The Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is an international reference observing network, designed to fill an important gap in the current global observing system. GRUAN measurements will provide long-term, high-quality climate data records from the surface, through the troposphere, and into the stratosphere. These will be used to determine trends, constrain and calibrate data from more spatially‐comprehensive observing systems (including satellites and current radiosonde networks), and provide appropriate data for studying atmospheric processes. GRUAN is envisaged as a global network of eventually 30-40 sites that, to the extent possible, builds on existing observational networks and capabilities.
Why do we need GRUAN?
Understanding our changing climate, and the underlying causes of these changes, requires an understanding not just of changes at the surface of the Earth but throughout the atmospheric column. Furthermore, high-quality measurements are needed to separate the climate change signal from natural variability.
A GRUAN reference observation
- Is traceable to a SI unit or an internationally accepted standard
- Provides comprehensive uncertainty analysis;
- Is documented in accessible literature
- Is validated (e.g. by intercomparison with complementary measurement systems)
- Includes complete metadata description
GRUAN data processing
- Corrects for all known errors and biases
- Is based on sensor character-isationfrom laboratory studies and field intercomparisons
- Provides best estimate of the – vertically resolved – measurement uncertainty
The GRUAN Measurement Strategy Aims to
- Maintain measurements over several decades to accurately quantify trends
- Provide data to validate and calibrate measurements from observing systems that are more spatially extensive, such as satellite systems and the global radiosonde network, leading to improved satellite data products
- Fully characterise the properties of the atmospheric column
- Measure essential climate variables using a number of different but com-plementary techniques to validate derived measurement uncertainties
- Characterise observational biases and estimate measurement uncertainties
- Describe measurements using extended metadata and comprehensive documentation of the observing technique
- Ensure long-term stability of measurement series by managing instrument changes, thus improving the overall upper-air observing network
- Tie measurements to SI units or internationally accepted standards
- Ensure that potential gaps in satellite programmes do not invalidate the long-term climate data record
- Further our understanding of climate variability and change
Scientific Imperatives Include
- Characterisation of changes in essential climate variables, in particular temperature, humidity, and wind
- Understanding the climatology and variability of humidity, particularly in the region around the tropopause since this is where changes have their largest effect on climate sensitivity
- Understanding changes in the hydrological cycle
- Understanding and monitoring tropopause characteristics
- Understanding the vertical profile of temperature trends
- Bringing closure to the Earth’s radiation budget and balance
- Understanding climate processes and improving climate models
Example: Water Vapour
Water vapour is the most important greenhouse gas as it is responsible for about 60% of the natural greenhouse effect. There are vigorous discussions within the research community regarding whether stratospheric humidity has changed and whether any further change is expected to influence the Earth’s energy budget. At the same time, water vapour measurements, particularly around the tropopause, are afflicted with high measurement uncertainties. Even key mechanisms controlling humidity in this region are not fully understood, leading in turn to significant deficiencies in the predictive skill of global climate models. Currently, satellites and research-quality instruments on aircraft and balloon platforms are the main sources of humidity measurements around the tropopause. Differences between these measurement systems have been difficult to reconcile.
Existing records of upper-air temperatures are insufficient to meet the growing range of needs for studying climate. They greatly lack continuity, homogeneity and representativeness of data, because past measurements were seldom intended for climate research, but mainly for short-term weather forecasting. It is likely that similar problems will persist in the future. Therefore, a way of separating climate change signals from the inevitable non-climatic effects, caused by measurement biases, instrument instabilities and network inhomogeneities, is essential.