We examine the performance of a widely used Lagrangian method for moisture tracking by comparing it with a highly accurate Eulerian tool, both operating on the same WRF atmospheric model fields. Although the Lagrangian approach is very useful for a qualitative analysis of moisture sources, it has important limitations in quantifying the contribution of individual sources to precipitation. These drawbacks should be considered by other authors in the future so as to not draw erroneous conclusions.

Two Lagrangian tracer tools are evaluated for studies on atmospheric moisture sources and pathways. Usual Lagrangian methods consider the initial moisture volume to remain constant and the particle to follow flow path lines exactly. In a different approach, the initial volume can be considered to depend on time as it is advected by the flow due to thermodynamic processes. Drag and buoyancy forces must then be considered.

For the first time, a climatology of Lyapunov exponents (FTLE) for the period 1979–2014 has been established. This climatology is able to reproduce the main large-scale structures observed in the troposphere. The mean FTLE has been shown to correlate with the baroclinic Eady growth rate within timescales of a few days. As a case study, the contribution of atmospheric rivers to this climatology and their contribution to rainfalls over the Sahara and UK regions have been studied.

This paper evaluates the connection between winter precipitation over the Iberian Peninsula and the large-scale tropospheric mixing over the eastern Atlantic Ocean. Finite-time Lyapunov exponents (FTLEs) have been calculated from 1979 to 2008 to evaluate this mixing. Our study suggests that significant negative correlations exist between summer FTLE anomalies and winter precipitation over Portugal and Spain.

The present study aims to improve the calculus of finite-time Lyapunov exponents (FTLEs) applied to describe the transport of inertial particles in a fluid flow. To this aim, the deformation tensor is modified to take into account that the stretching rate between particles separated by a certain distance is influenced by the initial velocity of the particles. Results are presented for two different flows and compared with the classical method by Shadden (2005).