Vertical Variability of Relative Humidity and Its Relation to Cirrus Clouds in the Extratropical UTLS

Small changes in the equilibrium between water vapor and cirrus clouds in the upper troposphere and lower stratosphere (UTLS) can strongly influence atmospheric radiative forcing. Relative humidity with respect to ice (RH_ice) is a key parameter governing the formation and life cycle of cirrus clouds.

We present RH_ice measurements in the UTLS over the North Sea and Germany during the TPEx Learjet campaign (7–20 June 2024), conducted from Hohn, Germany, as part of the TPChange research program. Two IAGOS capacitive hygrometers (ICHs) were deployed: one mounted on the Learjet fuselage and the other on an AIRTOSS (AIRcraft Towed Sensor Shuttle) trailing approximately 70–180 m below the aircraft. This setup enabled the resolution of fine vertical structures of RH_ice that cannot be captured by weather and climate models.

By combining ICH RH_ice measurements with ECMWF ERA5 cloud ice water content along flight paths, we distinguish between cirrus and non-cirrus conditions, corroborated by the NIXE cloud probe onboard. Additionally, we examine the fine vertical structure of RH_ice across the extratropical tropopause and its relationship with cirrus clouds. These findings enhance our understanding of cirrus cloud occurrence and variability in this sensitive region, providing valuable insights for improving their representation in climate models.