Three-year high-resolution wind observations of the wind profiler have been utilized to characterize the diurnal and seasonal features of the monsoon Low-Level Jet (LLJ) over a tropical station, Gadanki (13.5° N, 79.2° E), with a focus on the diurnal variability of low-level winds. The Boreal summer monsoon winds show a conspicuously strong westerly LLJ with average wind speed exceeding 20 m s−1. The L-band wind profiler measurements have shown an advantage of better height and time resolutions over the conventional radiosonde method for diurnal wind measurements. An interesting diurnal oscillation of LLJ core has been observed. It is varying in the height range of 1.8±0.6 km with the maximum and minimum intensity noticed during the early morning and afternoon hours, respectively. The jet core (wind maxima) height is observed to coincide with the inversion height. Strong wind shears are normally located beneath the LLJ core. The sole wind profiler observations are capable of identifying the monsoon phases, such as onset, break and active spells, etc. The mutual influence between the LLJ and the boundary layer has been discussed. One notices that the observed LLJ diurnal structures depend on the local convective activity, wind shears and turbulence activity associated with boundary layer winds. The day-to-day change in the LLJ structure depends on the latitudinal position of the LLJ core.

Using four years of Rayleigh lidar data collected from three different northern hemisphere stations (Gadanki 13.5°N, 79.2°E ; Mt. Abu 24.5°N, 72.7°E and Observatoire de Haute Province OHP; 44°N, 6°E), the statistical characteristics of double (separated) stratopause are presented here. The normal stratopause (NS) is positioned between the levels of double stratopause (upper and lower level), and it is closer to the lower level of double stratopause (LDS). The mean stratopause heights (NS, LDS and UDS) are higher in sub-tropics (Mt.Abu) than tropics (Gadanki) and midlatitudes (OHP). The lower and upper levels of double stratopause are separated by ~2-8 km with high value for tropics and sub-tropics than mid-latitudes.

As an observational evidence, it is reported that long period oscillations near 21- and 30-days generated in the lower atmosphere (troposphere and lower stratosphere) in the Indian tropical regions have possible influences on the lower E region ionospheric currents (say equatorial electrojet [EEJ]) at these periodicities. Dynamical parameters like atmospheric wind velocities are measured in the lower atmosphere (4–20km) and mesosphere (80–98km) using the Indian MST radar located at Gadanki (a tropical station) and the MF radar located at Tirunelveli (Indian magnetic equatorial station), respectively, during the time interval of 18 January to 23 April 1999. Using Nd-Yag Rayleigh scatter lidar collocated at Gadanki, the atmospheric temperature profiles are measured in the intermediate height range of 25–80km for the same period of observation. As a lower E region ionospheric parameter, the EEJ current strengths are measured by determining the geomagnetic field strengths at the Indian EEJ and off-EEJ stations of Trivandrum and Alibag, respectively. Analyses of the data by using fast Fourier transform (FFT), wavelet transform (Morlet wavelet), maximum entropy method (MEM) and Lomb–Scargle periodogram analysis have indicated that it may be possible for the long period oscillations generated in the lower atmosphere to have influences on the E region ionosphere. Possible influences of the solar high energy radiations and magnetospheric origin on EEJ current systems in the periodicity range of ∼20–30 days are also discussed with illustrations.

Altitude profiles of middle atmospheric temperature data using Rayleigh Lidar at Gadanki have been utilized to study the gravity wave characteristics. The wave activity during the period November 2002-April 2005 is investigated. The vertical propagation characteristics show waves with maximum amplitude of ~5-8 K and vertical wavelength of ~10 km. Potential energy density of these two bands of periodicities in the altitude regions 30-60 km is estimated for different seasons. Equinoctial enhancement in the wave activity is observed. Momentum fluxes of these two bands of periodicities of gravity waves also exhibit seasonal variation with maximum around equinox and minimum in solstial months. A high correlation exists between the gravity wave activity and one of its major sources namely convection.

Abstract. Using four years of Rayleigh lidar data collected from three different northern hemisphere stations (Gadanki 13.5° N, 79.2° E ; Mt. Abu 24.5° N, 72.7° E and Observatoire de Haute Provence: OHP; 44° N, 6° E), the characteristics of double (separated) stratopause occurrence are presented here, for the first time. The characteristics are illustrated by a seasonal change during summer and winter and the variation in percentage of occurrence from place to place. It is found that the over-all mean normal stratopause (NS) positioned at the middle level of double stratopause (upper and lower level) with its location nearer to the lower level of double stratopause (LDS) than to the upper level of double stratopause (UDS). The frequency distribution of NS, LDS and UDS demonstrated variability with location, indicating role of dynamical activity. By making use of a quasi-continuous 40 days of lidar observations over Gadanki and OHP, the responsibility of Gravity Wave (GW) and Planetary Wave (PW) activity for the LDS and UDS occurrence are examined and presented.

MST radars are powerful tools to study the mesosphere, stratosphere and troposphere and have made considerable contributions to the studies of the dynamics of the upper, middle and lower atmosphere. Atmospheric gravity waves play a significant role in controlling middle and upper atmospheric dynamics. To date, frontal systems, convection, wind shear and topography have been thought to be the sources of gravity waves in the troposphere. All these studies pointed out that it is very essential to understand the generation, propagation and climatology of gravity waves. In this regard, several campaigns using Indian MST Radar observations have been carried out to explore the gravity wave activity over Gadanki in the troposphere and the lower stratosphere. The signatures of the gravity waves in the wind fields have been studied in four seasons viz., summer, monsoon, post-monsoon and winter. The large wind fluctuations were more prominent above 10km during the summer and monsoon seasons. The wave periods are ranging from 10 min-175 min. The power spectral densities of gravity waves are found to be maximum in the stratospheric region. The vertical wavelength and the propagation direction of gravity waves were determined using hodograph analysis. The results show both down ward and upward propagating waves with a maximum vertical wave length of 3.3 km. The gravity wave associated momentum fluxes show that long period,gravity waves carry more momentum flux than the short period waves and this is presented.

A multibeam VHF radar experiment, which was carried out to explore the mesoscale convective systems over Gadanki (13.5°N, 79.2°E), is discussed. These observations revealed an interesting pattern in the height horizontal distance section of radar reflectivity. A weak echo region (WER) is observed in the height region of ∼8–14 km well above the melting layer, which is believed to be first of its kind in VHF radar observations. A single vertical velocity core of ∼7 km vertical extent with ∼10 ms−1 vertical velocity is observed and it has been noticed that the WER is forming in the peripheral region of large vertical velocities. An attempt has been made to explain the plausible mechanism responsible for the formation of WERs. Significance of present results lies in showing the capability of VHF radar to scan the convective system in vertical plane and in providing observational evidence for existence of WER above the melting level.

Simultaneous observations made on four days using the MST radar and GPS-sonde at Gadanki (13.5° N, 79.2° E), a tropical station in India, are presented to address the aspect sensitivity of radar backscatters observed at different heights. The observations show that wherever stability parameter N2 is high, vertical shear of horizontal wind is low and Richardson number (Ri) is high, the aspect sensitivity is high indicating that the aspect sensitive radar backscatters are due to thermal structures in the atmosphere. Such a case can be seen very clearly in the upper troposphere and lower stratosphere. At some heights, where N2 is high, Ri is high, but shears are relatively weak, the aspect sensitivity is found to almost disappear, indicating that some amount of shear provides favorable conditions for causing aspect sensitivity. Aspect sensitivity does not occur at all where N2 is low or negative and Ri is low in spite of wind shear being either high or low, indicating that the regions are well mixed and hence turbulent. The study also shows a power difference in the symmetric beams. A case study on this aspect suggests that this asymmetry is due to the tilting of layers by the action of atmospheric waves. There is indication that these waves are generated through Kelvin-Helmholtz-instability (KHI).

Tropical convection plays an important role in enhancing rainfall and also creates uncertainty in the model-based predictions of weather in tropics due to the latent heat released into the troposphere. Ground-based radar systems are important tools available for the effective characterization of convective events. Availability of different radar systems ideally suited to study tropical convection in an area popularly known as the Golden triangle for weather observations in southern part of India, led to the organization of an interagency program for a campaign mode of observations using the different radar systems and associated rain gauges etc. to observe tropical convection during the period Oct-Dec, on a few days when the north east monsoon was prevalent over the east coast of peninsular India. The Golden triangle consists of Sriharikota Island where the Space Launch Complex of ISRO is located at the Satish Dhavan Space Center, the Cyclone Detection Radar site of IMD at Chennai and the National MST Radar Facility (NMRF) at Gadanki in close proximity to the temple city of Tirupati. An indigenously developed S band Doppler weather radar is commissioned at Sriharikota Island on the east coast of India in December 2002, as an interagency program between India Meteorological Department and Indian Space Research Organization. A siphoning type fast response rain gauge and a tipping bucket rain gauge are located within 10 km from the radar. Another S band DWR is located in Chennai, India by IMD. These radars have the capability to measure precipitation and Doppler velocity and provide in real time the 3 base products viz., reflectivity, velocity, and spectral width of the hydrometeors within radar sample volume with good accuracy up to 250 km. From these base products, other meteorological products like rainfall rate, rainfall accumulation, Cappi, echotop etc. are derived. Indian MST radar, a VHF profiler (at 53 MHz) normally used for estimating the winds and turbulence and an L band lower atmospheric wind profiler operating at 1357.5 MHz for estimating the winds, turbulence, and precipitating weather systems in tropical latitudes are operating at NMRF since 1990. Apart from these radar systems, a disdrometer and an optical rain gauge are also located at this facility as collocated instrumentations for the measurement of rainfall rate and rainfall accumulation. All these three locations are geographically located within 80-100 km from each other and form a triangle. These instrumentation systems provide an excellent ground-based network for the characterization of tropical convection. The paper describes the campaign details including the detailed characteristics of the radar systems used and provide intercomparison of the data obtained as the convective systems transited over the terrain which is essentially coastal for Chennai and SHAR and mountainous for Gadanki.

Middle atmospheric temperature profiles were presented from the Nd:YAG Lidar measurements over Chung Li (25°N,121°E), Taiwan, and Gadanki (13.5°N,79.2°E), India. The stratopause is observed in the height range of 45–50km with temperatures 265–280K over Chung Li and the stratopause width is broader at Gadanki with temperatures 260–270K. These data were compared with the coincidence UARS-HALOE profiles. The comparison shows fair agreement between the ground-based measurements and HALOE, but HALOE temperatures are colder by 2–10K in the stratosphere with maximum difference at stratopause altitudes. The stratospheric temperatures from the nearest radiosonde data show good agreement with NCU Lidar and HALOE data. The results from these two stations were compared and discussed with results available in the literature.

Long‐term mean vertical velocities observed by the Mesosphere‐Stratosphere‐Troposphere (MST) radar over a tropical station, Gadanki, India (13.5°N, 79.2°E), are presented for the first time in this paper. Profiles of mean vertical velocities show wave like structure with a vertical wavelength of ∼6 km, and the observed values range from 3 to 20 cm s −1 on average. During monsoon and postmonsoon seasons, larger magnitudes are observed up to 20 cm s −1 . From the present study, an interesting feature, reversal in vertical velocities from downward to upward between 5 and 10 km during the monsoon season, is observed. This upward motion in long term averaged vertical velocity is thought to be due to reversal in wind direction around this height range and also due to horizontal velocity convergence, which is frequently observed over this region. One more reversal is observed near the Tropical Easterly Jet (TEJ) (around 16 km), which is from downward to upward and always showing minimum vertical velocities close to zero. This reversal can be attributed to instabilities associated with the jet streams. Large negative values are observed in the lower troposphere below 6 km, which are attributed to large vertical wind variances observed in this region indicating gravity wave activity.

Retrieval of vertical profiles of temperature and humidity parameters using a VHF radar is described in this paper. For this, Indian MST radar located at Gadanki (13.5° N, 79.2° E) has been operated in a special mode. First, vertical velocities are collected continuously using the radar and are subjected to Fast Fourier Transform (FFT) analysis to obtain Brunt-Väisälä oscillations. From the measured Brunt-Väisälä oscillations, temperature profile is obtained from the radar observations following Revathy et al. (1996). The various terms required for the retrieval of vertical profiles of humidity are the eddy dissipation rate, ε, the volume reflectivity, η, and the potential refractive index gradient, M. The eddy dissipation rate, ε, is calculated from the spectral width after removing the effects due to non-turbulence. The volume reflectivity, η, of the turbulence scattering is calculated using the signal-to-noise ratio as a function of height. The potential refractive index gradient, M, is evaluated using the measured Brunt-Väisälä oscillations, the eddy dissipation rate and the volume reflectivity, η. Vertical profiles of humidity are retrieved following Tsuda (1997) using the radar derived temperature as well as the balloon measured temperature and are compared with the humidity as measured by the radiosonde. The sign of the potential refractive index gradient, M, is taken from the simultaneous measurements of balloon soundings. The retrieved vertical profiles of temperature and humidity have been compared with the radiosonde data, which are released simultaneously with the radar observations at the radar site. A fairly good comparison is seen between the two measurements on some days and there are some discrepancies on some other days. The strengths and limitations in estimating the vertical profiles of temperature and humidity from the radar observations are discussed.Key words. Atmospheric composition and structure (pressure, density and temperature; enhancements and techniques)

For the first time at Gadanki, India, an L-band (1357.5 MHz) Lower Atmospheric Wind Profiler (LAWP) has been in operation since August 1997. Gadanki-LAWP provides continuous high-resolution wind measurements in the first few kilometers of the atmosphere and offers excellent opportunity to diagnose convective planetary boundary layer and precipitating cloud systems. To check the reliability winds observed with the Gadanki-LAWP system are compared with the Indian MST radar measurements. Though the echo mechanism is different, the comparison of wind speed shows fairly good agreement. Gadanki-LAWP can provide continuous convective atmospheric boundary layer (CBL) height measurements with very good time resolution (5 minutes or less), allowing for detailed understanding of the growth and fluctuations of the CBL. In this paper we also examine the application of LAWP to the diagnosis and classification of precipitating cloud systems at Gadanki.

Seasonal and diurnal variation of turbulence parameters such as refractivity structure constant Cn2 and eddy dissipation rate ε is presented using the data collected with the Indian mesosphere‐stratosphere‐troposphere (MST) radar over 3 years. The log Cn2 values estimated from signal‐to‐noise ratio (SNR) are found to be in the range of −17 to −19 m−2/3 in the height range of 7.5–21 km. Monthly mean values of log Cn2 show a maximum variation below 12 km with a magnitude of 12–15 dB during the course of annual cycle. A maximum variability of ∼7–10 dB is observed below 12 km in seasonal mean values of log Cn2. The diurnal variation of log Cn2 at different heights is also given. Cn2 is found to be more in the region of strong shears, generally observed in the boundaries of jet streams. Different methods for the estimation of eddy dissipation rate and their limitations are discussed. For the present study, spectral width method is used after correcting the observed spectral width from beam and shear broadening effects. The observed median log ε is on the order of −3 to −4 m2 s−3. Monthly variation of log ε is found to be ∼5–7 dB. Below 10 km the magnitude of ε is more in the postmonsoon than that observed in other seasons. The interannual variation of ε is less in winter than in other seasons. The diurnal variation of log ε is found to be small in postmonsoon at most of the heights. To facilitate a comparison with the other results, we have estimated the eddy diffusivity K and the inner and outer scales of turbulence. The observed values of Cn2, ε,K, and the inner and outer scales of turbulence are largely consistent with the results available in the literature.

We have been working on rainfall observation project at tropical site in India, in order to study tropical storm structure and raindrop size distribution (DSD) characteristics for improving the current PR rainfall retrieval algorithm, and for making the ground validation of TRMM PR observation. At Gadanki (53-MHz VHF MST radar, an L-band lower atmospheric wind profiler (LAWP), a disdrometer and an optical rain gauge (ORG) are set up to obtain more knowledge on vertical properties of DSD and rain structure during monsoon season. Measurements of drop size distribution (DSD) with disdrometer have been providing the information to study basic DSD characteristics in tropical India. We have found a clear seasonal dependence in Reflectivity (Z) - Rainfall (R) relations (i.e. DSD characteristics) in India. Our results indicate that there are about 3-times differences peak-to-peak in estimates of rain rate using a single Z-R relation. It seems that this type of seasonal dependence should be taken into account to improve the accuracy of the PR algorithm. Drop size distribution characteristics were retrieved in moderate/heavy precipitation using VHF wind profiler. The retrieved drop size parameters were compared to corresponding disdrometer data and found that there is reasonably good agreement between the measurements, lending credence to the profiler retrievals of DSD parameters. Preliminary study on the ground validation of TRMM PR shows fairly good agreement between the disdrometer and TRMM precipitation radar measurements.

VHF and UHF Doppler radars provide a unique database to estimate the refractivity structure constant Cn2, eddy dissipation rate ∈, and vertical flux of horizontal momentum. Using the data collected from the Indian MST radar, these parameters are studied at a tropical latitude. The refractivity turbulence structure constant is estimated from the backscattered power of the received echoes. Cn2 (radar) and Cn2 (model), derived from radiosonde observations, are compared, and a fairly good agreement is seen. Diurnal and seasonal variations of Cn2 are also presented. The eddy dissipation rate is estimated from the radar echoes employing the power and spectral width methods. A fairly good agreement is seen between the two methods. Values of ε are found to vary from 10−6 to 10−3 m2 s−3 in a height range of 4–19 km. Cn2 and ε are observed to be minimum during a moderate jet stream wind of 50–60 m s−1. Vertical flux of horizonal momentum is computed using the symmetrical two‐beam method. Significant fluxes of westward and northward momentum are observed, and the values lie in the range of −1 to +1 m2 s−2. The implied accelarations are also estimated. The results presented are largely consistent with the results available in the literature.

Investigations of the effect of path inclination on fading, based on the data collected from three line-of-sight (LOS) microwave links over the Indian subcontinent, are reported. It is observed that as the path inclination increases, the number and depth of fades caused by layers are reduced. In regions where temperature inversions prevail for a considerable percentage of the time, the design of LOS links should incorporate the concept of path inclination to overcome fading