| dc.description.abstract | Water vapor is one of the most important constituents of the free atmosphere since it is the principal mechanism by which moisture and latent heat are transported and cause “weather.”(Gutman et al., 1989). Its significance arises from the fact that it is a primary contributor to greenhouse effect which influences the temperature average of the Earth’s surface resulting in globe warming(Alshawaf et al., 2012). One of the important goals in modern weather prediction is to improve the accuracy of short-term cloud and precipitation forecasts, but our ability to do so is severely limited by the lack of timely water vapor data(Alshawaf et al., 2012). Most of the water vapor in the atmosphere resides in the troposphere, which ranges from an altitude of approximately 9 kilometers at the poles to more than 16 kilometers at the equator. The measurement of atmospheric water vapour is essential for weather and climate research as well as for operational weather forecasting(Ie, 2001). Atmospheric water vapour is characterized by high temporal and spatial fluctuations which calls for continuous determination of its content.
Conventional meteorological devices such as radiosondes and water vapour radiometers are employed for measuring atmospheric water vapour. Radiosondes provide measurements at fine vertical resolution but they are not normally used in observing temporal and horizontal variations of water vapour because of their expensive nature hence increasing operational costs, the high costs also lead to infrequent launches, large distances between launching points compared with the lateral scale of variations in water vapour, and data not acquired at the assumed point of acquisition limits their usage especially in the developing countries(Gutman et al., 1989). Another alternative for measuring water vapour is by using water vapour radiometers but these do not measure the fine spatial variations of water vapour and are more suitable for use over oceans but the high price of these instruments limits their usage and they require constant calibration and do not function well under all weather conditions(Gutman et al., 1989).
Another method to measure atmospheric water vapour is by use of the Global Positioning System (GPS) satellites. These can be used to monitor precipitable water vapor with millimeter accuracy and sub-hourly temporal resolution (Rocken et al. 1993). However, these can only be used at sparse sites which limits the spatial resolution of water vapour maps derived based on GNSS (Gutman et al., 1989).
Just like GNSS observations, water vapour reduces the propagation of the INSAR signals in the atmosphere (Alshawaf et al., 2012). This effect could be exploited to determine the amount of water vapour in the atmosphere. One of the characteristics of the INSAR phase observations is that they have a high spatial coverage over a wide area (Alshawaf et al., 2012). | en_US |
