Evaluating the Application of Reanalysis Wind Products in a Trajectory Model Study of Convective Transport

An important application of analysis and reanalysis products is the use of their wind fields in trajectory model calculations for transport studies. This study evaluates the performance of four (re)analysis products, the ERA5 and ERA-Interim reanalyses and the GFS and ECMWF operational analyses, in representing convective transport over the Tropical Western Pacific (TWP) with the aid of a trajectory model. The evaluation is made possible by two observation-based diagnostics: a database of satellite-derived convective cloud tops and airborne observations of chemical species taken over the TWP during boreal winter 2014. Cloud top information is used to characterize the spatiotemporal distribution and representation of convection within the (re)analysis products kinematic vertical velocity () fields. The airborne chemical observations have recently been used to derive transit time distributions (TTDs) based on species chemical lifetimes, which characterize the transport time scales associated with the sampled air masses. The present study derives trajectory-based TTDs from backward trajectories initiated at the same airborne measurement points, permitting direct evaluation of the ability for trajectories driven by (re)analysis wind fields to represent convective transport processes over the TWP. Results indicate that TTDs derived from trajectory experiments represent 60-75% of convective processes, despite the limitation of (re)analysis wind fields to contain information on the fastest convective transport pathways. The ERA5 reanalysis is shown to best represent convective transport processes over the TWP, while the ERA-Interim is shown to be an outlier in its comparative underrepresentation of such processes, pointing toward the value of higher temporal resolution for representing convective processes. This study attests to the meaningful information contained within the kinematic vertical velocity fields of (re)analysis wind products, and showcases a unique diagnostic for the evaluation of observations and models alike.