ABSTRACT

The Global Positioning System (GPS) is a space-based radio positioning system, which is capable of providing continuous position, velocity, and time information to users anywhere on, or near, the surface of the Earth. The main objective of this work was to estimate the integrated precipitable water vapour (IPWV) using ground GPS and Low Earth Orbit (LEO) Radio Occultation (RO) to study spatial-temporal variability. For LEO-GPS RO, we used Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) datasets. We estimated the daily and monthly mean of IPWV using six selected ground-based GPS stations for the years 2012 to 2016. The main perspective for selecting the years 2012 to 2016 is that, continuous data were available during these periods at all Ethiopian GPS stations. We studied temporal, seasonal, diurnal, and vertical variations of precipitable water vapour using GPS observables extracted from the precise geodetic GAMIT-GLOBK software package. Finally, we determined the cross-correlation of our GPS-derived IPWV values with those of the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40 Interim reanalysis and of the second generation National Oceanic and Atmospheric Administration (NOAA) model ensemble Forecast System Reforecast (GEFS/R) for validation and static comparison. There are higher values of the IPWV range from 30 to 37.5 millimetres (mm) in Gambela and Southern Regions of Ethiopia. Some parts of Tigray, Amhara, and Oromia regions had low IPWV ranges from 8.62 to 15.27 mm. The correlation coefficient between GPS-derived IPWV with ECMWF and GEFS/R exceeds 90%. We conclude that there are highly temporal, seasonal, diurnal, and vertical variations of precipitable water vapour in the study area.

摘要

全球定位系统 (GPS) 是一种基于空间的无线电定位系统，能够为地球表面上或附近任何地方的用户提供连续的位置、速度和时间信息。这项工作的主要目标是使用地面 GPS 和低地球轨道 (LEO) 无线电掩星 (RO) 来估计综合可降水汽 (IPWV) 以研究时空变异性。对于 LEO-GPS RO，我们使用了气象、电离层和气候星座观测系统 (COSMIC) 数据集。我们使用 2012 年至 2016 年的六个选定的地面 GPS 站估算了 IPWV 的日和月平均值。选择 2012 年至 2016 年的主要观点是，在这些时期内，埃塞俄比亚所有 GPS 站都可以获得连续数据。我们研究了时间的、季节性的、昼夜的、和使用从精确大地测量 GAMIT-GLOBK 软件包中提取的 GPS 观测值的可降水水汽的垂直变化。最后，我们确定了 GPS 衍生的 IPWV 值与欧洲中期天气预报中心 (ECMWF) ERA-40 中期再分析和第二代国家海洋和大气管理局 (NOAA) 模型集合的互相关Forecast System Reforecast (GEFS/R) 用于验证和静态比较。埃塞俄比亚的甘贝拉和南部地区的 IPWV 值较高，范围为 30 至 37.5 毫米 (mm)。提格雷、阿姆哈拉和奥罗米亚地区的部分地区 IPWV 范围较低，为 8.62 至 15.27 毫米。GPS 推导的 IPWV 与 ECMWF 和 GEFS/R 的相关系数超过 90%。我们得出结论，存在高度时间性、季节性、昼夜性、