The Sensitivity of Atmospheric River Identification to Integrated Water Vapour Transport Threshold, Resolution, and Regridding Method

Abstract:

Atmospheric Rivers (ARs) are elongated narrow bands of enhanced water vapour that can cause intense rainfall and flooding. ARs only appeared in the literature within the last 30 years and there has been much debate about how to define ARs, and how to identify them. As a result, a wide range of AR identification algorithms have been produced with variations in the conditions required for an object to be classified as an AR, and differences in the input data. One of the key conditions in most AR identification algorithms is a minimum threshold of water vapour or water vapour flux, along with geometric criteria. In this study, we conduct a sensitivity analysis of a commonly-used AR identification algorithm to different Integrated Water Vapour Transport (IVT) thresholds, input data resolutions and regridding methods during the Years of Tropical Convection operational analysis (May, 2008 – April, 2010). We found that the resolution and regridding method affects the number of ARs identified but the seasonal cycle is maintained. AR identification is highly sensitive to the choice of IVT threshold, which can impact the number of ARs identified and modify the AR spatial distribution. Importantly, the commonly used 250 kgm^-1s^-1 IVT threshold is not appropriate for global studies with detection methods that also include a restrictive geometric condition as this combination can lead to the strongest systems failing to be identified. Additionally, we apply this method to the understudied Australasia region and present our evidence showing that ARs are associated with 70-100% of extreme rainfalls days and 90% of the most expensive floods between 2007-2017 in New Zealand.