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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 (

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.

Using the MST radar data of vertical wind, the characteristics of the tropical tropopause, following four different definitions, depending on 1) temperature lapse rate, 2) cold point, 3) convective outflow and 4) potential temperature lapse rate minimum, are studied. From the vertical wind data of the radar, the altitude profiles of temperature and horizontal divergence are derived, from which the tropopause levels corresponding to i) the lapse rate ii) cold point iii) convective outflow level and iv) potential temperature lapse rate minimum are determined. The convective outflow level and hence the convective tropopause altitude is determined, for the first time using the MST radar data. The tropopause altitudes and temperatures obtained following the four definitions are compared on a day-to-day basis for the summer and winter seasons. Winter and summer differences in the tropopause altitude and temperature are also studied. Keywords. Meteorology and atmospheric dynamics (convective process; middle atmosphere dynamics; tropical meteorology)

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.

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.

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.

Using long-term data (1998–2008) collected from mesosphere–stratosphere–troposphere (MST) radar and Rayleigh Lidar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, vertical flux of the momentum from troposphere to mesosphere associated with the gravity waves of periods in the range 20 min to 2 h is investigated for the first time. The emphasis is on seasonal variability of mean zonal and meridional momentum fluxes in mesosphere and troposphere and vertical flux of horizontal momentum in the stratosphere. At tropospheric altitudes of 11–16 km large enhancement in flux is noticed during equinoxes. In the lower mesosphere in the altitude region 58–62 km the maximum values of flux (∼2.8 m2/s2) observed are pragmatic in winter and spring. Interestingly, the vertical flux of horizontal momentum estimated from lidar is in the range of those estimated from radar data in the overlap altitude region, though the estimates are from two different techniques. In the mesosphere, large variations with altitude in zonal momentum flux are noticed with a magnitude ∼0–4 m2/s2 in summer. In winter and summer the zonal wind direction is opposite to the momentum flux direction between 73 and 80 km and in equinoxes zonal wind follows the momentum flux. The meridional fluxes in the mesosphere are higher in equinoxes (∼10–12 m2/s2).

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.

Association between semitransparent cirrus (STC) and tropospheric dynamics during the two contrasting seasons (Summer-monsoon and Winter) is studied using lidar and MST radar observations at Gadanki (13.5°N, 79.2°E) along with data from Geo-stationary satellite (KALPANA-1). The tropical tropopause layer (TTL) is found to be very conducive for the formation of STC. For those thicker STCs forming within TTL, the cloud top remains mostly steady with the top of the TTL, while the cloud base varies significantly in accordance with the altitude extent of tropospheric convective outflow. Based on depolarization characteristics of STCs (from lidar) along with regional distribution of tropospheric clouds derived from satellite data, altitude profile of horizontal wind, wind field at 150 hPa and air mass back trajectories, it can be reasonably inferred that, while the multi-layered and highly structured STCs during the monsoon period originate mostly from the deep convections, the sub-visual/ultra thin STCs during winter could mostly be of in situ origin.

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.

The physical properties of high altitude cirrus occurring near the tropical tropopause are studied at the tropical station Gadanki (13.5°N, 79.2°E) using a monostatic lidar. This study indicates that the altitude region 14–16 km is more favourable for the formation of cirrus. The cloud depth varies from 0.4 to 4 km. Clouds with optical depth less than 0.03 (subvisual cirrus) is found to occur quite frequently at this tropical region. They introduce significant depolarisation for backscatter radiation indicating the presence of non-spherical ice crystals. Formation and persistence of these clouds is found to be closely associated with the strength of tropospheric turbulence, estimated from vertical wind data obtained from MST radar.

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.

High-resolution Rayleigh lidar observations of temperature in the altitude region of 27–60 km over Gadanki (13.5°N, 79.2°E) during the period of January 18 to March 5 1999 and February 29 to March 31 2000 were used to study the gravity wave characteristics in the tropical middle atmosphere. The vertical wavenumber spectra of gravity waves and the altitude variation of the potential energy per unit mass (Ep) associated with gravity wave activity are found to be similar during both these years. Altitude profile of E p indicates that significant wave damping occurs in the stratosphere while nearly non-dissipating gravity wave growth is observed in the lower mesosphere. The slope of the vertical wave number spectra is observed to be about −2.1, which is less than that expected for saturated spectra (−3). The gravity wave activity in the stratosphere and lower mesosphere is found to undergo temporal variations, which are correlated with the temperature fluctuations in the middle atmosphere. A longer period wave with a mean vertical wavelength of about 12.8 km and wave period of about 8–14 hours is also observed on several nights.

General features of high-altitude cirrus clouds observed below the tropical tropopause are studied using lidar at Gadanki (13.5°N, 79.2°E). These clouds occur quite frequently over this tropical region. Thin clouds with optical depth 0.3. The preferred altitude of these clouds is between 14 and 16 km, with a vertical extent ranging from 0.4 to 4 km. The backscattered lidar signal from the thin cirrus clouds exhibits significant depolarisation, indicating the presence of randomly oriented nonspherical ice crystals in abundance.

Using altitude profiles of temperature in the range 4 to 70 km derived from Mesosphere–Stratosphere–Troposphere radar and lidar observations at Gadanki (13.5°N,79.2°E) from 18 January 1999 to 5 March 1999, characteristics of equatorial waves are studied. Two-dimensional Fourier-transform analysis of the temperature profiles is carried out to identify the periodicities and their vertical wave numbers. From the characteristics obtained, equatorial slow Kelvin waves with periodicities 15.7 d, 9.4 d, 7.8 d and 6.7 d are identified in the troposphere and stratosphere regions and among these 7.8 d and 6.7 d periodicities are found to penetrate into the mesosphere. Equatorial waves with smaller periodicities in the range 5.2 d to 3.6 d are also observed. The vertical flux of horizontal momentum (zonal) of the identified slow Kelvin-wave periodicities in the altitude region 4–25 km is estimated. It is found that equatorial waves modulate tropical tropopause temperature and altitude. Copyright © 2002 Royal Meteorological Society.

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.

Using the MST radar located at Gadanki (13.5°N, 79.2°E) winds were measured every four hours in the troposphere and lower stratosphere during a 72-hour period from 23 to 26 September 1997 when the tropical tropospheric easterly jet (TEJ) was present south of Gadanki with its core at a height of ∼16 km. Analysis of the radar data shows the presence of ∼56 hour oscillations (close to local inertial period of ∼51 hours) in the wind components with significant amplitudes. Examination of the corresponding horizontal wind vector shows clockwise rotation with altitude (upward energy propagation) in an altitude region (17.7–20.8 km) above the TEJ and anticlockwise rotation (downward energy propagation) in another altitude region (8.3–10.5 km) below. This indicates the generation of Inertia Gravity Waves (IGW) in the TEJ and their propagation both upwards and downwards from the source region. The inferred vertical and horizontal wavelengths of the IGW are ∼0.7 and ∼75 km respectively in the stratosphere and ∼1 and ∼143 km respectively in the troposphere.

An equatorial wave campaign was conducted at Trivandrum (8.5°N, 77°E), Minicoy (8.3°N, 73°E) and Port Blair (11.7°N, 92.7°E) during June-July 1988. The campaign provided balloon-measured daily wind profiles at all the three stations for 48 days in the 0–30 km altitude range and rocket-measured daily wind profiles at Trivandrum for 42 days in the 31–60 km altitude range. Using these daily wind data a study was made on different equatorial wave modes present in this region. The study revealed evidence of Kelvin waves with period 12–16 days and vertical wavelength ∼ 10 km in the lower stratosphere, with period 6–9.6 days and vertical wavelength of ∼ 10–15 km in the stratospheric-lower mesospheric region and MRG waves with periods 4–4.4 days and vertical wavelength of 10 km in the upper troposphere and lower stratosphere.

[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.

Characteristics of aerosol extinction in lower stratospheric altitudes over a tropical station are studied using the ruby lidar data at Trivandrum (8°55′N, 77°E) for the period 1986–1987. The integrated stratospheric aerosol extinction as well as the aerosol extinction at fixed altitudes in this region showed a significant negative correlation with tropopause temperature. This is attributed to tropospheric convective activity. Though the direct correlation between ambient temperature and aerosol extinction is insignificant at these altitudes, the partial correlation eliminating the influence of tropopause temperature becomes increasingly significant at altitudes far from the tropopause. This indicates the increasing influence of stratospheric microphysical processes at higher altitudes, that is, near the altitude of stratospheric aerosol peak.

[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] Global distribution of mesopause using observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) aboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite is studied with special emphasis on tropical latitudes. Temperature structure of the mesosphere and lower thermosphere over Gadanki (13.5°N, 79.2°E), a tropical latitude location in India, has been studied in detail. The mesopause altitude at Gadanki is observed to be in the range of 87–105 km with a mean of ∼98 km, and the temperature is in the range of 160–205 K with a mean of ∼170 K. A secondary temperature minimum is observed in the height region of 75–80 km with temperature in the range of 175–195 K. Although the mesopause temperature and altitude show pronounced short-term variability, on a seasonal mean basis, the variation in these two parameters is rather small. There is no significant long-term variation in mesopause altitude. The height of the secondary minimum, however, is found to show semiannual variation with a maximum in solstices and a minimum in equinoxes. Thus, the variabilities in the height and temperature of the mesopause and the secondary minimum reported here are different from the two-level mesopause reported previously for middle and high latitudes. Another significant result that comes from the time series analysis of mesopause temperature is a decreasing trend with a rate of ∼0.72 ± 0.05 K/yr. Furthermore, the long-term mean profile of temperature does not show the secondary minimum, providing strong support that it could be due to shorter scale tides and gravity waves. The possible implications of the mesopause temperature structure on various physical processes occurring in the mesosphere and the lower thermosphere are discussed.

The characteristics of the quasi-biennial oscillation in zonal wind and temperature at Trivandrum (8.5°N, 77°E) have been studied using data covering 16 years. Similar study has been carried out for zonal wind at Balasore (21.5°N, 87°E) using data covering 9 years. The cycle to cycle variation of amplitudes, their altitude variation, periods and descent rates of the westerly and easterly regimes have been studied.

Using rocket wind data at a tropical station, Balasore (21.5°N, 86.9°E), the diurnal and semidiurnal tidal amplitudes and phases of the zonal and meridional components have been obtained over an altitude range of 20–65 km for equinox, summer and winter seasons. Comparison with the theoretical values revealed some important differences between the two and the implications of these are discussed.

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.

Characteristics of cirrus clouds forming just below the tropical tropopause and associated atmospheric turbulence in this region are studied using a monostatic lidar and Mesosphere Stratosphere Troposphere (MST) radar. The lidar data shows quite frequent occurrence of cirrus clouds near the tropical tropopause. The vertical extent is confined to the region just below the tropopause with a width of about 1-2 km. Based on cloud optical depth these cirrus clouds can be classified as; the subvisual cirrus with optical depth (tau) c c < 0.05. Frequency of occurrence of subvisual clouds is larger than that of the thick clouds. While the subvisual cirrus occurs more frequently at an altitude approximately 16 km, the optically thick clouds occur more frequently around approximately 14 km. The lidar signal scattered by these clouds shows significant depolarization, indicating the presence of abundant amount of non-spherical particles. The MST radar observations of vertical wind during the lidar observation revealed that the region between 10 km to 16 km, where these clouds are observed, is highly turbulent. A study of the Turbulent Kinetic Energy profile has shown high er values during the period of cloud occurrence. altitude profile of the vertical gradient eddy diffusion coefficient in the latitude region 10 to 16 km shows a sharp positive peak followed by a sharp negative peak above, when the cloud is strong and continuous. On other periods when the cloud is weak and discontinuous in this region is relatively low and the peaks are rather broad. This means that a region of divergence followed by a region of convergence above is favorable for the cirrus cloud formation.

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)

The authors report recent observations of the effect of turbulence on the altitude behavior of the equatorial electrojet. Their results show that with an increase in observed turbulence that the altitude of maximum in the electron drift velocity shifts to higher altitudes. This is consistent with recent theoretical work which shows that small scale turbulence can produce large-scale changes in the dynamics of the equatorial electrojet.

Aerosol extinction profiles obtained from lidar data at Trivandrum (8.6 deg N, 77 deg E) are compared with corresponding Stratospheric Aerosol and Gas Experiment II extinction profiles. The agreement between the two is found to be satisfactory. The extinction profiles obtained by both the experiments showed a prominent peak at 23-24 km altitude in the stratosphere. The study revealed large variability in upper tropospheric extinction with location (latitude).

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.

Altitude distribution of aerosols in the mixing region in a tropical coastal environment is studied using a bistatic continuous-wave lidar. It is found that aerosols remain fairly well mixed—their number density showing little variation with altitude up to an altitude of approximately 300 m from the surface, and above this their number density, in general, decreases with an increase in altitude. The aerosol number density shows a significant dependence on the near-surface wind speed. This dependence, could be represented fairly well by an exponential function of wind speed. The wind contribution to aerosol content is found to be at its maximum during the southwest monsoon period.