Search

[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.

[1] A comprehensive study on the ducting conditions prevailing over the Indian tropical station of Gadanki (13.5°N, 79.2°E) is made using more than 5 years (April 2006 to September 2011) of high-resolution GPS radiosonde observations. In the present study, the characteristics of ducts occurring over Gadanki were examined statistically using the modified refractivity. Strong diurnal and seasonal variation in the percentage occurrence of the ducts was found with the highest and lowest occurrences during winter and monsoon seasons, respectively. Duct strength is very strong during midnight to the morning hours than in the evening hours particularly during winter. The mean thickness of the duct and the duct strength were found to be the highest and the strongest in postmonsoon and winter seasons, whereas they are the lowest and the weakest in the monsoon season. Elevated ducts were found to be stronger than the surface and the surface-based ducts. The characteristics of ducts during stable and unstable conditions were also investigated, and it was found that the duct altitude is higher during the stable conditions and is higher during premonsoon followed by postmonsoon, winter and minimum in the monsoon. The maximum wavelength being trapped was investigated, and it was found that wave trapping occurs for the radars with frequencies of 60 GHz and above.

[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] In this study, long-term (2.5 years) observations of a high vertical resolution radiosonde are used for the first time to identify the atmospheric boundary layer (ABL) height over a tropical station, Gadanki (13.5°N, 79.2°E). An alternative method of detecting ABL height from refractivity (N) profiles is proposed, which includes both temperature and water vapor information, and several advantages were found. The identified height using N is compared with that detected by traditional methods like potential, virtual potential temperature, and mixing ratio during different background meteorological conditions. Very good correlations in all weather conditions indicate that N can also be used as an indicator for detecting the ABL height. The ABL height thus obtained is compared with independent measurements of N from the Constellation Observing System for Meteorology Ionosphere and Climate GPS radio occultation (RO), and very good correlation is found between the two. ABL height is found to be higher during premonsoon, followed by monsoon and postmonsoon, and is minimum in winter. In addition, radiosondes launched four times a day during different seasons have been used to study the diurnal variation. These results were compared with GPS RO data collected during different times in a day for a given season, and very strong diurnal variation was found. For studying the global distribution of ABL height from GPS RO data, it is suggested that one considers the GPS RO data for a fixed time or range of time (as RO data are very sparse in tropical regions) and the profiles reaching down to 0.5 km, particularly during nighttime.

[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.