The Potential of Observing African Weather with GNSS Remote Sensing

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Climate System and Water Vapour

Climate is defined as the combined averaged weather conditions (i.e., « temperature, air pressure, humidity, precipitation, sunshine, cloudiness, and winds » etc.) over a place or region for a succession of years (Goosse et al., 2010). The climate system consists of five components: the atmosphere, hydrosphere, cryosphere, land surface and biosphere. The atmosphere is composed of oxygen (20.95%), nitrogen (78.09%), carbon dioxide (0.039%), argon (0.93%), and some other gases (Lutgens and Tarbuck, 2004). Temperature varies in the atmosphere and this has led to the atmosphere being divided into five regions, namely the troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The lowest part of the earth’s atmosphere is the troposphere. The troposphere starts from the surface of the earth to about 9 km at the poles and 17 km at the equator. The lowest part of the troposphere is known as the planetary boundary layer (PBL) and is directly influenced by the activities on the earth’s surface. Above the PBL is the free atmosphere where only some internal air turbulence occurs. The top boundary of the troposphere is known as the tropopause and its height is an indicator of tropospheric warming.
Within the troposphere, the temperature decreases with height and the value at the tropopause is usually the lowest in the troposphere (Mason et al., 2001; Li, 2013). The region from the tropopause upwards to about 50 km is the stratosphere. In the stratosphere, the temperature increases with height. Although the climate change in the stratosphere does not directly affect the biology of the earth, it may affect the height of the tropopause and thus affect the warming or the cooling of the troposphere. The mesosphere starts from the top of the stratosphere to about 85 km upwards.
In the mesosphere, the temperature decreases with altitude. The top of the mesosphere is known as the mesopause and is the coldest layer of the earth’s atmosphere. From the mesopause to about 700 km above the earth is the thermosphere. In the thermosphere, the temperature rises with altitude. Above the thermosphere is the exosphere, which is the outermost layer of the earth’s atmosphere. Since this layer is close to the sun and due to low pressure, the temperature is rather high. Gases in this layer are hardly affected by the earth’s gravity anymore, thus their atoms and molecules can move into outer space. A unique region of interest in the earth’s atmosphere is the ionosphere.
The ionosphere ranges from about 60 km to about 1500 km, and it includes the thermosphere and parts of the mesosphere and exosphere (Odijk, 2002). The ionosphere contains ions (charged particles) due to effects from the sun (solar radiation). The ionosphere has a significant effect on electromagnetic waves travelling through it. Figure 1.1 depicts a schematic illustration of the vertical structure of the atmosphere. The earth’s climate is transforming, and the transformations are expected to have a massive effect on the inhabitants of our planet, ecosystems, cities and energy use.
At the start of the 20th century the global air temperature was about 1.5 degrees Fahrenheit lower than it is today; the rise in the last 30 years has been about 1 degree (IPCC, 2001a). The global temperature is again likely to go up by another 2 to 8.6 degrees Fahrenheit by 2100, as reported by the Intergovernmental Panel on Climate Change (IPCC). The substantial boost in the earth’s average near surface temperature in the lower atmosphere is due to heat preservation caused by the build-up of greenhouse gases (i.e., water vapour, carbon dioxide, methane, nitrous oxides, sulphur hexafluoride, hydro- fluorocarbons, perfluorocarbons and chlorofluorocarbons).
These gases form a coverlet around the earth that lets the inward-bound rays of the sun (short-wave radiation) pass through but obstructs the reflected heat rays (long-wave radiation) from being released into space (see, e.g., www.kmaheshwari.com; www.climate.thinkaboutit.eu). This atmospheric heating phenomenon is often referred to as the « greenhouse effect ». Although the aforementioned climate change could be caused by natural reasons, such as eruption of volcanoes, the main contributions to the change are made by anthropological factors. Among these anthropological factors, the most important one is the combustion of fossil fuels, which results in an increase in the amount of greenhouse gases in the atmosphere.
Water vapour is a major natural greenhouse gas, which is the source of about 36-70% of the greenhouse effect on Earth excluding clouds; other gases such as carbon dioxide contribute about 9-26%, methane contributes about 4-9%, and ozone contributes 3-7% (IPCC 2001b). It is not feasible to declare that a particular gas causes a definite fraction of the greenhouse effect, for the reason that the influences of the diverse gases are not additives. Water vapour is one of the more important components of the atmosphere, given that it is the means by which humidity and energy (as latent heat) are transported through the troposphere and lower stratosphere to influence weather. Furthermore, apart from the function of water vapour in balancing the atmospheric heat budget, water vapour plays an essential task in the global hydrological cycle and global climate system (Boutiouta and Lahcene, 2013).

Table of Contents :

• Title Page
• Declaration
• Certification
• Acknowledgement
• Abstract of the Thesis
• Author’s Literature and conference Contributions
• Table of Contents
• List of Figures
• List of Tables
• List of Acronyms
• Chapter 1: Introduction
  • 1.1 Climate System and Water Vapour
  • 1.2 Water Vapour Observing Systems
  • 1.3 GNSS and its Scientific Applications
  • 1.4 Research Objectives
  • 1.5 Research Methodology
  • 1.6 Scope and Limitation of Research
  • 1.7 Organisation of Thesis
• Chapter 2 : The Potential of Observing African Weather with GNSS Remote Sensing
  • 2.1 Introduction and Background
  • 2.2 Operational Requirements and Standards for GNSS Meteorology
  • 2.3 Status of the African GNSS Network
  • 2.4 Preliminary Results on Monitoring of GNSS ZTD Variability on the African GNSS Network
  • 2.4.1 GNSS Data Processing and Tropospheric Product Descriptions
  • 2.4.2 GNSS ZTD Variability on the African GNSS Network
  • 2.5 Steps to Improve GNSS Meteorology within the African GNSS Network
  • 2.5.1 Densification of GNSS Networks through Collaborative Initiatives
  • 2.5.2 Meteorological Parameter Modelling from Global/Regional Weather Models
• • 2.5.3 Filling the Gaps with GNSS Radio Occultation (RO)
  • 2.6 Concluding Remarks
  • Acknowledgment
• Chapter 3: Evaluation of Spatial and Temporal Characteristics of GNSS Derived ZTD Estimates in Nigeria
  • 3.1 Introduction and Background
  • 3.2 Material and Methods
  • 3.2.1 GNSS Data Processing
  • 3.2.2 Spatial Auto-Correlation Analysis
  • 3.2.3 Temporal Characteristics (Diurnal and Intra-seasonal Trends)
  • 3.2.3.1 The Mann Kendall Test
  • 3.2.3.2 Spectral Analysis and Testing for White Noise
  • 3.2.3.3 Test for Stationarity
  • 3.3 Results and Discussions
  • 3.3.1 Spatial Characteristics
  • 3.3.2 Temporal Characteristics (Diurnal and Intra-seasonal Trends)
  • 3.4 Concluding Remarks
  • Acknowledgment
• Chapter 4: Performance Evaluation of Blind Tropospheric Delay Correction Models over Africa
  • 4.1 Introduction and Background
  • 4.2 Description of Tropospheric Correction Models Adopted in This Study
  • 4.2.1 Saastamoinen Model
  • 4.2.2 UNB3m Hydrostatic Delay Model
  • 4.2.3 Global Pressure Temperature wet (GPT2w) Model
  • 4.3 Assessment of the Accuracies of the UNB3m and GPT2w Models
  • 4.4 Concluding Remarks
  • Acknowledgment
• Chapter 5: Evaluation of Surface Variables from Global Reanalysis Models and their Application in Precipitable Water Vapour Retrieval from GNSS
  • Observations over Nigeria
  • 5.1 Introduction and Background
  • 5.2 Data and Methods
  • 5.2.1 Reanalysis Datasets
  • 5.2.2 Observational Data
  • 5.2.3 Validation of Reanalysis Dataset using AWOS Over Nigeria
  • 5.2.4 Application of Reanalysis Dataset in GNSS Atmospheric Precipitable Water Vapour Estimations
  • 5.3 Results and Discussions
  • 5.3.1 Results on the Validation of the ERAI and NCEP/NCAR Reanalysis Models
  • 5.3.2 Analysis of Derived PWV from GNSS and ECMWF/NCEP/GPT Dataset
  • 5.3.2.1 Comparison of ZHD from AWOS, ECMWF, NCEP, and GPT2 Dataset
  • 5.3.2.2 Comparison of Tm Estimated from Ts of AWOS, ECMWF, NCEP, and GPT2 Dataset
  • 5.3.2.3 Comparison of PWVs from AWOS, ECMWF, NCEP, and GPT2 Dataset
  • 5.4 Concluding Remarks
  • Acknowledgment
• Chapter 6: Modelling Weighted Mean Temperature in the West African Region: Implication for GNSS Meteorology
  • 6.1 Introduction and Background
  • 6.2 Materials and Methods
  • 6.2.1 Data and Location
  • 6.2.1.1 Radiosonde Data
  • 6.2.1.2 NWP Data
  • 6.2.2 Estimation of Tropopause Height from the NCEP/NCAR Reanalysis Model and Sounding Data
  • 6.2.3 Determination of Tm from NCEP/NCAR Reanalysis Model and Sounding Data (Integral Method)
  • 6.2.3.1 Harmonic Model from Global NCEP/NCAR Reanalysis Data
  • 6.2.3.2 UNB3m Model
  • 6.2.3.3 GPT2w Model
  • 6.2.4 Estimating Precipitable Water Vapour from GNSS Observations
  • 6.3 Results, Model Validation and Discussions
  • 6.3.1 Results of the Models
  • 6.3.2 Validation of Models
  • 6.3.3 Spatiotemporal Variation of TmModels
  • 6.4 Implication of TmModelling for GNSS Meteorology
  • 6.4.1 Precision of PWV from Tm Models
  • 6.4.2 Analysis of Derived PWV from GNSS and the Various Tm Models
  • 6.5 Concluding Remarks
  • Acknowledgment
• Chapter 7: Retrieval and Analysis of Precipitable Water Vapour based on GNSS, AIRS, and Reanalysis Models over Nigeria
  • 7.1 Introduction and Background
  • 7.2 Overview of Data sets and PWV Retrieval Procedures
  • 7.2.1 GNSS Observations
  • 7.2.2 Atmospheric Infrared Sounder
  • 7.2.3 ERA-Interim Reanalysis
  • 7.3 Methodology
  • 7.4 Results and Discussions
  • 7.4.1 Diurnal Relations of GNSS PWV with other Retrievals (AIRS and ERAI)
• • 7.4.2 Monthly Relations of GNSS PWV with other Retrievals (AIRS and ERAI)
• • 7.5 Concluding Remarks
  • Acknowledgment
• Chapter 8 : Assessing the Meteorological Impact of Variations in Atmospheric Water Vapour Content over Nigeria from GNSS Measurements
  • 8.1 Introduction and Background
  • 8.2 Data and Methodology
  • 8.3 Variability of GNSS PWV over Nigeria
  • 8.4 Investigation of GNSS PWV and Rainfall Events over Nigeria
  • 8.5 Investigation of GNSS PWV and Solar Events over Nigeria
  • 8.6 Concluding Remarks
  • Acknowledgment
• Chapter 9 : Conclusion, Outlook and Recommendations
  • 9.1 Summary
  • 9.2 Conclusion
  • 9.3 Outlook and Recommendations
  • List of References

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