Assessment of the origin of moisture for the precipitation of North Atlantic extratropical cyclones: Insights from downscaled ERA5

Abstract

Extratropical cyclones (ECs) are large-scale synoptic weather systems characterised by complex mesoscale precipitation-generating features. Unresolved questions remain regarding how their water budget varies depending on the origin of the rainfall. This study aims to address this gap by examining the variability in moisture uptake patterns across EC precipitation structures. To achieve this, 237 cases were selected during the maximum deep phase over the North Atlantic (NATL). The study utilizes ERA5 downscaled simulations and Lagrangian moisture source diagnostics to identify key moisture parameters. We compare different precipitation-targeting approaches, including the radius, Warm Conveyor Belt (WCB) footprint, and square root spiral contours centred on the EC. The radius approach captures central precipitation and innermost rainbands, while the WCB target better represents the full extent. The NATL region was divided into sectors to facilitate a detailed analysis of geographic influences and dynamic factors, including moisture availability. Using the radius approach, moisture sources in the western/W and northern/N NATL are predominantly aligned along the Gulf Stream. When considering WCB and spiral targets, the 95th percentile moisture uptake pattern expands into the subtropics. Moisture patterns shift southward and intensify for broader targets. In NNATL ECs, the spiral region's moisture source shows peak uptake at ∼3000 km, exceeding the radial target's 1500 km. WNATL ECs exhibit intense moisture uptake, with a southwest (southeast) source extending up to 4500 (4000) km. In eastern/E NATL, EC-relative composites indicate evenly distributed moisture uptake, constrained within a 2000 km radius.