The role of radiation in the Northern Hemisphere troposphere-to-stratosphere transport

Abstract. The upper troposphere and lower stratosphere (UTLS) region of the atmosphere is greatly impacted by the exchange of mass and constituents between the two layers. The two-way transport, both stratosphere-to-troposphere (STT) and troposphere-to-stratosphere (TST), has been studied climatologically to quantify the exchange globally or in the context of specific weather systems. However, the local physical processes responsible for the potential vorticity (PV) modification required for crossing the dynamical tropopause have been studied only in detailed case studies. In this study, we introduce a method of quantifying the role of radiative processes leading to TST and apply this method to 10 years of TST trajectories identified from ERA5 reanalysis. This approach combines a Lagrangian TST identification with a process-specific PV framework. The combination allows us to attribute the processes that contribute to the TST. We focus on the period with a significant PV increase of 1 pvu prior to TST and study the contribution of radiation to that increase. Radiation is present in 84 % of TST trajectories, and for every fourth trajectory crossing the 2 pvu dynamical tropopause, radiation is responsible for at least a quarter of the considered 1 pvu increase prior to TST. Focusing on radiatively dominated TST trajectories, i.e., TST events where radiation contributes more than 50 % to the 1 pvu increase, we find large variability in terms of how PV accumulates, ranging from short to long accumulation times and strongly varying values of the associated PV rates. We show that the high PV rates along the radiatively dominated TST trajectories are mainly produced by temperature tendencies resulting from cloud-top cooling, with maximum values occurring at the top of mixed-phase clouds with high hydrometeor contents. However, a similar magnitude of radiative PV rates can also be produced by vertical gradients in specific humidity. The accumulation time for the 1 pvu increase is determined not only by radiation but mainly by the amplitude of the PV rates due to turbulence and cloud microphysical processes. In summary, this study provides new insight into the complex interplay of processes and pathways that air parcels experience on their way to the stratosphere.