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Diurnal variation of atmospheric water vapour in the troposphere over two tropical stations Trivandrum (8.5°N, 76.9°E) and Gadanki (13.5°N, 79.2°E) are studied using radiosonde observations for three years (December 2010–March 2014) carried out as part of Tropical Tropopause dynamics (TTD) campaigns under CAWSES (Climate And Weather of the Sun-Earth System) India Phase II program. Trivandrum is a coastal station and Gadanki an inland station in the Indian Peninsula. Even though the absolute value of columnar integrated water vapour content (IWV) is higher over Trivandrum in all the seasons, the amplitude of its diurnal variation is almost the same (∼± 5–8 kg m−2) at both the stations with lower values during the day and higher values during night. Though the absolute humidity decreases exponentially with altitude, the amplitude of normalized diurnal anomaly is more-or-less the same (15–20%) from the surface up to 10 km in all the seasons. There exists a time difference in the peaking hours of water vapour density between the two stations with Gadanki always leading Trivandrum. While the water vapour density maximizes around midnight over Trivandrum, it peaks in the late evening over Gadanki. Harmonic analysis shows that the diurnal and semi-diurnal components together account for almost all the sub-daily variations in IWV and absolute humidity. The contribution of semi-diurnal component is only 25% of that of the diurnal component. Diurnal variation of surface wind speed and direction together with the vertical motion associated with convection regulated by the diurnal variation in temperature seems to play the lead role in causing the diurnal variation in water vapour. While the diurnal variation over Gadanki is mainly controlled by the convectively induced vertical motion, the diurnal variation in the rate of evaporation due to surface wind and local circulation also plays a significant role over Trivandrum.

Influence of convection on the thermal structure of Troposphere and Lower Stratosphere (TLS) is investigated using radiosonde data, obtained from Trivandrum (8.5°N, 76.9°E), Gadanki (13.5°N, 79.2°E), Bhubaneswar (20.25°N, 85.83°E), Kolkata (22.65°N, 88.45°E) and Singapore (1.37°N, 103.98°E), collected during different convective categories classified based on the altitude of deep convective cloud tops (CT) in the period 2008–2014. During deep convective events, the temperature showed lower tropospheric cooling, an upper tropospheric warming and an anomalous cooling (warming) below (above) the cold point tropopause (CPT) with respect to the clear-sky value. While warming in the upper troposphere is strongest (∼2–4 K) around 10–12 km, anomalous cooling (warming) below (above) the CPT is maximum around 15.5 km (17.5 km) with values in the range of−2 to −4 K (3–6 K). These temperature perturbations are observed 5–6 days prior to the convective events. In response to deep convection, surface cooling up to ∼ −4 K is also observed. This study showed that the magnitude of cold and warm anomalies increases with strength of convection. During deep convection, the potential temperature (θ) shows a decrease (

The effect of deep convection on the thermal structure and dynamics of the tropical tropopause at sub daily scales is investigated using data from radiosondes launched over two sites in the Indian Monsoon region (Gadanki (13.5°N, 79.2°E) and Trivandrum (8.5°N, 76.9°E)) conducted between December 2010 and March 2014. The data from these soundings are classified into 5 convective categories based on the past, present and future cloudiness over the launching region after the radiosonde has reached tropopause altitude. They are denoted as category 1 (no convection), category 2 (convection may occur in any of the next 3 h), category 3 (convection occurred prior 3 h), category 4 (convection terminated within 3 h of launching) and category 5 (convection persistent throughout the considered period). The anomalies from the background in temperature, relative humidity and wind speed are grouped into the aforementioned five different convective categories for both the stations. Cooling and moisture anomalies are found during the active convection (category 5). The horizontal wind speed showed a strong anomaly indicating the presence of synoptic scale features. Vertical wind obtained simultaneously from the MST radar over Gadanki clearly showed strong updraft during the active convection. The ozone profiles from ozonesondes launched during the same period are also segregated according to the above convective categories. During the active convection, high and low ozone values are found in the upper troposphere and the lower troposphere, respectively. The cross tropopause ozone mass flux and vertical wind at the tropopause and convective outflow level estimated from the ozonesonde, and MST radar/ERA-Interim data showed positive values indicating the transport of ozone between troposphere and stratosphere during deep convection. Similarly, the total mass flux crossing the cold point tropopause over Gadanki showed upward flux during the active convection. The variability of the cross tropopause mass flux is found to be higher over Gadanki compared to Trivandrum.

Using 8 years (May 2006 to March 2014) of high resolution and high accuracy GPS radiosonde observations available from a tropical station Gadanki (13.5°N, 79.2°E), India, we have investigated the climatology of gravity wave energy and zonal momentum fluxes in the lower stratosphere. We also obtained best fit spectrum model for the gravity waves (GWs) for this tropical station. In general, strong annual variation in the energy and momentum flux with maximum during Indian summer monsoon is observed in the lower stratospheric region (18–25 km). By considering different source spectra, we have applied Gravitywave Regional or Global RAy Tracer (GROGRAT) model run on monthly basis using the source spectrum values at different altitudes on the ERA-Interim background fields to obtain the kinetic energy and zonal momentum fluxes for each of the spectra considered. These simulated fluxes are compared with the observed fluxes to arrive at the best fit spectrum model. It is found that the spectrum which represents the convection transient mountain mechanism that is purely anti-symmetric and anisotropic in nature is the best fit model for Gadanki location. This information would be useful in parameterization of the GWs in numerical models over Indian region.

The structure of tropical tropopause layer (TTL) is delineated based on static stability criteria. It is defined as the region extending from the level of minimum static stability to the level of maximum static stability. The static stability and dynamic instability steadily increase from the base to the top of the TTL. Radiosonde data from three tropical stations and Constellation Observing System for Meteorology, Ionosphere, and Climate GPS radio occultation (COSMIC) data over the tropics are used for this investigation. This study suggests that in terms of stability, the TTL can further be considered as a composite of three sublayers: a bottom layer (BL), a middle layer (ML), and an upper layer (UL) embedded between the potential temperatures ~350–360 K, ~360–380 K, and 380–420 K, respectively. While the BL is significantly influenced by frequent convective outflows, the influence of convection declines across the ML. Ozone and water vapor mixing ratios show a discernible change in their gradients across these sublayers. Occurrences of small-scale turbulence and cirrus are maximum in the BL and decrease in the ML and UL. Longitudinally, the BL is broad and the ML and UL are narrow over the deep convective cores. Strength of static stability in the UL (BL) is relatively strong (weak) centered about the equator, with pronounced features over the deep convective regions. These features of static stability centered about the equator in the BL could be attributed to the convective outflows in the troposphere and that in the UL to the dynamic and radiative processes in the upper troposphere and lower stratosphere.

We analyzed 17 years (1993–2009) of horizontal winds measured by the medium frequency (MF) radar located at Tirunelveli (8.7°N, 77.8°E) and 10 years (2005–2014) of horizontal winds measured by a meteor radar located at Thumba (8.5°N, 77°E) to examine the seasonal, inter-annual, and solar cycle variability of the Quasi-Two Day Wave (QTDW) in the mesosphere and lower thermosphere region. These two radars are nearly co-located, but differ in their measurement technique. Comparison of the estimated QTDW amplitudes by the two radars shows that the amplitudes are larger in the meteor radar than those in the MF radar. The difference between the amplitudes is larger in May in the zonal component and in April and September in the meridional one. Furthermore, the differences are larger in the meridional component. The QTDWs in both the radars show a strong semi-annual oscillation (SAO). In addition, the meridional QTDW amplitudes of both the MF and meteor radars show a distinct enhancement in the month of October. While the whole spectra of QTDWs contribute to the SAO amplitudes, only 45–50 h waves contribute to the October enhancement. The amplitudes of the QTDWs, in general, show large inter-annual variability. The QTDW amplitudes from both the radars show modulation at period of quasi-biennial oscillation. The QTDWs of the MF radar show a small negative correlation with solar activity while those of meteor radar do not show any correlation. The above aspects are discussed in the light of current understanding of the QTDWs.

In the present study, sources for the inertia–gravity waves (IGWs) that are observed in the troposphere and lower stratosphere during different seasons over a tropical station Gadanki (13.5°N, 79.2°E), India, have been identified. For this we have used long-term high resolution radiosonde observations of horizontal winds and temperature during May 2006 to March 2014. The IGW parameters are extracted using Stokes method. Rotary analysis is applied to the wind data to find the vertical direction of propagation. Dominant vertical wavelengths for these waves are obtained from FFT in the altitude domain as well as from apparent frequency method. The vertical and horizontal wavelengths are found to be in the range of 2–5 km (1–3 km) and 500–1500 km (1000–2000 km) in the troposphere (stratosphere), respectively. Using Stokes parameter method we have obtained the statistics of the number of the waves which are monochromatic. GROGRAT model with ERA-Interim data as the background atmosphere is used to identify the terminal (and source) points (sources) for the observed waves. In general, 53% of the waves observed over this location have convection as source and for only 38% of the cases vertical shear in the horizontal wind is identified as a source. For the rest of the 9% of the cases, these two sources are not found to be active.

In the present work, characteristics of 30–50 day oscillations (referred to as the MJO) in tropospheric and lower stratospheric wind and temperature have been studied using long-term high resolution radiosonde observations at a tropical station, Gadanki (13.5°N, 79.2°E) for the period 2006–2012. Wind and temperature perturbations showed clear features of the MJO with higher amplitudes between 10 and 18 km altitude. Interestingly, the MJO signal is confined vertically to different altitudes in different seasons. Variability in the perturbations of wind and temperature similar to that of outgoing long-wave radiation (OLR) with a few cases showing an out of phase relation. The amplitudes of these oscillations are larger in the winter and pre-monsoon seasons than in the monsoon season where the largest amplitudes are confined below the Tropical Easterly Jet (~16 km). There also exists a large inter-annual variability in the MJO. Spatio-temporal variability of OLR not only showed the features of the MJO but also northward and eastward propagation in the monsoons and winter seasons, respectively, in a few cases. It is found that convection leads the MJO in the zonal wind by 8–12 days in all the seasons except in winter. One intriguing result observed is the vitiation of the MJO pattern by the presence of strong wind shears during monsoon season. We expect this study would be helpful in representing the MJO features in the vertical in the general circulation models (GCMs) which is still a major challenge.

Vertically resolved Black Carbon (BC) measurements are obtained in situ up to 10 km during the summer monsoon season using a micro-aethalometer on-board balloon platform from Hyderabad (17.48oN, 78.40oE, 557 m msl), India. Simultaneous (on-board) aerosol size-segregated mass concentration measurements are made at particle sizes 0.5, 1.0, 2.5, 5.0 and 10.0 μm using a Particle plus Optical Particle Counter for estimating the fractions of BC in size segregated (SS) PM2.5 and the fraction of SSPM2.5 in SSPM10. A prominent aerosol layer with large BC mass concentrations in the range of 10–25 μg/m3 is found below 4 km (above mean sea level) which matches well with the measured SSPM2.5 concentrations showing consistency in the two independent measurements. Thin layers of BC are also detected at mid-tropospheric region at altitude ~7 km and another between 8 and 10 km. FLEXPART analysis for back trajectories suggests that these high concentrations of BC are transported from over the west Asian region of Somalia, Gulf of Aden and Oman. Meteorological parameters along with ozone concentrations obtained simultaneously using radiosonde and ozonesonde together with BC concentrations are used to estimate the actual BC heating rates for the first time. Shortwave BC heating rates as high as 4 K/day are observed in the lower thick layer. It is suggested that this strong heating rate can act as a feedback in weakening the stable layer and well mixing of pollutants up to 4–5 km altitude. Heating rates obtained using standard tropical atmosphere model is found to be overestimated in this case suggesting the use of real time atmospheric conditions for actual estimates.

Characteristics of turbulence in the troposphere and lower stratosphere at Trivandrum (8.5°N, 76.9°E) and Gadanki (13.5°N, 79.2°E), two tropical stations located in the Indian Peninsula, are studied using GPS-radiosonde observations during the period of December 2010 to March 2014 as part of the Tropical Tropopause Dynamics (TTD) Experiment under the CAWSES-India program. This study relies on the detection of turbulence applying Thorpe analysis to the temperature profile, taking into account the impact of atmospheric moisture and instrumental noise on static stability. In general, the tropospheric turbulence is largely intermittent in space and time. The altitude region very close to the convective tropopause (COT), 10–15 km, is relatively more turbulent than the lower troposphere from 3 to 8 km. Though the occurrence of turbulence decreases significantly above the COT, occasionally a rather thin layer of turbulence (thickness 2 km, are the persisting features that can be observed in the 5–15 km altitude region in multiple observations at both the sites at least during Asian Summer Monsoon (ASM) season, prominent multiple thin layers of stratified turbulence in the lower troposphere lasting for a day or less are observed only at Trivandrum in all seasons. In general, the turbulence strength in the 5–15 km altitude region at Gadanki is generally larger than that at Trivandrum. Below 15 km, while the turbulence is mainly governed by the convective instability at Gadanki, wind-shear driven (dynamic) instability also contributes considerably for the generation of turbulence at Trivandrum. While the generation of turbulence above 15 km is dominated by dynamic instability, in the lower stratosphere (LS) it is mainly due to strong wind shears.

In the present study, vertical distribution of ozone observed using ozonesonde measurements obtained during 2010–2014 from a tropical station Gadanki (13.48°E, 79.18°E) is presented with special emphasis on the variability within the atmospheric boundary layer (ABL), in the free troposphere and in the lower stratosphere. In general, a clear semi-annual oscillation in ozone is seen within ABL and free-troposphere with peak during pre- and post-monsoon seasons. Whereas annual oscillation with peak during monsoon season dominates in the lower stratosphere. Interestingly, there is a delay of one month in the peak in the ozone mixing ratio (OMR) in the free troposphere when compared to the ABL during pre-monsoon and these features are quite different compared to other stations in the Indian region. One of the main objectives of the present study is to examine how well the present satellite (MLS and SABER) and re-analysis (ERA-Interim) ozone data sets represent the variability of ozone over the tropical station. In general, MLS and ERA-Interim overestimate the OMR by 97% and 80%, respectively, near the tropical tropopause, in comparison with ozonesonde data. Thus, caution is to be exercised in using these data sets for investigating the stratosphere–troposphere exchange (STE) process. However, all the three data sets show very good agreement with ozonesonde measurements above 22.5 km. ERA-Interim data set is found to agree well in the free troposphere but not in the ABL. Interestingly, the ozone tropopause is found to be more associated with the convective outflow level than cold point tropopause height. Back trajectory analysis reveals that the observed ozone in the troposphere over Gadanki region is mostly transported from the western region and from the higher altitudes. These features are discussed in the light of current understanding of the generation and transport of the ozone in the troposphere.

The role of deep convection on the tropical tropopause parameters at sub-daily scales using radiosonde observations at two locations in South-India affected by monsoon has been investigated. Special experiments were conducted under the Tropical Tropopause Dynamics (TTD) campaigns from two stations, (Gadanki (13.5°N, 79.2°E) and Trivandrum (8.5°N, 76.9°E) as a part of CAWSES India Phase-II programme during December 2010 to September 2013. In addition, data from regular radiosonde launches available from April 2006 to September 2013 are also utilized in the present study. Using satellite based infrared brightness temperature data, convection is classified into six categories based on the life cycle of the convection prevailing 3 h before and after the balloon reaching the tropical tropopause. Cold-point and lapse rate tropopause altitudes (CPH, LRH) and temperatures (CPT, LRT), convective outflow level (COH) and tropical tropopause layer (TTL) thickness extracted from individual soundings are grouped into six convection categories. Large amount of water vapour with diabatic cooling prevailed near the CPH during active convection leading to STE processes. At the same time, decrease in TTL thickness is observed not only because of pushing up of the COH but also due to decrease of CPH. On an annual basis a decrease (increase) in CPH and LRH (CPT and LRT) is noticed during active convection. This feature is more significant at Gadanki compared to Trivandrum. During the monsoon and pre-monsoon periods when the convection is rather widespread, CPH (CPT) shows a decrease (increase) at Gadanki while it increases (decreases) over Trivandrum. Large seasonal variation is noticed in the tropopause parameters even when they are segregated into different convective categories mainly due to intensity of the convection being different. During active convection, diabatic and adiabatic processes seem to be dominant at Gadanki and Trivandrum, respectively.

Sources and propagation characteristics of high-frequency gravity waves observed in the mesosphere using airglow emissions from Gadanki (13.5° N, 79.2° E) and Hyderabad (17.5° N, 78.5° E) are investigated using reverse ray tracing. Wave amplitudes are also traced back, including both radiative and diffusive damping. The ray tracing is performed using background temperature and wind data obtained from the MSISE-90 and HWM-07 models, respectively. For the Gadanki region, the suitability of these models is tested. Further, a climatological model of the background atmosphere for the Gadanki region has been developed using nearly 30 years of observations available from a variety of ground-based (MST radar, radiosondes, MF radar) and rocket- and satellite-borne measurements. ERA-Interim products are utilized for constructing background parameters corresponding to the meteorological conditions of the observations. With the reverse ray-tracing method, the source locations for nine wave events could be identified to be in the upper troposphere, whereas for five other events the waves terminated in the mesosphere itself. Uncertainty in locating the terminal points of wave events in the horizontal direction is estimated to be within 50–100 km and 150–300 km for Gadanki and Hyderabad wave events, respectively. This uncertainty arises mainly due to non-consideration of the day-to-day variability in the tidal amplitudes. Prevailing conditions at the terminal points for each of the 14 events are provided. As no convection in and around the terminal points is noticed, convection is unlikely to be the source. Interestingly, large (~9 m s−1km−1) vertical shears in the horizontal wind are noticed near the ray terminal points (at 10–12 km altitude) and are thus identified to be the source for generating the observed high-phase-speed, high-frequency gravity waves.

Radiosondes are widely used to obtain basic meteorological parameters such as pressure (P), temperature (T), relative humidity (RH), and horizontal winds during the balloon ascent up to the altitude of balloon burst, usually ∼32–35 km. Data from the radiosondes released from Gadanki (13.5° N, 79.2° E), a tropical station in India, has been collected during the ascent and during the descent as well without attaching any parachute or its equivalent since the year 2008. In the present study an attempt has been made to characterize the radiosonde descent data with the main objective of exploring its usefulness and reliability for scientific purposes. We compared the data obtained during ascent and descent phases of the same sounding. The mean differences in T, RH and horizontal winds between ascent and descent data are found to be small and are sometimes even within the uncertainty of the measurements and/or expected diurnal variation itself. The very good consistency observed between the ascent and the descent data shows that one more profile of the meteorological parameters can be constructed within 3 h of time of balloon launch practically at no additional cost. Further checks are done by utilizing the 3 hourly radiosonde observations collected during the Tropical Tropopause Dynamics campaign conducted at Gadanki. In the process of checking the consistency between the radiosonde ascent and descent data, several new findings are arrived at and are reported in this study. In general, it has taken more than half-an-hour for the balloon to reach the ground from the burst altitude. It is also observed that the fall velocity is close to 10 m s−1 near the surface. Finally, it is suggested to record also the observations when the balloon is descending as this information is also useful for scientific purposes.

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, ~60% of the Thorpe scale (LT) 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.

In the present study, long-term trends in the diurnal tide in the troposphere and stratosphere over a tropical station Gadanki (13.5°N, 79.2°E) are investigated using ERA-Interim wind and temperature products available since 1979. Suitability of the ERA-Interim data for the present study is ascertained using simultaneous radiosonde and MST radar observations over Gadanki and good consistency was found between the two. In general, diurnal tide amplitudes are found to increase from troposphere to stratosphere, as expected. Amplitude of the diurnal tide shows a long-term linear increasing trend, which becomes prominent in the stratosphere. Interestingly, convection over Gadanki also exhibits an increasing trend suggesting that they are related. Role of solar cycle on the diurnal tide is investigated by separating the tidal amplitudes during minimum and maximum of solar cycles 21, 22 and 23. Significantly higher amplitudes in the recent extended solar minimum are noticed though no consistent relation is found between solar activity and tides, in general. These results are discussed in the light of role of convection on the generation of the diurnal tide and their propagation to the higher altitudes, coupling lower and middle atmospheres. Special emphasis is made on the observed large amplitudes of the diurnal tide in the extended solar minimum while relating the observed changes to the background circulation.

The structure and variability of tropical tropopause over Gadanki (13.5°N, 79.2°E) are delineated using data obtained from Indian MST radar operated in the vertical mode as a part of intense Tropical Tropopause Dynamics (TTD) campaigns conducted under the CAWSES India Phase II (Theme 3) program. Radar measurements for 72 h in each month from December 2010 to September 2013 have been considered. The identified tropopause altitude with radar (RTH) is compared with the cold point (CPH) and lapse rate tropopause altitudes (LRH) obtained from simultaneous radiosonde data at three hourly intervals during these campaigns. Most of the time, a very good agreement between the RTH and CPH and/or LRH from radiosonde measurements is observed. The mean difference between RTH and CPH and RTH and LRH is found to be 0.1±1 km and 0.5±1 km, respectively. The smaller differences between RTH and CPH noticed in the present work when compared to other mid- and polar latitudes might be due to the well defined tropopause structure in the tropical latitudes. As the radar provides reliable data on the tropopause, its long-term variability is investigated using the data from 2007 to 2012 available from the MST radar.

It is widely accepted that the tropical tropopause is closely linked to climate change. Several campaigns have already been conducted and also are being planned to address various issues related to the tropical tropopause layer (TTL). Despite many campaigns, several scientific issues still remain unexplained including Indian summer monsoon dynamics, cirrus clouds and the trace gas distribution across the tropopause etc. In order to address some of the issues, particularly over the Indian region, intensive observational campaigns called ‘tropical tropopause dynamics (TTD)’ are being conducted since December 2010 at two stations namely Gadanki (13.5°N, 79.2°E) and Trivandrum (8.5°N, 76.9°E) under CAWSES India Phase-II programme. This overview article aims to bring out the current understanding on the tropical tropopause, issues addressed through the TTD campaigns and the details of the data collected in these campaigns using collocated instruments as well as complementary satellite data. So far 32 campaigns have been completed successfully and in this paper main focus is given for describing the systematic data collected using various techniques (MST radar, Mie lidar, Radiosonde, ozonesonde) simultaneously in each month. In general, over the study region affected by the monsoon, a prominent updraft prevails in the middle and upper troposphere regions covering TTL affecting the transport of minor species across the tropopause. The behavior of the cold point tropopause (CPT) at Gadanki and Trivandrum reveals that there are significant differences in the CPT characteristics even within the monsoon region. Cold point tropopause shows stronger sub-daily scale variation over Trivandrum than Gadanki though no indication of deep convection is present at the former location particularly in winter.

The present study deals with using long-term database for upper tropospheric water vapour (UTWV) variability studies over three tropical stations (Gadanki, Singapore and Truk), where different climatic conditions prevail. Over Gadanki (13.5°N, 79.2°E) strong seasonal variation in UTWV is revealed but not over Singapore (1.37°N, 103.98°E) and Truk (7.46°N, 151.85°E) except at 100 hPa. It is examined whether high resolution radiosonde measurements represent well the UTWV by comparing with different satellite based (Atmospheric Infrared Sounder (AIRS), Advanced Microwave Sounding Unit-B (AMSU-B) and Microwave Limb Sounder (MLS)) water vapour measurements. Very good comparison in the nature of variations of UTWV is observed between radiosonde data and satellite data, except over Singapore particularly with AIRS and MLS data, on long-term basis. An attempt is also made to examine the source for UTWV. A close relationship is found between UTWV and deep convection over Gadanki indicating that the source for UTWV is convection particularly during the summer monsoon season.

[1] For better understanding the life cycle of the convective systems and their interactions with the environment, a joint Indo-French satellite mission named Megha-Tropiques has been launched in October 2011 in a low-inclination (20°) orbit. In the present study, we show the first results on the comparison of relative humidity (RH) obtained using a six-channel microwave sounder, covering from surface to 100 hPa, from one of the payloads SAPHIR (Sounder for Atmospheric Profiling of Humidity in the Inter-tropical Regions). The RH observations from SAPHIR illustrated the numerous scales of variability in the atmosphere both vertically and horizontally. As a part of its validation, we compare SAPHIR RH with simultaneous observations from a network of radiosondes distributed across the world (±30° latitude), other satellites (Atmospheric Infrared Sounder, Infrared Atmospheric Sounder Interferometer, Constellation Observation System for Meteorology Ionosphere and Climate (COSMIC)), and various reanalysis (National Center for Environmental Prediction (NCEP), European Center for Medium-Range Weather Forecasts reanalysis (ERA)-Interim, Modern-Era Retrospective Analysis for Research and Application (MERRA)) products. Being at a low inclination, SAPHIR is able to show better global coverage when compared to any other existing satellites in the tropical region where some important weather processes take place. A very good correlation is noticed with the RH obtained from a global radiosonde network particularly in the altitude range corresponding to 850–250 hPa, thus providing a valuable data set for investigating the convective processes. In the case of satellite data sets, SAPHIR RH is well comparable with COSMIC RH. Among the reanalysis products, NCEP shows less difference with SAPHIR followed by ERA-Interim, and the MERRA products show large differences in the middle and upper troposphere.

In a previous work (Mehta et al., 2011. J. Geophys. Res. 116, D20105, doi:10.1029/2011JD016637), we emphasized the limitations of the WMO definition of multiple tropopauses (MTs) which was based on the lapse rate in identifying them sometimes, and provided an alternate method called cold point method, to delineate the MTs over the tropics effectively. In the present work we described the detailed characteristics of the MTs over the tropics identified using the cold point method. We have used COSMIC GPS RO data obtained during August 2006–August 2008 and radiosonde data from 5 stations during 1999–2008 in the ±15° latitude for the present study. There is a marked latitudinal variation within the tropics in the MTs and cold point tropopause (CPT) in different seasons. These MTs can occur both above and below the CPT. The saturation water vapor mixing ratio corresponding to the lower tropopause (MTs which occur below CPT) from November to May is in general ∼3 ppmv indicating dry conditions even below the CPT altitude. The latitudinal variation of altitudes of the MTs and CPT showed quite different features. We also examined the role of equatorial waves in the occurrence of MTs. This wave mechanism could explain the MT occurrences that are in groups but not in the individual cases suggesting other mechanisms could also exist.

[1] In recent years, the mesosphere (50 to 85–100 km) has evoked great scientific interest as long-term changes due to global warming can be clearly captured due to the large perturbation amplitudes at these altitudes. In the present study, zonal wind observations between 70 and 80 km over the Indian region provided by rocketsonde (1977–1991), HRDI/UARS (1991–1999), and MST radar (1995–2010) are used to construct a long-term data set from 1977 to 2010. Using this unprecedented data set, a decreasing trend of 2 m/s/yr is found, changing from strong eastward winds during the 1970s to weak westward winds in recent years. On the other hand, between 80 and 98 km using medium frequency radar observations during 1993–2009, no perceptible trend is found. Simulations of NCAR TIME-GCM also showed a similar change in the circulation when CO2 in the atmosphere is doubled, suggesting role of anthropogenic changes in the dynamics of the mesosphere.

In the present study, characteristics of inertia gravity waves (IGWs), associated momentum and heat fluxes, and their source mechanisms have been studied using five years (2006–2011) of high resolution radiosonde observations collected from the tropical station, Gadanki (13.5°N, 79.2°E), India. The deduced horizontal wavelengths are of the order of a few 1000s km in the troposphere and stratosphere in contrast to those low horizontal wavelengths reported earlier from this location. The estimated horizontal wavelengths match well with those reported very recently using satellite (HIRDLS and SABER) measurements. Clear link between the fluxes and background wind are noticed with enhanced values during the westward phase of zonal wind. Although two sources for the generation of IGWs namely strong convection and wind shears coexist during monsoon season, wind shear is found to be mainly responsible.

Until now most of the gravity waves (GWs) characteristics reported using radiosonde observations are of low frequency waves. In the present study, a method to detect high frequency GWs using radiosonde observations has been presented. Making use of this method, long-term high resolution radiosonde data at Gadanki (13.5°N, 79.2°E), a tropical station in India, has been analyzed. The vertical (horizontal) wavelengths of the GWs lie in the range of 6–12 km (100–300 km) and 3–7 km (100–500 km) in the troposphere and lower stratosphere, respectively. From the simultaneous MST radar observations the periods of these GWs are found to be in the range of 2–6 h. The propagation direction is towards south-east/north-west and south-east in the lower troposphere and lower stratosphere, respectively. These characteristics are quite different from those reported for the inertial period GWs. This analysis, if extended to the global network of radiosonde observations, will help to parameterize the high frequency GWs in the global models.

The three component wind data from MST radar at Gadanki ( 13 . 5 ∘ N , 79 . 2 ∘ E ) are used to study the modulation of the tropical tropopause by wave disturbances in the winter seasons of 2000 and 2002 and in the summer season of 2001. Wave disturbances with periodicities in the range 18 days (d)–3 d have been observed to modulate the tropopause altitude and temperature. The zonal and meridional wind components and temperature at fixed altitudes around the tropopause are analyzed to delineate the nature of the wave disturbances. It is found that in the summer season, wave disturbances with periodicities in the range 18–7 d modulating the tropopause are manifestations of equatorial Kelvin waves. However, in the winter seasons, there is no clear evidence of modulation by equatorial Kelvin or Rossby gravity waves.

Cirrus clouds forming just below the tropical tropopause and associated atmospheric turbulence are studied using a monostatic lidar and Mesosphere–Stratosphere–Troposphere (MST) radar. Lidar data show that the vertical extent of these clouds is confined to the region 14–16 km. Based on their visible optical depth, they can be classified as sub-visual (SVC), thin (TC) and dense cirrus (DC). Physical and optical properties of the cirrus clouds show significant temporal and spatial variability. While on some days, the clouds are strong and persistent throughout the night, they are weak and intermittent on some other days. Lidar backscattered signal from the clouds shows significant depolarisation indicating presence of non-spherical particles in abundance. The mean altitude of the cloud increases with decrease in tropopause temperature. The MST radar observations of vertical wind revealed that the region where these clouds are observed is highly turbulent. The vertical gradient of eddy diffusion coefficient shows a sharp discontinuity in the altitude region where the cirrus is strong and persistent, indicating that a region of divergence followed by a convergence above is conducive for cirrus formation.

Using ground-based techniques of MST radar and Lidar, temperature profiles in the entire height range of 4 to 75 km are obtained for the first time at a tropical location. The temporal resolution of the profiles is ∼1 h in the lower altitudes and 12.5 min in the higher altitudes and altitude resolution is ∼300 m. The errors involved in the derived values are presented. Preliminary analysis of temperature variations in a night revealed fluctuations with characteristics resembling those of large-scale gravity waves.

A method of computing the vertical flux of zonal momentum (associated with equatorial waves) from the zonal and vertical components of the winds measured by the Indian MST radar at Gadanki (13.5°N, 79.2°E) is presented. The application of the method to the radar data gives flux values of 16×10 −3 , 8.0×10 −3 and 5.5×10 −3 m 2 s −2 for slow Kelvin (12-day period), fast Kelvin (5.33-day period) and Rossby-gravity (RG) (3.43-day period) waves, respectively, in the upper troposphere. These flux values compare quite well with the values 4×10 −3 m 2 s −2 and 1×10 −3 m 2 s −2 obtained from radiosonde zonal wind and temperature data by Wallace and Kousky, 1968 for slow Kelvin and RG waves, respectively. An estimate of the error in the fluxes gives a value of ∼ 1.2×10 −3 m 2 s −2 .

Night-time equatorial F -region plasma drifts are deduced from VHF backscatter radar observations of F -region irregularities. The zonal drifts reveal large vertical shears. It is found that the irregularity polarization electric field (though small compared to the ambient field) is significant in affecting the observed zonal drifts.

[1] Using long-term data (January 1998 to December 2009) collected from Mesosphere-Stratosphere-Troposphere (MST) radar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, variability of low-latitude mesospheric vertical wind is investigated for the first time. The emphasis is on sub-daily, monthly, seasonal and annual variabilities in vertical wind in the altitude region 65–85 km. Possible sources of errors in the vertical wind measurement at mesospheric altitudes from the MST radar are discussed. The observed mesospheric vertical wind is generally upward in all the seasons. During winter and summer, significant sub-daily variations are noticed followed by spring and fall equinoxes. The vertical wind can reach occasionally values as high as 5 ms−1 but most of the time (95%) it is, in general, less than ∼2.63 ms−1. The present observations are consistent with the general circulation features in recent models for low latitude locations with northward and upward wind prevailing throughout the year representing part of meridional circulation.

[1] New criteria for the identification of multiple tropopauses (MTs) in the tropics are proposed using COSMIC GPS RO data during 2006–2008 in the tropical belt (±30° latitude) and radiosonde data from 5 tropical stations during 1999 to 2008 in the ±15° latitude. In the present study, we emphasized that many times the WMO definition is not able to identify all the complex structures, such as MTs, and developed alternate criteria to delineate the MTs over the tropics effectively. The new criteria are based on the cold point tropopause (CPT) and its neighborhood points of inflection in the temperature profile rather than on lapse rate, which is used in the WMO definition. The method using the new criteria (called the cold point method) identifies MTs that occur below (referred to as Lower Tropopause, LT) and above (referred to as Second Tropopause, ST; and Third Tropopause, TT) the CPT. The percentage occurrences of MTs determined using the cold point method differ from those found using the WMO definition. The percentage occurrence of MTs within the tropics maximizes at the equator, reaching higher values in the northern hemisphere (NH) summer and lower values in the NH winter. Among the MTs, the percentage occurrence of the ST is the highest, followed by that of the LT. The percentage occurrence of MTs within the tropics also shows large longitudinal variability.

[1] Extensive observations on various atmospheric parameters were carried out from Gadanki (13.48°N, 79.18°E), a tropical station in India, during the annular solar eclipse of 15 January 2010. In the present study, we report changes observed in temperature and ozone concentration in the lower stratosphere associated with the eclipse. Unusual enhancements in the temperature of 12.5 K and in ozone concentration of 5.5. mPa are observed around 21 km just after the eclipse. Plausible mechanisms for the observed eclipse effects are discussed using data of simultaneous vertical and horizontal winds collected through co-located MST radar observations. From the preliminary analysis, the temperature and ozone enhancements are attributed to large subsidence and horizontal advection, respectively.

[1] The variations of the cold point tropopause (CPT), convective tropopause (COT), lapse rate tropopause (LRT), and tropical tropopause layer (TTL) from subdaily to seasonal timescales are studied using high-resolution radiosonde observations from April 2006 to December 2008 over a tropical station Gadanki (13.5°N, 79.2°E), in the Indian monsoon region. The correlations between temperatures and altitudes of the different tropopauses and between TTL thickness and satellite brightness temperature (TBB) are estimated. Diurnal behavior of CPT, LRT, and COT is also investigated. The seasonal variation of the CPT at Gadanki shows notable differences when compared to Western Pacific region, though major features remain similar. CPT temperature at Gadanki goes below 191 K more frequently during December to May than during June to November. The TTL thickness, which is the difference between CPT altitude and COT altitude, is small during summer monsoon and is strongly correlated with TBB on seasonal scale. The CPT altitude and temperature show large variation by as much as ∼1 km and ∼3 K, respectively, even on subdaily scale. During deep convection (TBB 240), increase in the CPT altitude with increase in CPT temperature is noticed on some occasions. Strong correlation between LRT (and COT) altitude and temperature from subdaily to seasonal timescale is noticed in contrast to CPT altitude and temperature. However, the correlation between TBB and CPT, LRT, and COT degrades from seasonal to subdaily timescale.

[1] Seasonal variation of the turbulence parameters, refractivity structure constant Cn2, eddy dissipation rate ɛ, and eddy diffusivity K, is presented using 3 years (April 2006 to March 2009) of high vertical resolution GPS radiosonde measurements over the tropical station Gadanki (13.5°N, 79.2°E). First, a correlation analysis was done in order to study the relative contributions of temperature (T) and relative humidity (RH) to the refractive index gradient (M). A strong positive (negative) correlation between M and RH (T) gradients is noticed up to 10 km. Above 10 km, a strong positive correlation between M and T gradients is seen, except at altitudes near the tropopause. In the present study we flag the turbulent layers using the Thorpe sorting method and estimate the structure constant Cn2 for the turbulent layers. The probability of occurrence of Cn2 is derived from the cumulative distribution at each height and is found to decrease from 10−7 to 10−17 m−2/3 with height. The probability of the occurrence of Cn2 conditioned to the occurrence of turbulence as a function of height is derived, and it is found that only the mean and the 90th percentile profile reach an altitude of ∼27 km. The demarcation between the boundary layer and the free atmosphere as well as between the troposphere and the stratosphere are quite clear from the turbulence profile, which allows us to identify the boundary layer and tropopause heights from turbulence profiles. Thus, the present analysis introduces an alternative approach to identify the height of the boundary layer as well as the tropopause and also to characterize the probability of turbulence and thickness of turbulent eddies in the atmosphere with a finer scale size (down to 5 m) for the first time from a tropical latitude.

[1] Several investigations of the dominant period gravity waves (GWs) were conducted earlier using Indian mesosphere-stratosphere-troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E). However, these works had their own limitations of unavailability of continuous data, low SNR, and lack of reliable data at the stratospheric heights (in the case of MST radar) and low spatial resolution (for routine balloon data). For the present study, high-resolution GPS radiosonde data for more than 2 years (2006–2008) has been used for the first time to characterize the dominant GWs and their associated source mechanisms. Particular attention is paid (1) to check the consistency in GW characteristics observed with MST radar, (2) to estimate potential energy, kinetic energy, and hence total energy, (3) to extend the analysis up to 25 km and check whether vertical wavelength is the same as that observed by MST radar in the lower stratosphere, and finally (4) to estimate the exact direction of propagation in horizontal, which was not possible from MST radar alone. Clear semiannual variation in GW energy, with maximum during monsoon and winter and minimum during premonsoon and postmonsoon in the troposphere, is noticed during 2006 but not clear in 2007. Annual variation is observed in the lower stratosphere with maximum during monsoon (winter enhancement is not significant) season. Strong eastward shear due to tropical easterly jet and orography is found to be responsible for generating the GWs during the monsoon and winter, respectively. Although several features are consistent with that observed earlier, a few new features have been observed by GPS radiosonde and are reported in the present study.

The altitude profiles of water vapor density, ρ, in the troposphere at low latitudes have been studied using radiosonde observations over nine stations in India. An attempt has been made to evolve a satisfactory model for the altitude variation of ρ in terms of the surface value. It has been found that a simple exponential relation with constant scale height parameter is not always adequate for this purpose. The scale height parameter shows significant variation with altitude. A double exponential profile having two different scale height parameters is found to be more suitable for most of the cases.

[1] In this paper, it is shown that the altitude of the tropopause can be determined directly from the profile of Bending Angle (α) of GPS RO (hereafter called as Radio Tropopause, RT) instead of from the derived parameter of temperature which needs further assumptions like hydrostatic equilibrium. Above the altitude range of moist atmosphere, the altitude of peak in α′(=dα/dh) profiles above which it decreases for at least 1 km is defined as the RT. The RT altitude is compared with the cold point tropopause (CPT)/lapse rate tropopause (LRT) altitude from independent measurements of temperature by the radiosonde at different stations. A very good agreement is observed between the tropopause altitudes from these two independent techniques. Particularly at polar latitudes, where it is often difficult to identify the tropopause altitude from temperature profiles, the present method is a more reliable means of identifying the tropopause.

The vertical velocity in the troposphere-lower stratosphere region measured using MST radar has been utilized to evaluate the temperature profile in the region. The diurnal variation of the tropospheric temperature on one day in August 1998 at the tropical station Gadanki (13.5° N, 79.2° E) has been studied using the MST radar technique. The diurnal variation of the temperature revealed a prominent diurnal variation with the peak in the afternoon hours increasingly delayed in altitude. The tropopause temperature and altitude exhibited a clear diurnal cycle.Key words. Atmospheric composition and structure (pressure, density and temperature; troposphere - composition and chemistry; instruments and technique)

Regular radiosonde measurements conducted by the India Meteorological Department at eleven stations spread over a latitude range of 8.5°–28.6°N for a period of nine years are used for the, study of the tropical tropopause characteristics, altitude (H), temperature (T) and pressure (P). The results are examined in the light of the hypothesis of Reid and Gage. It has been shown that large-scale cloud cover affects the tropopause parameters for annual and semiannual periods, while for longer periods (biennial) the cloud cover effect is not significant.

[1] Using the MST radar observations at Gadanki (13.5°N, 79.2°E), a tropical station, the altitude of major convective outflow in the troposphere is identified and is considered to represent the convective tropopause. This is found to match well with the altitude of local minimum of potential temperature lapse rate obtained from simultaneous radiosonde observations. The convective tropopause altitudes are also compared with the cloud top altitudes obtained using satellite brightness temperature (BT) data and are found to match in the case of deep convection. The thickness of the tropical tropopause layer follows very closely the convective tropopause altitude and has little dependence on the cold point tropopause altitude. The thickness of the tropopause layer is found to shrink when convection reaches high altitudes. This occurs mainly during the monsoon months of July, September, and October.