Assessment of humidity ground checks for GRUAN-compliant automatic radiosonde operations

Abstract. Automated Radiosonde Launchers (ARLs) are increasingly adopted for upper-air observations as they offer major operational advantages compared to manual launches, but they introduce new challenges for maintaining reference-quality humidity measurements within the GCOS Reference Upper-Air Network (GRUAN). In particular, the applicability of the pre-launch Standard Humidity Chamber (SHC) test, routinely used for manual radiosonde launches, has not been systematically assessed for radiosondes stored inside ARLs prior to launch.

This study investigates how residence time inside ARLs affects radiosonde humidity sensor performance and the adequacy of SHC tests for automated GRUAN operations. A coordinated experiment was conducted at six sites using three radiosonde models (Vaisala RS41, Meisei IMS-100, and MeteoModem M10) and three ARL systems (Vaisala AS15, Meisei ARS, and MeteoModem Robotsonde). Radiosondes were tested in an SHC prior to storage and again after residence times of 7 and 14 days. Relative humidity biases at saturation (RH = 100 %) were analyzed using aggregated statistics, including a paired RH bias difference—i.e., the change in bias between pre-storage and post-storage measurements, to quantify temporal drift. A non-parametric bootstrap resampling analysis was first applied to assess the robustness and confidence intervals of the paired RH bias difference estimates. Statistical significance of the paired RH bias difference distributions was subsequently evaluated using the Wilcoxon signed-rank test. A normalized storage-effect metric, combining the systematic drift and variability of the paired RH bias differences, was compared with the GRUAN humidity uncertainty contribution associated with ground preparation.

Results indicate that radiosonde humidity performance remains stable for residence times up to 14 days under the tested storage conditions, particularly for the RS41–AS15 configuration, where only small negative drifts were observed (approximately ~ −0.1 % RH week^-1). In contrast, the IMS-100–ARS configuration exhibited a pronounced increase in drift with storage time (approximately ~ −0.8 % RH week^-1), which was amplified under high-humidity and temperature seasonal conditions, while the M10–Robotsonde configuration showed a reversed drift with partial bias reduction during storage (~ +0.12 % RH week^-1). These findings demonstrate that radiosonde behavior during storage strongly depends on the combined radiosonde–ARL system.

Based on the experimental and statistical evidence, an operational upper limit of 14 days for radiosonde storage inside ARLs is recommended for GRUAN-compliant operations, with residence times up to 7 days providing optimal reliability. The study confirms the feasibility of implementing SHC tests for ARL operations while highlighting the need for manufacturer- and site-specific procedures to ensure long-term traceability and consistency of reference humidity measurements. The results demonstrate that residence time inside ARLs constitutes a system-dependent source of uncertainty, warranting explicit inclusion as an additional component in the GRUAN Data Product (GDP) humidity uncertainty budget.