The frosty frontier: redefining the mid-latitude tropopause using the relative humidity over ice

The tropopause represents a central feature of the atmospheric vertical structure, marking the transition between the troposphere and stratosphere. While common definitions rely on quantities conserved under adiabatic changes, diabatic effects, resulting from radiation, cloud processes or turbulence are also decisive for the tropopause structure. Therefore, we propose a new definition based on the vertical gradient of the relative humidity with respect to ice (RHi). RHi is the key variable for ice cloud formation and incorporates both diabatic and adiabatic processes. Based on high-resolution radiosonde data, we can show that our RHi-GT-based definition is generally consistent with, and often provides a clearer characterization than the thermal tropopause. This is not only evident in individual vertical profiles, but also when looking at statistics of many profiles with a tropopause-relative height axis. Last but not least, the robust and simple calculation of our definition makes it an ideal tool for studies involving the tropopause.