From Glossary of Meteorology
In the Lagrangian description, the independent variables are time and a set of particle labels (such as the initial position of each particle). Although the Lagrangian description is employed less frequently than the Eulerian description, it can be particularly useful in combined chemistry/ dynamics problems such as stratosphere/troposphere exchange along isentropic surfaces that intersect the tropopause.
Salmon, R. 1998. Lectures on Geophysical Fluid Dynamics. Oxford University Press, . 5–6.