Assessment of the origin of moisture for the precipitation of North-Atlantic extratropical cyclones

This study uses high-resolution simulations and Lagrangian diagnostics to identify the sources of moisture contributing to precipitation at the deepest stage of extratropical cyclones (ECs) over the North Atlantic (NATL). Precipitation was associated with target regions defined by a radius, warm conveyor belt (WCB) footprint, and square root spiral contours centred on the cyclone. The NATL region was divided into sectors for detailed analysis. In the northern North Atlantic (NNATL), moisture sources extend westward across the ocean. Subtropical moisture supports precipitation in non-central areas of ECs, which intensify over the central and western NNATL. The moisture uptake patterns of ECs in the higher latitudes of the western North Atlantic (WNATL) are similar to those in the NNATL, with southwestward extension and moisture uptake from the eastern American coast. For ECs in the lower latitudes of the WNATL, moisture uptake is more symmetric around the cyclone centre, with major contributions from the Caribbean and limited moisture flow from the Gulf of Mexico due to migrating anticyclones. For ECs in the eastern NATL, moisture comes from the surrounding ocean. Overall, 75% of the moisture gain occurs below 600 hPa, with a significant concentration observed around 800 hPa. Continental mass influence is observed for ECs deepening near the coasts of East America and Western Europe. ECs at higher latitudes in the WNATL and NNATL exhibit extensive synoptic-scale disturbances, with moisture sources for WCB and spiral precipitation extending 3,000 to 4,000 km southwest of their centres. The most intense moisture uptake occurs over the WNATL, particularly for lower latitude ECs.