Tagging moisture sources with Eulerian and Lagrangian tracers: Application to an intense atmospheric river event.

One Eulerian and two Lagrangian tracers tools are evaluated for studies on atmospheric moisture sources and pathways. The first method has been recently implemented into the Weather Research and Forecasting (WRF) mesoscale model, while the Lagrangian methods are described here. In these methods, a moisture volume is assigned to each particle, which is then advected by the wind flow. Usual Lagrangian methods consider this volume to remain constant and the particle follows flow path lines exactly. In a different approach,, the initial moisture volume can be considered to depend on time as it’s advected by the flow, due to thermodynamic processes. In this case, the tracer volume drag must be taken into account. Equations have been implemented and moisture convection was represented in both Lagrangian models. We apply these methods to evaluate the intense atmospheric river that devastated the Pacific North West region of the United States, with flooding rains and intense winds in early November 2006. We note that the usual Lagrangian method underestimates moisture availability in the continent while active tracers (both Eulerian and Lagrangian) achieve more realistic results.