Implementation and Initial Application of an Atmospheric River Detection Tool Based on Integrated Vapor Transport

Abstract:

Atmospheric rivers (ARs) are narrow, filamentary regions of enhanced water vapor transport whose presence has been observed to coincide with extreme precipitation and major flooding events along the west coast of North America. Criteria for their visual identification were previously developed based on fields of integrated water vapor (IWV) from either satellite retrievals or numerical weather prediction (NWP) models. This approach, including thresholds on the width, length, and IWV content, was then incorporated into an automated, objective tool termed the Atmospheric River Detection Tool (ARDT) for IWV. The ARDT-IWV has been demonstrated to be successful in reproducing manually derived climatologies of landfalling AR events and was employed in evaluations of the ability of NWP models to forecast the characteristics and landfall of ARs along the west coast of North America. While highly valuable for its ability to be employed on fields directly available from satellite retrievals, the technique does not address the integrated vapor transport (IVT) that most directly characterizes an AR.

An enhanced version of the ARDT has now been developed for application to fields of IVT derived from NWP models and reanalyses. The 2 cm threshold for IWV has been replaced with regionally varying thresholds on IVT and the maximum width and minimum length criteria of 1000 km and 2000 km, respectively, have both been relaxed to 1500 km. Further invoking the river analogy, an additional requirement for the IVT to be aligned with the primary axis of the feature has been imposed. This presentation details implementation of the new ARDT for IVT, its initial evaluation over the northeastern Pacific, and comparison of statistics of landfalling ARs based on IWV and IVT criteria. Regional modification of the technique for application within the southeastern U.S. is also described and the function of the tool in that region demonstrated.