Atmospheric rivers are narrow, filamentary structures of water vapor in the atmosphere responsible for 90% of the meridional poleward water vapor transport. Recent studies demonstrated that these atmospheric rivers were present and played an important role during many recent major flooding events along the California coast. Previous work at the NOAA Earth System Research Laboratory developed objective characteristics for the identification of atmospheric river events in integrated water vapor (IWV) retrievals from the Special Sensor Microwave Imager (SSM/I). These techniques were applied interactively to the development of an 8-year climatology of events striking the west coast of North America. Further application to forecast studies and model verification, however, require full automation of the atmospheric river identification.

This presentation describes the development and application of an automated atmospheric river detection procedure based on the previous objective characteristics and 12-hour imagery of IWV retrievals. Potential atmospheric rivers are first identified through thresholding and location of linear regions of strong gradients in the IWV data. The river features are further distinguished through the image processing technique of skeletonization and determination of feature width and length. The procedure results in identification of the river axis, its width, and an indication of strength based on the IWV magnitude. Through accumulation of results from sequential images, information is derived on the frequency and lifetime of the events. The technique is also applicable to numerical weather prediction (NWP) fields of IWV. Graphical examples of the use of the technique and its limitations are presented.

Initial results are also presented for application of the procedure to two projects related to improving forecasting of the events and associated precipitation. First, the technique is applied to multiple seasons of observations from the SSM/I and corresponding forecast fields from several of the NWP models included in the THORPEX Interactive Grand Global Ensemble (TIGGE) to evaluate and compare the ability of the models to accurately reproduce the frequency and scale of atmospheric river events. Second, the approach is integrated into the development of a satellite-derived atmospheric river threat indicator for use as a tool in California forecast offices in the short-term prediction of severe precipitation events.