Some aspects of Indian northeast monsoon as derived from TOVS data

The Tiros Operational Vertical Sounder (TOVS) is a popular satellite sounding system. In this paper certain features of Indian northeast monsoon have been studied with the help of three years of TOVS data received through the satellite ground station located at the Regional Meteorological Centre, Chennai. The TOVS based latitudinal and longitudinal profiles of Outgoing Longwave Radiation (OLR) and Precipitable Water Vapour (PWV) were derived for various phases of northeast monsoon activity, over coastal and interior Tamilnadu and oceanic regions. These were consistent with the known spatial rainfall characteristics of northeast monsoon. The average vertical temperature profiles derived for the various phases of northeast monsoon for the different regions revealed that the lowest layer and upper troposphere are warmer and mid troposphere colder during active northeast monsoon compared to dry phase. The diurnal variation of OLR and PWV and the comparability of TOVS derived data with conventional upper air data and INSAT data have been briefly discussed.

Atmospheric water vapour and its effect on aerosol Extinction at a coastal station – Visakhapatnam

Integrated atmospheric water vapour content. has been evaluated from the spectral optical depths around the PaT band of water vapour by making directly transmitted solar flux measurements at 800, 935 and 1025 nm. The temporal variation of the total precipitable water vapour shows significant seasonal variation with maximum during~ pre-monsoon and monsoon months and minimum during winter months. The integrated content shows a positive correlation with surface humidity parameters and the correlation is better during monsoon months compared to other seasons. The experimentally derived variations of water vapour are compared with the model variations formulated using radiosonde data. The aerosol extinctions derived from the, multi-spectral solar flux measurements in the visible and near IR regions increase with increasing atmospheric water vapour and this increase shows .a seasonal dependence the surface temperature also seems to affect the, aerosol extinction probably through Its effect on the mixing heights.

IPWV estimation and data quality analysis from different GNSS antenna

Global Navigation Satellite System (GNSS) is widely used now days in variety of applications. The observation file for the near realtime estimation of Integrated Precipitable Water Vapour (IPWV) received at the ground-based receiver is mixed with ambiguities. Multi-path effects affect the positional accuracy as well as range from satellite to ground based receiver of the system. The designing of the antenna suppress the effect of multi-path, cycle slips, number of observations, and signal strength and data gaps within the data streams. This paper presents the preliminary data quality control findings of the Patch antenna (LeicaX1202), 3D Choke ring antenna (LeicaAR25 GNSS) and Trimble Zephyr antenna (TRM 39105.00). The results shows that choke ring antenna have least gaps in the data, cycle slips and multi-path effects along with improvement in IPWV. The signal strength and the number of observations are more in case of 3D choke ring antenna.

Variation of aerosol optical thickness with atmospheric water vapour : A case study over a continental station Mysore, India

Atmospheric measurements in a continental, low latitude station Mysore (12.3° N) has been carried out, for the period December 2003 to June 2006. Measurements were made using a sunphotometer with five bands in the visible and near-infrared range of the solar spectrum. To bring out the wavelength dependence of Aerosol Optical Thickness (AOT) on atmospheric water vapour, typically two wavelength channels are being used, one at 500 nm and the other at 1020 nm. A linear dependence between AOT and water vapour on meteorologically calm days is the important observation made. Growth rate of AOT is found to be larger at shorter wavelength (500 nm) than that of the longer wavelength (1020 nm). A mass-plot representation is followed on monthly basis, which is nothing but the graphical plot of spectral AOT versus water vapour of the scans for all the clear sky days of a particular month. Further investigations reveal that some months exhibit a single trend of growth of AOT with water vapour whereas double trend is the scenario for other months. These results provide insight into the changes in the atmospheric aerosol characteristics with precipitable water vapour, which is the subject matter of this paper.

Error handling in GPS data processing

We are aware that the processing of GPS data through GAMIT processing software is not free from errors. Some of them are generated due to different modules involved in processing. The data quality depends so many factors, like quality of met-instrument, which supplies the meteorological data, algorithm of processing which based on the network homogeneity or heterogeneity and location of the site, whether it is free from multi-path etc. The root mean square errors for New Delhi, Mumbai, Kolkata, Guwahati and Chennai GPS stations are spatially correlated and observations are weighted according to the satellite elevation angle. Diurnal variability of Integrated Precipitable Water Vapour (IPWV) has been shown its range from 45 mm to 65 mm for New Delhi during the monsoon season, 2008.

Retrieval of column-integrated water vapour from MODIS and analysis of its monthly and seasonal variability over several typical cities in China

Column-integrated water vapour also called Precipitable Water Vapour (PWV), is one of the main parameters influencing the global climate change. Due to its high spatial and temporal variability PWV has been found to be a good tracer of atmospheric motions. Retrieving PWV from Moderate Resolution Imaging Spectroradiometer (MODIS) data has the merits of high spatial resolution and low cost. In this paper, an algorithm for retrieving PWV using several MODIS near-IR channels data is first presented. Six typical cities in China with different climate are selected for study. These are Beijing, Shanghai, Guangzhou, Chengdu, Wuhan and Lanzhou. The variations of PWV in recent13 years (2001-2013) over six cities have been analyzed. The study brings out an increasing trend of annual average of water vapour over these cities in recent 13 years. The results also indicate that PWV reaches the highest value in summer, decreases in autumn, further decrease in spring, and is lowest in winter. PWV in summer over the six cities have been increasing in recent 13 years, but PWV in autumn and winter have been decreasing over inland cities, such as Wuhan and Beijing. Possible reasons for such observed trends are given in this paper.

Water Vapour Assessment Using GNSS and Radiosondes over Polar Regions and Estimation of Climatological Trends from Long-Term Time Series Analysis

The atmospheric humidity in the Polar Regions is an important factor for the global budget of water vapour, which is a significant indicator of Earth’s climate state and evolution. The Global Navigation Satellite System (GNSS) can make a valuable contribution in the calculation of the amount of Precipitable Water Vapour (PW). The PW values retrieved from Global Positioning System (GPS), hereafter PWGPS, refer to 20-year observations acquired by more than 40 GNSS geodetic stations located in the polar regions. For GNSS stations co-located with radio-sounding stations (RS), which operate Vaisala radiosondes, we estimated the PW from RS observations (PWRS). The PW values from the ERA-Interim global atmospheric reanalysis were used for validation and comparison of the results for all the selected GPS and RS stations. The correlation coefficients between times series are very high: 0.96 for RS and GPS, 0.98 for RS and ERA in the Arctic; 0.89 for RS and GPS, 0.97 for RS and ERA in Antarctica. The Root-Mean-Square of the Error (RMSE) is 0.9 mm on average for both RS vs. GPS and RS vs. ERA in the Arctic, and 0.6 mm for RS vs. GPS and 0.4 mm for RS vs. ERA in Antarctica. After validation, long-term trends, both for Arctic and Antarctic regions, were estimated using Hector scientific software. Positive PWGPS trends dominate at Arctic sites near the borders of the Atlantic Ocean. Sites located at higher latitudes show no significant values (at 1σ level). Negative PWGPS trends were observed in the Arctic region of Greenland and North America. A similar behaviour was found in the Arctic for PWRS trends. The stations in the West Antarctic sector show a general positive PWGPS trend, while the sites on the coastal area of East Antarctica exhibit some significant negative PWGPS trends, but in most cases, no significant PWRS trends were found. The present work confirms that GPS is able to provide reliable estimates of water vapour content in Arctic and Antarctic regions too, where data are sparse and not easy to collect. These preliminary results can give a valid contribution to climate change studies.

Precipitable water vapour monitoring using ground based GPS system

The sensing of near real time Precipitable Water Vapour (PWV) using Global Positioning System (GPS) over Indian region were analyzed. GPS data collected from five stations at hourly interval were utilized to determine near real time PWV using GAMIT software. Sliding window technique was used to derive near real time PWV. The PWV determined from GPS observations of each site were compared with respective radiosonde measurements. The results shows that the derived GPS precipitable water well agree for some stations with the independent radiosonde measurements. We have also examined the variation of hourly GPS-PWV with hourly rainfall observation and found that PWV increases significantly before the event take place and decreases after the event.

Effect of broadcast and precise satellite orbits in the estimation of Zenith tropospheric delay and integrated precipitable water vapour from GPS

Global Positioning System (GPS) estimates the total delay in zenith direction by the propagation delay of the neutral atmosphere in presence of water vapour present in the troposphere. This total delay has been treated as a nuisance parameter for many years by the geodesists. The above delay have two parts dry delay and wet delay and known as Zenith Hydrostatic Delay (ZHD) and Zenith Wet Delay (ZWD) respectively. The Integrated Precipitable Water Vapour (IPWV) is estimated through ZWD overlying the receiver at ground-based station. The accuracy of the above said estimates depends on the quality of the predicted satellite orbits, which are not the same for each individual satellite. India Meteorological Department (IMD) is operationally estimating the IPWV on near real time basis at five places and matches fairly well (error ~6.7 mm) with Radisonde (RS) data. This paper examine the effect of International GPS Service (IGS) predicted precise orbits and near real time predicted rapid or broadcast orbits supplied by the Scripps Orbit and Permanent Array Center (SOPAC) on Zenith Total Delay (ZTD) and IPWV estimates by calculating the mean Bias and Root Mean Square Error (RMSE) for ZTD and IPWV in mm for all the five stations. The observed bias for ZTD is almost of the order of less than 1 mm in most cases and RMSE is less than 6 mm. Similarly the bias observed in the case of derived IPWV is almost negligible and RMSE is less than 1 mm.