Ta-Kang Yeh

Precipitable Water Vapor Estimates in the Australian Region from Ground-Based GPS Observations

We present a comparison of atmospheric precipitable water vapor (PWV) derived from ground-based global positioning system (GPS) receiver with traditional radiosonde measurement and very long baseline interferometry (VLBI) technique for a five-year period (2008–2012) using Australian GPS stations. These stations were selectively chosen to provide a representative regional distribution of sites while ensuring conventional meteorological observations were available. Good agreement of PWV estimates was found between GPS and VLBI comparison with a mean difference of less than 1 mm and standard deviation of 3.5 mm and a mean difference and standard deviation of 0.1 mm and 4.0 mm, respectively, between GPS and radiosonde measurements. Systematic errors have also been discovered during the course of this study, which highlights the benefit of using GPS as a supplementary atmospheric PWV sensor and calibration system. The selected eight GPS sites sample different climates across Australia covering an area of approximately 30° NS/EW. It has also shown that the magnitude and variation of PWV estimates depend on the amount of moisture in the atmosphere, which is a function of season, topography, and other regional climate conditions.

Identifying the Relationship between GPS Data Quality and Positioning Precision: Case Study on IGS Tracking Stations

AbstractThe number of global positioning system (GPS) tracking stations is increasing, primarily because the stations are multifunctional. In civil engineering, they can be used for precision positioning; in the earth sciences, they can be used to monitor faults and earthquakes; and in the atmospheric sciences, they can be applied to predict perceptible water vapor. Currently, there are more than 400 GPS stations in Taiwan; however, the data obtained through such stations are not being assessed carefully. Experienced scientists and engineers examine the data in advance to see if they qualify for research purposes, but inexperienced users can adopt poor quality data that eventually lead to inaccurate research results. Of the observation stations with receivers that were renewed between 2006 and 2008 in the International GNSS Service Network, four stations (ZIMM, BOR1, NRC1, and NICO) were selected to be the subjects of this research. Six indexes of data quality were observed to calculate the quality of dat...

Comparisons Between Air and Subsurface Temperatures in Taiwan for the Past Century: A Global Warming Perspective

Air and sea surface temperature increases due to global warming have been widely observed around the world at various rates. This temperature rising has also been documented in many subsurface records recently. The air-ground temperature coupling system introduces an important factor in disturbing the original thermal balance and provides a new dimension to comprehend the effects of global warming on the Earth system. Ten meteorological stations of Central Weather Bureau in Taiwan that have been routinely measured for air (1.5 m above the ground) and subsurface (at depths of 0, 5, 10, 20, 30, 50, 100, 200, 300 and 500 cm below the ground) temperatures are used for in-depth comparison in this study. These stations have a mean observation period of 82 years (as of 2008) to provide good coverage for a preliminary examination of air-ground temperature coupling relationship in a century scale. Results show that patterns and variations of air and subsurface temperature are quite different among stations in Taiwan. In general, air and subsurface temperatures exhibit consistent linear trends after 1980 due to accelerating global warming, but display complex and inconsistent tendencies before 1980. When surface air temperature is subtracted from subsurface one, the differences in the eastern Taiwan are generally larger than those in the western Taiwan. This observation is possibly caused by (1) heat absorption of dense high-rise buildings, and/or (2) cut off heat propagating into deep depths in the urban area of western Taiwan. By comparing temperature peaks at various layers from shallow to deep, rates of thermal propagation can be estimated. The distinct time shifts among stations suggest that thermal propagations have to be taken into account when constructing historical temperature records.

Investigation into the atmospheric parameters retrieved from ROPP and CDAAC using GPS radio occultation measurements over the Australian area

GPS Radio Occultation (RO) is a space-based technique for sounding the Earths atmosphere. This technique has demonstrated great potential for improving numerical weather prediction and climate monitoring. This investigation utilises FORMOSAT-3/COSMIC RO data to identify the differences between the results obtained using two different data processing packages – the Radio Occultation Processing Package (ROPP) and the COSMIC Data Analysis and Archive Center (CDAAC) software package. The introduced RO measurements for two software packages are all obtained from CDAAC dataset. This study analyses 20,210 events located in the Australian region in the year 2010. The results of this study show a negative bias between the two software packages in the bending angle at heights below approximately 5 km. The refractivity also shows a negative bias, which is consistent with previous results from the Global Navigation Satellite System Receiver for Atmospheric Sounding Satellite Application Facilities (GRAS SAF) report. A negative bias between the two processing software packages also appears for the pressure parameter at all heights and for dry temperatures from 5 to 16 km in height. The height intervals showing differences less than 1% in the bending angle, refractivity, pressure and dry temperatures are 0–25 km, 0–16 km, 0–16 km and 6–23 km, respectively. In general, the differences between the ROPP and CDAAC processing methods in the bending angle, refractivity, and pressure increase with altitude but are always less than 3%. The difference in dry temperature is also less than 3% at heights greater than 5 km, but it is larger at heights below 5 km.

Analyzing the Variation of Precipitable Water Vapor with Ground-Based GPS Over Taiwan

Water vapor plays an important role in weather prediction. Thus, it would be helpful to use Precipitable Water Vapor (PWV) data from Global Positioning System (GPS) signals to understand weather phenomena. Approximately 100 ground GPS stations that cooperate with approximately 500 ground weather stations were used in this study. The hourly Zenithal Wet Delay (ZWD) values during the observation period of between 2006 and 2011 were estimated. The PWV which was converted from the ZWD variations were compared with the rainfall observations. The results indicated that the PWV amplitudes were between 10.98 and 13.10 mm and always occurred at the end of July. The magnitudes of the PWV annual growth rate were between 0.68 and 0.83 mm/year. Although the end of July with the greatest monthly average PWV values, the rainfall magnitude on this period is smaller than that during the typhoons, which only occurred for a few days. The PWV also increased during typhoons. Because this affect was short-term, it did not contribute to the PWV monthly average seriously.