The impact of temperature resolution on trajectory modeling of stratospheric water vapour

Lagrangian trajectories driven by reanalysis meteorological fields are frequently used to study water vapour (H₂O) in the stratosphere, in which the tropical cold-point temperatures regulate H₂O amount entering the stratosphere. Therefore, the accuracy of temperatures in the tropical tropopause layer (TTL) is of great importance for trajectory studies. Currently, most reanalyses, such as the NASA MERRA (Modern Era Retrospective-Analysis for Research and Applications), only provide temperatures with ∼ 1.2 km vertical resolution in the TTL, which has been argued to introduce uncertainties in the simulations. In this paper, we quantify this uncertainty by comparing the trajectory results using MERRA temperatures on model levels (traj. MER-T) to those using temperatures in finite resolutions, including GPS temperatures (traj. GPS-T) and MERRA temperatures adjusted to recover wave-induced variability underrepresented by the current ∼ 1.2 km vertical resolution (traj. MER-Twave). Comparing with traj. MER-T, traj. GPS-T has little impact on simulated stratospheric H₂O (changes ∼ 0.1 ppmv), whereas traj. MER-Twave tends to dry air by 0.2-0.3ppmv. The bimodal dehydration peaks in traj. MER-T due to limited vertical resolution disappear in traj. GPS-T and traj. MER-Twave by allowing the cold-point tropopause to be found at finer vertical levels. Despite these differences in absolute values of predicted H₂O and vertical dehydration patterns, there is virtually no difference in the interannual variability in different runs. Overall, we find that the finite resolution of temperature has limited impact on predicted H₂O in the trajectory model.