GRUAN depends critically on a wide variety of instrumentation for measuring upper-air essential climate variables. In addition, different instruments are often used to measure the same climate variable as such a variety of approaches allows us to validate different measurement systems through intercomparisons. The core GRUAN principle of ensuring traceability of measurements to SI units or community accepted standards, requires a deep understanding of how every instrument used in GRUAN functions.

Radiosondes

Radiosondes provide high resolution in-situ data of the first priority essential climate variables temperature, humidity, and pressure, as well as of wind speed and direction in the form of vertical profiles through the troposphere and into the lower stratosphere. For the GRUAN reference measurements based on balloon soundings, radiosondes quantitatively form the backbone. They also serve as transmitter for data from additional sensors or devices launched together with the sondes in combined setups, which provide measurements of the essential climate variables or additional quantities based on other or more specialized technologies. These sensors include:

Ozone sensors

Additional sensors for measuring ozone concentrations through the troposphere and into the stratosphere. These are typically ozonesondes.

Water vapour sensors

Humidity sensors on radiosondes are seldom suitable for measuring the very low water vapour concentrations found in the stratosphere. Therefore, specialized instruments are often flown to measure water vapour concentrations in the stratosphere.

Temperature sensors

Although radiosondes do measure temperature, they may not always be suitable for very specific applications e.g. measuring extremely rapid changes in temperature that result from atmospheric turbulence. Therefore, additional fast response temperature sensors are sometimes flown together with radiosondes to provide such bespoke temperature measurements.

Aerosol sensors

Measuring aerosols size distributions, composition and concentrations is essential for understanding how the atmosphere absorbs and transmits radiation. Aerosols also play key roles in cloud formation by providing cloud condensation nuclei. Specialized sondes are often flown on balloons to make the required measurements to understand aerosols in Earth’s atmosphere.

GNSS-PW

Global Navigation Satellite Systems (GNSS) satellites transmit radio waves captured by the antennas of the ground-based GNSS receivers. The radio waves are delayed by the ionosphere and the neutral atmosphere. Total delay along the zenith, called zenith total delay (ZTD), is estimated from GNSS observations and is used to derive atmospheric precipitable water (PW). The advantages of the GNSS PW data include continuous measurements, global coverage, availability under all weather conditions, high accuracy, long-term stability and low cost.

Lidar

The lidar (LIght Detection And Ranging) technique is an atmospheric remote sensing technique utilizing the scattering of light by the atmospheric molecules and particles to infer properties of the atmosphere. This high vertical and temporal resolution technique is an excellent “ground-reference” for the validation of satellite measurements, and an excellent technique to study atmospheric variability for a large spectrum of timescales (from minutes to decades).