The Lagrangian moisture source and transport diagnostic WaterSip V3.2

Abstract

WaterSip is a diagnostic software tool that identifies the evaporation sources and transport pathways of precipitation or water vapour over a target area based on Lagrangian model output. In addition to the geographic location, WaterSip identifies select thermodynamic properties of the moisture sources, during atmospheric transport, and arrival over the target area based on Lagrangian particle dispersion or trajectory model output. WaterSip software thereby employs the Lagrangian diagnostic algorithm for quantitative moisture source accounting of Sodemann et al. (2008b). Moisture sources are identified from changes in specific humidity along trajectories at each output time step. The ratio between changes in specific humidity and the specific humidity of the air parcel allow to estimate the quantitative contribution of a moisture source to an air parcel at a specific time and location. This contribution and its temporal sequence provides the basis for identifying moisture source contributions to the final precipitation. WaterSip is designed to operate on large datasets of trajectories filling regional to global domains. The results are provided as gridded information in a variety of output files in netCDF format. This paper revisits the relevant methodological foundations, describes the configuration and technical set-up, and provides a consistent example case study to illustrate the use and interpretation of the software tool and its results. A short sensitivity study supports the choice of the main parameters of the diagnostic. Key uncertainties and caveats are described and discussed throughout the text. Users of WaterSip should be aware of these uncertainties to obtain a valid and reliable interpretation of the diagnostic results.