HAO Colloquium - Mapping Material Transport in the Upper Atmosphere with Lagrangian Coherent Structures

Speaker: Seebany Datta-Barua, Illinois Institute of Technology

In 2003, water vapor exhaust expelled during a space shuttle launch reached the lower thermosphere, Earth's neutral atmosphere above 85 km dominated by neutral gas dynamics and driven by diurnal heating.  Two to three days later noctilucent clouds appeared in the upper atmosphere [Stevens et al., 2005]. These observations raise an intriguing question: was there coherent structuring in the thermosphere that could have predicted the transport?

Coherent structuring is known to occur in the upper atmosphere, e.g., in the form of auroral arcs and curls in the ionosphere, the charged particle layer embedded in the thermosphere subject to electrodynamic forcing.  Identification of ionospheric and thermospheric structuring requires sustained observation of 2D or 3D flow fields over broad regions.  Models typically provide the most consistent way of specifying flow fields of the upper atmosphere.  These provide the means to apply advanced fluid advection analysis for predictive ability in material transport in the upper atmosphere.

Identifying Lagrangian coherent structures (LCSs) is a way of analyzing fluid that has been used in other fields of geophysical science.  In the Lagrangian frame, which flows with the particles (in contrast to an Eulerian fixed-mesh frame), barriers in material transport can be objectively identified.  Given a sequence of flow fields, particles are traced and their maximum stretching after a finite time quantified through the finite time Lyapunov exponent (FTLE).  The manifolds (surfaces) of maximum FTLE over the domain define barriers within the flow domain across which fluid material cannot cross.  We have implemented this calculation in the ionosphere-thermosphere algorithm for LCSs (ITALCS).

In this presentation I review our findings so far about the presence of LCSs at global scale in the ionosphere and thermosphere.  For the ionosphere, we have used the Weimer (2001) electric potential model for high latitude ion drifts, and SuperDARN derived convection fields.  For the thermosphere, an early study used the Horizontal Wind Model (2014).  More recently, we used NCAR thermosphere-ionosphere-electrodynamic general circulation model (TIEGCM) to look at the relationship between ionospheric and thermospheric LCSs.  In the lower thermosphere we have used the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (both SD-WACCMX and WACCMX+DART versions) for examining the behavior of NOx and of the space shuttle water vapor.  We find there is currently significant inter-model variation in the lower thermosphere.  Ongoing plans will extend ITALCS to compute the FTLE for flows only known over a finite domain, and for three dimensions, such that we can potentially examine vertical transport in the future.

Date and time: 
Thursday, October 28, 2021 - 2:00pm to 3:00pm