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Illustration of cosmic satellites
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| Radio occultation - when radio signals from GPS satellites pass through the atmosphere, the signals' paths are bent and their progress is slowed. The rate of these changes depends on the atmosphere's density along the path. COSMIC's low-Earth-orbiting satellites take advantage of this effect by intercepting the GPS radio signals just above Earth's horizon; measuring the bend and signal delay along the signal path.
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A team of meteorology experts from Japan, the US, the University of Pretoria and the CSIR recently conducted a study into the efficiency of the temperature estimates derived from the US/Taiwanese Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC/FORMOSAT-3) satellite. The latter, launched on 15 April 2006, is used for measuring lower atmospheric temperatures.
The team was headed by Dr Pangaluru Kishore from the Department of Physics and Atmospheric Science at Dalhousie University in Canada.
Through different forecast models, the scientists have validated and found that there is a large discrepancy in the magnitude over polar latitudes. Dr Sivakumar Venkataraman of the CSIR National Laser Centre involved in laser atmospheric remote sensing studies and collaborator in this study says, "This study has proven that even new satellites can provide inaccurate temperature measurements over polar regions where they rely mainly on in situ measurements."
In their paper published this year in Atmosphere Chemistry and Physics, Kishore says, "Temperatures are important in many aspects of atmospheric research and many studies have investigated the accuracy of derived temperature profiles. Knowledge of the upper tropospheric and lower stratospheric temperatures is important for understanding the structure and dynamics of the region; and is linked to matters concerning global climate change and the stratosphere-troposphere exchanges."
By April 2007, high-quality temperature profiles in the troposphere and lower stratosphere had been collected by nearly half a million COSMIC radio occultations (a satellite remote sensing technique). Venkataraman says, "Our analysis is based on the radio occultation data samples collected during the first year of observation from April 2006 to April 2007."
For the validation, the researchers used operational stratospheric analyses from the National Centres for Environmental Prediction - Reanalysis (NCEP), the Japanese 25-year Reanalysis, and the United Kingdom Met Office data sets. Spatially, the largest deviations are noted in the polar latitudes; height-wise, the tropical tropopause region noted the maximum differences. Venkataraman says, "We found that among the three reanalysis data sets, those from the NCEP resemble the COSMIC measurements best."
He adds, "Despite our findings, the COSMIC/FORMOSAT-3 satellite has an important role to play in the use of global positioning systems (GPS) radio occultation data for atmospheric and ionospheric (the upper atmosphere, mainly useful for telecommunications) research; and ultimately improve global weather forecasts and climate change-related studies."
Kishore explains that a variety of observational techniques have been used to measure temperatures in the troposphere and lower stratosphere; and their variations in time and space. He says, "These techniques include lidar, but most of these observations are mainly over land areas of the northern hemisphere and the period of observations has been limited." Venkataraman says, "The future plan for measuring lower atmosphere temperature and water vapour by lidar will ensure that my work at the CSIR National Laser Centre is based on thoroughly corroborated data."
The team plans to perform further investigations with additional COSMIC and other GPS radio occultation measurements to gain a better understanding of global warming and global climate change.
Enquiries: CSIR Communication
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