Lower Thermospheric Material Transport via Lagrangian Coherent Structures

Abstract

We show that inter‐model variation due to under‐constraint by observations impacts the ability to predict material transport in the lower thermosphere. Lagrangian coherent structures (LCSs), indicating regions of maximal separation (or convergence) in a time‐varying flow, are derived in the lower thermosphere from models for several space shuttle water vapor plume events. We find that inter‐model differences in thermospheric transport manifest in LCSs in a way that is more stringent than mean wind analyses. LCSs defined using horizontal flow fields from the Specified Dynamics version of the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (SD‐WACCMX) at 109 km altitude are compared to Global Ultraviolet Imager (GUVI) observations of the space shuttle main engine plume. In one case, SD‐WACCMX predicts an LCS ridge to produce spreading not found in the observations. LCSs and tracer transport from SD‐WACCMX and from data assimilative WACCMX (WACCMX + DART) are compared to each other and to GUVI observations. Differences in the modeled LCSs and tracer positions appear between SD‐WACCMX and WACCMX + DART despite the similarity of mean winds. WACCMX + DART produces better tracer transport results for a July 2006 event, but it is unclear which model performs better in terms of LCS ridges. For a February 2010 event, when mean winds differ by up to 50 m/s between the models, differences in LCSs and tracer trajectories are even more severe. Low‐pass filtering the winds up to zonal wavenumber 6 reduces but does not eliminate inter‐model LCS differences. Inter‐model alignment of LCSs improves at a lower 60 km altitude.

Key Points

• Model thermospheric Lagrangian coherent structures (LCSs) are computed for five space shuttle water vapor deposition events

• A model constrained only below 60 km predicts LCS ridges that indicate spreading, but is not observed in Lyman‐alpha emissions

• Models yield LCSs that differ even when mean winds are similar, and more at 109 km than 60 km even when small‐scales are filtered out