The Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is an international reference observing network of sites measuring Essential Climate Variables (ECVs) above Earth's surface, designed to fill an important gap in the current global observing system.

At present, the GCOS Surface Network (GSN) includes 1023 sites, GUAN includes 177 sites, of which China has a total of 33 GSN sites and 7 GRUAN sites.

A bird's view of the site Photoed by WANG Zhenzhen

GRUAN measurements provide long-term, high-quality climate data records from the surface, through the troposphere, and into the stratosphere.

In 2021, the China Meteorological Administration (CMA) Meteorological Observation Centre (MOC) took on the responsibility of hosting the GSRN lead center, which is responsible for managing the overall operation and development of the GSRN. It will provide strong technical support for the standardization of global reference climate observations, which will facilitate the establishment of a global coordinated surface climate observation network.

Xilinhot site of GCOS Reference Upper-air Network (hereinafter referred to as XIL GRUAN site) was accepted as a GRUAN candidate site in October 2008. In March 2023, XIL GRUAN site began on GRUAN observation with the operational data on GTH3 radiosonde (Twice a day), RS41 radiosonde (Twice a week) and CYT-1 ECC ozone sonde(Once a week).

XIL GRUAN site Photoed by LI Huirong

In June 2025, XIL GRUAN site obtained certification GRUAN, becoming the 15th GRUAN site globally, and the 3rd in Asia.

Located in northern China's ecologically fragile grassland zone along a key cold air transit corridor, it serves as a critical node for studying climate change in mid-temperate semi-arid regions.

The journey of being certified

To quantify climate change precisely, the World Meteorological Organization (WMO) and GCOS launched GRUAN in 2007.

Selected as a pioneer site for its unique geographical and climatic representativeness, XIL GRUAN site filled the gap in reference upper-air observations across north-central Asia.

XIL GRUAN site features an integrated observation platform, data processing center, and hydrogen production equipment (including a QDQ4A automatic water electrolysis system). Laboratories house specialized devices such as standard humidity chambers and ozonesonde detectors, ensuring the observational precision.

Experts from WMO visited the field experiment base in 2024. Photoed by JIA Kehan

Scientific practice and international exchange

XIL GRUAN site commenced trial observations in March 2023, achieved GRUAN data global sharing in August 2024 and received official certification in June 2025. It has delivered three key breakthroughs:

Data Accuracy: Exchanging highest-precision data with 14 global GRUAN sites to continuously monitor essential climate variables (temperature, humidity, pressure, wind, ozone) with high reliability.

Satellite Calibration Foundation: Providing ground validation for FENGYUN meteorological satellites and international space-based systems while constraining global vertical observation network quality.

Technology Diffusion: enhancing ground-based remote sensing capabilities.

Upper-air sounding - L-band radar Photoed by LI Huirong

Ruud Dirksen, head of the GRUAN Lead Center, acclaimed that XIL GRUAN site's high-quality operations have significantly contribute to global reference upper-air observations.

Looking into the future

XIL GRUAN site will prioritize advancing Beidou-based sounding and GRUAN data product certification, and develop numerical weather prediction and satellite validation systems to enhance support for climate change research.

In collaboration with Inner Mongolia Autonomous Region Meteorological Service

Special thanks to LI Hongmei with the Service

Planned by/Editor:LIU Shuqiao