Moisture sources and transport pathways of summertime intense extratropical cyclones in the North-Atlantic

Extratropical cyclones are essential for the redistribution of energy, moisture, and momentum from the equator to the poles. Although wintertime extratropical cyclones are relatively well studied, less is known about summertime cyclones. Therefore, this research aims to improve our understanding of how summertime extratropical cyclones in the Northern Atlantic shape the global water cycle. More specifically, we focused on determining the moisture sources of these cyclones and analysed how precipitating air parcels were transported to the cyclone center. Changes in the moisture uptake and transport characteristics during the cyclone life cycle were also evaluated. To this end, 8-day backward trajectories were computed for the 20% most intense storms for three different stages in their life cycle: intensification, time of maximum intensity, and decay. Trajectory calculations were performed for all precipitating air parcels in a 500 km radius surrounding the cyclone center using the Lagrangian analysis tool LAGRANTO. Subsequently, moisture uptakes along the trajectories of only precipitating parcels were identified using the moisture source diagnostic WaterSip. We find that the bulk of the precipitation falls close to the cyclone center and along the fronts, mostly during the intensification phase. The origins of this moisture correspond to areas of high evaporation, with hotspots over the Gulf Stream region and its northeastern extension, and continental sources for cyclones in the Labrador Sea. The source distance is large during intensification, while local evaporation becomes more important during decay. Finally, we discuss the differences between summer and winter, as they have different dependencies on preceding "parent" cyclones for moisture supply.