Local Finite-Amplitude Wave Activity of Water Vapor as a Diagnostic of Atmospheric River Events

We generalize the formalism of local wave activity of quasi-geostrophic potential vorticity to water vapor, defined as LWA-V, and derive the LWA-V budget equation. LWA-V measures the waviness of moisture contours from the zonal symmetry in the eddy-free atmosphere. It delineates well the northward moisture intrusion and the filamentary feature of AR events. Compared with the traditional moisture budget, the climatological LWA-V and LWA-V budget terms are more consistent in structure and are able to delineate the climatological AR. Moisture flux convergence is the dominant process for the intensification and movement of the LWA-V center associated with the AR event while the combined effect of evaporation and precipitation is mainly a sink of LWA-V, which is consistent with traditional moisture budget analysis. Furthermore, utilizing the Lagrangian aspect of LWA-V, we demonstrate quantitatively that the original latitudes of the largest precipitable water of the AR event are in the tropical region.