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

The tropopause represents a central feature of the vertical structure of the atmosphere, marking the transition between the troposphere and stratosphere. While common definitions such as the thermal tropopause (TTP) defined by the WMO primarily rely on quantitates that  are conserved under adiabatic processes, diabatic effects resulting from radiation, cloud processes, or turbulence are also decisive for the tropopause structure.

We propose a new definition of the tropopause based on the vertical gradient of the relative humidity with respect to ice (RHi), named the RHi Gradient Tropopause (RHi-GT). The RHi-GT is determined using a simple, robust gradient method. We demonstrate that the RHi-GT definition is generally consistent with the TTP but often provides a clearer characterization. In individual profiles, the RHi-GT coincides more closely with regions that mark a clear transition in atmospheric structure, such as sharp gradients in absolute humidity or increases in static stability. Furthermore, when examining mean profiles over the 10-year period relative to the RHi-GT, both RHi and static stability show a more coherent and distinct transition between the moist troposphere and the very dry stratosphere compared to when referenced to the TTP.