. The WV varies 17.1% (30.3%) and the AOT varies 53.8% (57.5%) in 1.1 km (1.67 km), whereas no noteworthy variations are observed in the TCO in this field campaign.

We present D region ionospheric response to 22 July 2009 total solar eclipse by modeling 19.8‐kHz signal from NWC very low frequency (VLF) navigational transmitter located in the Australia. NWC VLF signal was received at five stations located in and around eclipse totality path in the Indian, East Asian, and Pacific regions. NWC signal great circle paths to five stations are unique having eclipse coverage from no eclipse to partiality to totality regions, and the signal is exclusively confined in the low and equatorial regions. Eclipse‐induced modulations in NWC signal have been modeled by using long‐wave propagation capability code to obtain D region parameters of reflection height (H′) and sharpness factor (β). Long‐wave propagation capability modeling showed an increase in H′ of about 2.3 km near central line of totality, 3.0 km in the region near to totality fringe, and 2.4 to 3.0 km in the region under partial eclipse. Using H′ and β, Wait ionosphere electron density (Ne) profile at the daytime altitude of 75 km showed a decrease in Ne by about 58% at a station near totality central line, whereas at totality fringe and in partial eclipse region decrease in the Ne was between 63% and 71% with respect to normal time values. The eclipse associated variations in the H′, β, and Ne are less in low‐latitude region as compared to midlatitude. The study contributes to explain observations of wave‐like signature in the D region during an eclipse and difference in the eclipse effect in the different latitude‐longitude sectors.

In this study, we investigate the causative processes responsible for the observed enhancement in the tropospheric and surface ozone during December 09–11, 2008 orography induced gravity wave event over Himalayan region. The analysis is done using surface ozone measurements and satellite datasets from Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit-A (AIRS/AMSU-A), COSMIC, TES and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Observations depict a two fold increase in surface and tropospheric ozone during the event as compared to normal days in both AIRS and TES ozone measurements. COSMIC temperature perturbations show generation of shorter vertical wavelengths efficient for the sub-tropical tropopause folding due to orography induced gravity waves. Moreover, the intense tropopause folding as evidenced by upward-downward vertical velocities couplet could trigger the intrusion of stratospheric ozone rich air into upper tropospheric ozone poor air as also confirmed by high values of potential vorticity during the observational period. Hence, present study reemphasizes the importance of wave induced atmospheric dynamics on atmospheric constituents' especially tropospheric ozone over Himalayan region.

Abstract. We present the measurement of cloud base height (CBH) derived from the Doppler Lidar (DL), Ceilometer (CM) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite over a high altitude station in the central Himalayan region for the first time. We analyzed six cases of cloud overpass during the daytime convection period by using the cloud images captured by total sky imager. The occurrence of thick clouds (> 50 %) over the site is more frequent than thin clouds (< 40 %). In every case, the CBH indicates less than 1.2 km, above ground level (AGL) observed by both DL and CM instruments. The presence of low level clouds in the height-time variation of signal to noise ratio of DL and backscatter of CM shows a similar diurnal pattern on all days. Cloud fraction is found to be maximum during the convective period. The CBH estimated by the DL and CM showed reasonably good correlation (R2 = 0.76). The DL observed updraft fraction and cloud base vertical velocity also shows good correlation (R2 = 0.66). The inter-comparison between DL and CM will have implications in filling the gap of CBH measurements by the DL, in absence of CM. More deployments of such instruments will be invaluable for the validations of meteorological models over the observationally sparse Indian regions.

This study examines the link between upper-tropospheric planetary-scale Rossby waves and surface meteorological parameters based on the observations made in association with the Ganges Valley Aerosol Experiment (GVAX) campaign at an extratropical site at Aryabhatta Research Institute of Observational Sciences, Nainital (29.45° N, 79.5° E) during November–December 2011. The spectral analysis of the tropospheric wind field from radiosonde measurements indicates a predominance power of around 8 days in the upper troposphere during the observational period. An analysis of the 200 hPa meridional wind (v200 hPa) anomalies from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis shows distinct Rossby-wave-like structures over a high-altitude site in the central Himalayan region. Furthermore, the spectral analysis of global v200 hPa anomalies indicates the Rossby waves are characterized by zonal wave number 6. The amplification of the Rossby wave packets over the site leads to persistent subtropical jet stream (STJ) patterns, which further affects the surface weather conditions. The propagating Rossby waves in the upper troposphere along with the undulations in the STJ create convergence and divergence regions in the mid-troposphere. Therefore, the surface meteorological parameters such as the relative humidity, wind speeds, and temperature are synchronized with the phase of the propagating Rossby waves. Moreover, the present study finds important implications for medium-range forecasting through the upper-level Rossby waves over the study region.

We present the measurements of cloud-base height variations over Aryabhatta Research Institute of Observational Science, Nainital (79.45 degrees E, 29.37 degrees N, 1958 m amsl) obtained from Vaisala Ceilometer, during the nearly year-long Ganges Valley Aerosol Experiment (GVAX). The cloud-base measurements are analysed in conjunction with collocated measurements of rainfall, to study the possible contributions from different cloud types to the observed monsoonal rainfall during June to September 2011. The summer monsoon of 2011 was a normal monsoon year with total accumulated rainfall of 1035.8 mm during June-September with a maximum during July (367.0 mm) and minimum during September (222.3 mm). The annual mean monsoon rainfall over Nainital is 1440 +/- 430 mm. The total rainfall measured during other months (October 2011-March 2012) was only 9% of that observed during the summer monsoon. The first cloud-base height varied from about 31 m above ground level (AGL) to a maximum of 7.6 km AGL during the summer monsoon period of 2011. It is found that about 70% of the total rain is observed only when the first cloud-base height varies between surface and 2 km AGL, indicating that most of the rainfall at high altitude stations such as Nainital is associated with stratiform low-level clouds. However, about 25% of the total rainfall is being contributed by clouds between 2 and 6 km. The occurrences of high-altitude cumulus clouds are observed to be only 2-4%. This study is an attempt to fill a major gap of measurements over the topographically complex and observationally sparse northern Indian region providing the evaluation data for atmospheric models and therefore, have implications towards the better predictions of monsoon rainfall and the weather components over this region.

This work presents a detailed study of the dynamical processes triggering the occurrence of the two heavy dust storms which occurred between 18 and 22 March 2012 over the Middle East. The dynamics of this event are related to the coupling of subtropical jet and polar jet over the Saudi Arabia region, resulting in massive dust storm generation and dust transport through Rub’ al Khali and the Persian Gulf to the UAE region. AOD and PM10 values showed a fourfold increase during the event reaching a maximum of 1.8 and 1653μg/m3 respectively. The spatial extent of the dust storm is evident from high values of MODIS AOD (~1.5) and OMI aerosol index (4.5) covering the entire Middle East. The total attenuated backscatter at 550nm from CALLIPSO showed the vertical extent of dust up to 8km. In addition, surface temperature showed a decrease of almost 15°C during the event signifying the intensity of the dust storm. Aerosol radiative forcing estimates during the dust storm showed a cooling at the surface and warming in the atmosphere, with a maximum forcing value reaching up to ~−210Wm−2 (185 Wm−2). Hence, it is evident from the present study that the dust layer caused an additional warming of ~150Wm−2 in the atmosphere over this region. The present event showcases the importance of dust storm induced aerosol optical and physical processes, and associated atmospheric dynamics over UAE as well as other affected regions.

In this report, we attempt to quantify direct aerosol radiative forcing by considering the diurnal variation of aerosols over central Himalayan region. The measured day time aerosol optical depth (AOD) values are higher by a small magnitude ≤15% during forenoon and reached as high as 90% by late afternoon when compare to night time AOD. The above observation gives clue about transport of regional polluted aerosols to the observational site. Our results show, 10 (16) % increment in atmospheric radiative forcing due to diurnal variation of aerosols instead of average aerosols during winter (post-monsoon) season.