Tracking Atmospheric Rivers Globally: Spatial Distributions and Life Cycle Evolutions

Atmospheric rivers (ARs) are narrow, elongated, synoptic jets of water vapor that play important roles in the global water cycle and regional weather and hydrology. A number of different and largely complementary techniques have been developed in the community for the identification of ARs. Such techniques have facilitated the investigation of the most basic characteristics of ARs, including considerations of variability, weather forecast skill, climate change projections, etc. However, most of the existing techniques only identify AR conditions at each time step, and do not track ARs across time steps as they propagate. The need for characterizing AR life cycles (i.e., from genesis to termination) and understanding their representation in global weather/climate models motivate the development of a new AR tracking algorithm suitable for global studies. Based on AR shapes (i.e., AR conditions) identified in our initial algorithm (Guan and Waliser 2015, with refinements in Guan et al. 2018) using the 85% percentile of integrated water vapor transport (IVT) and other geometric requirements, the new algorithm constructs AR life cycles by associating AR shapes between two adjacent time steps based on their spatial proximity and, in more complicated cases, morphological similarity. The algorithm works sequentially in time (i.e., “future” data are not needed for track assignment at the current time step), tracks all ARs over the globe simultaneously, and records key attributes of each AR life cycle as soon as it terminates. Results from the application of the new algorithm to global, 6-hourly IVT from ERA-Interim and MERRA-2 reanalyses will be presented. Global distributions of AR genesis/termination locations (including those associated with separations and mergers), the temporal evolution of key characteristics over the AR life cycle (location, geometry, IVT intensity/direction/coherence, etc.), and their seasonal/interannual variations, will be discussed.