The Global Observing System

by Sue Barrell¹, Lars Peter Riishojgaard², Jochen Dibbern³ with contributions from many others

Three jewels shine particularly brightly in the crown of the World Weather Watch: the Global Observing System (GOS), the Global Telecommunication System (GTS) and the Global Data-processing and Forecasting System (GDPFS). As envisaged in the original, visionary plan for the WWW, these individual components deliver their achievements not in isolation but through their connection from end-to-end and through the strong user requirements process that underpins them. The combined achievement is far greater than the sum of the parts, and no single component would be able to deliver the intended benefits on its own. However, GOS is arguably unique in being the foundation on which the others stand, in providing the essential observations subsequently disseminated through the GTS and assimilated and processed into forecasting products through the GDPFS.

GOS is an extremely complex undertaking, and it is perhaps one of the most ambitious and successful instances of international collaboration of the last 100 years. It consists of a multitude of individual observing systems owned and operated by a plethora of national and international agencies with different funding lines, allegiances, overall priorities and management processes. And yet the overall requirements and reporting practices are almost universally adopted, and through the combination of GOS and GTS billions of observations are obtained and exchanged in real time between WMO Members and other partners every single day. Without GOS and GTS, not a single WMO Member would be able to serve the weather needs of its citizens as well as they do today.

In spite of the technological proficiency of modern society – and perhaps contrary to popular belief – our dependence on weather has increased rather than decreased over the last five decades. This is due to a combination of factors, including an exponential growth in travel and transportation by air and sea, a larger than ever part of the population living along the coastlines of the world and in other vulnerable areas such as flood plains, and increased reliance of intensive farming methods to feed growing populations, etc. The increasing demand for and dependence on weather information, as well as the emergence of new observational capabilities, have forced GOS since its inception to constantly evolve, and it will need to continue to do so in the future.

Components of GOS

At the outset, GOS included polar orbiting satellites, some 8 000 weather stations on land and roughly 4 000 merchant ships routinely providing meteorological observations to the WWW. About 800 of the land stations were also making upper-air soundings up to altitudes of 30 km. In addition, manual observations were made from about 3 000 commercial aircrafts.

Today the surface-based observing systems of GOS includes about 11 500 land stations making at least three-hourly and often hourly observations of meteorological parameters, 1 000 weather radars, 1 300 upper-air stations plus about 15 ships making upper-air profiles over the ocean, over 3 000 automatic observing systems onboard aircrafts, 4 000 routinely reporting ships, 1 250 drifting buoys, more than 500 moored buoys, and many other types of observing stations (e.g. wind profilers, lightning detection systems, tide-gauges, etc.) – all contributing to the WWW. Some 4 000 of the land stations comprise the Regional Basic Synoptic Networks and over 3 000 stations comprise the Regional Basic Climatological Networks both drawn up by the six WMO Regional Associations. A subset of these surface stations are used in the Global Climate Observing System (GCOS) Surface Network (GSN) and a subset of the upper-air stations composes the GCOS Upper-Air Network (GUAN).

While the advent of the space era was a key driver behind the creation of the WWW and GOS, it is the combination of the space-based and surface-based components that remains the key to its operational success, to the participation and commitment of all WMO Members and to the translation of data and information into needed and usable end-products.

Role of WMO in GOS

The components of GOS belong to the National Meteoro­logical and Hydrological Services of WMO Members, to other national and international agencies or to private entities. The role of WMO is to coordinate and guide GOS, both in its every day operations and in its strategic evolution. WMO thus maintains a Rolling Review of Requirements (RRR) aimed at continuously assessing user requirements and matching them against current and evolving observational capabilities. The Commission for Basic Systems has overall responsibility for the RRR. The bulk of the work is done through the Open Programme Area Group on Integrated Observing Systems (OPAG-IOS), which consists of a number of Expert Teams and formal and informal interfaces to other relevant entities inside and outside the WMO structure, e.g. technical commissions and regional associations.

The RRR process builds on two cornerstones:

• A regularly updated database listing observational data requirements for all application areas supported by WMO Programmes; and
• An updated database of all observational capabilities available to WMO Members and partners through the WMO Integrated Global Observing System (WIGOS).

The contents of these two databases are matched against each other annually to provide a gap analysis, the results of which are provided in Statements of Guidance (SoG) for each application area. The SoGs information is of a tactical nature and useful for both application experts and observing system developers and providers. For the former user group they serve as a brief introduction to the relevant observational capabilities, while for the latter they serve as an easily accessible reference listing the most important shortfalls of GOS at any given time.

Complementing the tactical guidance of the SoGs are two key documents of a more strategic nature: The WMO Vision for the GOS and the Implementation Plan for the Evolution of the Global Observing System (EGOS-IP). The former provides a broad outline of observational capabilities that are expected to be available to the operational users roughly 15 years into the future (the current version at the time of writing addresses the 2025 time frame), while the latter provides a detailed list of actions that will need to be taken in order to realize the Vision. Both documents were endorsed by the Commission for Basic Systems and subsequently adopted by the Executive Council as official WMO positions.

 

Evolution of GOS components and future outlook

Aircraft-based observations and AMDAR

The ties between the meteorological and aviation communities are long-standing, close and mutually beneficial. While it is obvious to most that Air Traffic Management and airline operations depend heavily on meteorological information and forecasts to ensure both passenger safety and economic efficiency of flight operations, it is not common knowledge that the aviation industry provides valuable data and information in support of meteorological and climatological applications.

Initially this provision of data was limited to little more than logged information from simple meteorological instruments and oral feedback from pilots regarding weather phenomena and conditions encountered during flight. Later, with the advent of radio communication and more sophisticated onboard equipment and avionics, such reporting was standardized and eventually automated into AIRcraft REPorts (AIREPS).

For decades, atmospheric scientists have utilized aircraft as platforms to collect upper-level atmospheric data. In the early 1970s, this led to the development of continuously automated, operational programmes of measurement of particular atmospheric variables from commercial aircrafts in a collaborative effort between aviation and meteorology.

The first measurement programme of this type was the aircraft to satellite data acquisition relay (ASDAR), which involved the fitting of a data acquisition and communications package to the aircraft fuselage. This gave way in the 1980s to the aircraft meteorological data relay (AMDAR), which provides observations from sensors and avionics and communications systems that are integrated into the aircraft’s systems – no modification to the airframe or its systems are required. This major advantage greatly facilitated AMDAR’s rapid growth over the last two decades to become an important component of GOS. Around 40 airlines and over 3 000 aircraft now contribute more than 300 000 high-quality observations per day of temperature and winds and other important quantities, including humidity. AMDAR observations are supplemented by additional automated meteorological observations made from the aircraft platform as a by-product of systems instituted by the International Civil Aviation Organization such as the Automatic Dependent Surveillance.

The availability of such valuable data from the aircraft platform is expected to grow exponentially in the future to provide improved global upper-air coverage. The expected increase in humidity measurements from aircrafts could potentially improve airline operations and have environmental benefits through applications such as water vapour contrail avoidance, possible icing warnings and fuel efficiency improvements.

Measurements of other atmospheric constituents of significance to aviation and the environment, such as volcanic ash, carbon dioxide and methane, are in the early stages of implementation and are expected to develop further.

Weather radars (precipitation)

Weather radar networks have continued to grow – approximately 1 000 are operated by National Meteorological and Hydrological Services – yet significant land areas remain uncovered. On the technology side, signal processing software and capacity have improved considerably, and the shift towards using polarimetry in all new installations is ongoing. After several decades of intensive research work, polarimetry is now ready for operational use with expected benefits in the classification of echoes, in the removal of non-meteorological echoes from data, and in providing better correction for hydrometeor attenuation at the C- and X-bands, which will lead to improved rain rate estimation.

An important factor contributing to the growth of the weather radar network is the growing interest in using X-band radars as a part of operational systems to improve coverage, for example, in mountainous regions or within urban areas, especially in hydrological applications related to flash flooding. One of the benefits of X-band is the lower cost of systems and infrastructure.

The use of radar data in NWP models has increased, and radar-derived rain intensities, radar reflectivities, radial wind data and derived vertical wind profiles are now used for both assimilation and verification, and have been shown to improve the predictive skill of the models. The international exchange of these data is a prerequisite for further developments

Wind profilers

Ground-based remote measurements of the vertical profile of the horizontal wind vector in the atmosphere by wind profiler were first demonstrated in the early 1970s and have undergone continued development and improvement since. The main advantage of wind profilers is their ability to provide vertical profiles of the horizontal wind at high temporal resolution in nearly all meteorological conditions, whether cloudy or clear, without the need for additional, a priori, information. No other remote sensing instrument has similar capabilities. Comparisons have shown that the accuracy of a well-operated and maintained wind profiler is at least comparable to that of radiosonde wind data, if not better. Wind profilers are now widely used both in operational meteorology and for research purposes.

The US National Oceanic and Atmospheric Administration set up the first operational profiler network in the mid-1990s using UHF radars (404 and 449 MHz). Since then, additional networks have been installed in Europe and Asia. The growing number of installations has led to an increase in the use of radar wind profiler data in data assimilation for NWP purposes in the last decade. A study by the UK Met Office showed that the assimilation of radar wind profiler data clearly reduced the forecast error in both global and high-resolution models, with a positive overall impact even exceeding that of radiosonde observations.

The high-resolution observations provided by radar wind profilers are especially well-suited to describe the atmospheric state at the mesoscales and finer, where other observational data tend to fall short. It is expected that the impact of radar wind profiler data will be even higher for mesoscale models.

 

 

 

Ground-based Global Navigation Satellite System (GNSS) data

There is a continued lack of humidity observations in the meteorological observing system, and in recent years ground-based GNSS data (Zenith Total Delay, or ZTD) has been gaining ground as a remedy. Most GNSS sites are installed for positioning purposes, and for the operators and primary users the atmospheric delay is thus a noise term. However, the ZTD measurement scan can be converted into estimates of column atmospheric water vapour above the GNSS site, which can then be delivered in near real-time for operational meteorological applications.

Production of near real-time ZTD estimates requires a close collaboration between geodesy and meteorology. In the future, it is likely that benefits will flow both ways in that meteorological information will help further improve GNSS positioning. The two most familiar GNSS systems are the US GPS system and the Russian GLONASS. However, new European and Asian systems are on the way. This will improve the quality of ZTD estimates, and enable new, more detailed GNSS atmospheric products to be produced for meteorological usage (ZTD gradients, slant delays, tomographic reconstruction of the water vapour field).

Since the start of the millennium the “GNSS NRT ZTD network” has increased tremendously. It is estimated that data from 4 000 to 5 000 sites are currently available, with the majority coming from high-density networks in Western Europe, North America and Japan. A strong increase is expected over the next few years. European ZTD data are distributed via GTS as additional data; however, the exchange of such data still needs substantial improvement on the global level.

ZTD observations are unique among meteorological data, in that the quality of GNSS ZTDs improve with time, from real-time, to near real-time, to post processing, and finally to re-analysis. This is due to the fact that additional and better information about the state of the GNSS system itself becomes available over time, based on measurements rather than on predictions. This opens the possibility of generating higher quality products with higher data latencies, specifically intended for climate applications.

Marine observations

Both dedicated marine meteorological applications and weather and climate applications, more generally, rely heavily on in situ and satellite meteorological and oceanographic observations in the marine environment. In situ marine observations also provide ground truth for the validation of satellite observations and make measurements not yet obtainable by other means.

For decades ships were the only means of obtaining such observations, but with the advent of WWW other types of observing platforms were developed. These include dedicated ocean weather ships, drifting buoys, wave buoys, meteorological and oceanographic moored buoys, tide gauges, Tsunami monitoring platforms (monitoring both sub-sea earthquakes and surface Tsunami waves), and more recently Argo deep ocean profiling floats, ocean surface and sub-surface gliders, and High Frequency (HF) coastal radars monitoring waves and ocean surface currents.

Looking at the evolution of the availability of marine meteorological climate data from various observing platform types in the last 75 years (below), it can be seen that since the establishment of the WWW, there has been an impressive increase in the number of observations collected, from about 1.5 million to more than 9 million. While ship data has decreased substantially, this has been off-set by the increase of observations produced from drifting and moored buoys.

In the last ten years, substantial efforts were made internationally through WMO, but also in partnership with the Intergovernmental Oceanographic Commission (IOC) of UNESCO, to develop and implement marine meteorological and ocean observing systems in a more coordinated way. Today, 62 per cent of the initial composite ocean observing system has been completed, and three components have achieved their initial implementation target: the drifting buoy array (September 2005) with 1 250 units, the Argo profiling float programme (November 2007) with 3 000 units and the VOS Climate Project fleet (June 2007) with 250 VOSClim ships.

Looking into the future, there are emerging requirements for new technologies and for observations of additional quantities, especially biogeochemical variables. New types of ocean observing platforms are expected to be increasingly used, e.g. surface wave gliders, sub-surface gliders, marine animal mounted observing platforms, and bottom-based observing platforms connected to old submarine telecommunication cables.
 

Benefits to the end-users

The end-user benefits of the WWW are as diverse as improved agricultural efficiency and productivity, reduction of urban pollution, and human health improvements associated with better-managed environmental systems. Equally important are the scientific foundation of the product generation and the service delivery mechanisms that ultimately enable National Meteorological and Hydrological Services to provide improved and targeted outputs tailored to end-users. These must be jointly credited to all components of the WWW; however, there are some notable areas of achievement that can be more specifically attributed to the GOS.

All stages of the Disaster Risk Reduction (DRR) cycle – plan, prepare, predict, respond, recover, review – benefit from the GOS. Not only do the concepts of integration and network design stand behind the multi-hazard approach, but without the worldwide integration provided by the GOS, local observations would at best inform local decisions only. The ability to confidently mobilize regional and international responses to disasters hinges on the reliability and trust engendered by the routine observations provided 24 hours per day, 365 days per year by the GOS and their prompt exchange through the GTS.

The long-term observational data record provides the strongest evidence of global climate change. While the satellite system was given little credit in the past for its ability to monitor change extending beyond the life-time of a single satellite, both the space- and surface-based components of the GOS are now recognized as being capable of recording and documenting real measures of the temporal trends and geographic distribution of global climate change. The global, high-level political attention now focused on understanding and addressing the impacts of climate change provides tangible evidence of the value of international cooperation on observing systems. This level of attention, reflected also in international mechanisms to understand and address global climate change such as the Intergovernmental Panel on Climate Change and the United Nations Framework Convention on Climate Change, would not have materialized without internationally negotiated and agreed observing and reporting standards, and the establishment and maintenance of global observing systems, including high-quality baseline networks. Monitoring the nature and the impact of climate change requires the continued development and strengthening of internationally coordinated observing systems exemplified by GOS.

The fact that application areas well beyond weather prediction have benefited from the Global Observing System clearly shows that GOS is not just about weather: it also provides an example and a solid base on which to build even more comprehensive observational programmes.

GOS contributes directly and indirectly to economic growth and prosperity due to the many and diverse economic sectors that rely on timely and reliable weather and weather forecast information for decision-making on a wide variety of time scales. It plays a critical role in protecting life and property.
 

The way forward: Integration

The concept of integration has been central to GOS from its beginning, bringing together and optimizing the contributions of many diverse space-based and surface-based component observing systems into a composite system of systems. Over the last two decades, NWP has revolutionized the assimilation of observations on a variety of temporal and spatial scales and has delivered further value from integration at both a system and data level. This development has facilitated the extraction of added value from observations and the identification of those observations that carry the largest information content. The requirements of different application areas can now be met more efficiently and in an integrated way using a mix of composite and complementary systems that capitalize wherever possible on the “observe once, use many times” principle while the Rolling Review of Requirements process will help identify the unique additional measurements required to satisfy unmet needs.

GOS is now part of the WMO Integrated Global Observing System (WIGOS), an even more broadly integrated approach to observations, which will enable more Members and communities to benefit from its systematic and comprehensive approach to meteorological observations. WIGOS is aimed at providing a more coordinated approach to all observing systems supported by WMO and its Members, extending the system of systems approach already established within GOS to other systems such as the Global Atmosphere Watch (GAW), the World Hydrological Cycle Observing System (WHYCOS), the Baseline Surface Radiation Network (BSRN) and the Global Climate Observing System (GCOS). WIGOS is intended to be a comprehensive, coordinated and sustainable system of observing systems and, together with WIS, aims to improve the ability of WMO Members to provide a wide range of observation-based services and to better serve the needs of relevant research programmes. WIGOS is also an important WMO contribution to the Global Earth Observation System of Systems (GEOSS), to which WIGOS brings all the capability and experience of WWW, the integrated and systematic user-driven approach of the GOS, the connection to users, and the spirit of voluntary international cooperation and collaboration.

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¹ CBS Vice-president and Assistant Director, Observations and Engineering, Bureau of Meteorology, Australia

² Chair CBS OPAG-IOS and Director, Joint Centre for Satellite Data Assimilation, NOAA Science Centre

³ Co-Chair CBS OPAG-IOS and Director, Technical Infrastructure and Operations, Deutscher Wetterdienst, Germany