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

Extratropical cyclones are essential for redistributing energy, moisture, and momentum from lower latitudes to higher latitude regions. Although extratropical cyclones during the winter season are relatively well studied, less is known about summer cyclones and their moisture sources. Therefore, this research focuses to enhance our understanding of how summertime extratropical cyclones in the Northern Hemisphere shape the characteristics of the global water cycle. More specifically, the study focused on determining the moisture sources of these storms and analyses how precipitating air parcels are transported to the cyclone center. 

For this purpose, 8-day backward trajectories were calculated for a subset of the 20 % most intense summertime cyclones over the North-Atlantic and for all air parcels within the vicinity of cyclone center, using the Lagrangian Analysis Tool LAGRANTO. Subsequently, moisture uptakes along the trajectories of precipitating air parcels were identified using the moisture source diagnostic WaterSip. Using this approach, we find that the bulk of the precipitation associated with summertime cyclones falls close to the cyclone center within the WCB, mainly during the intensification phase. The origins of this moisture correspond to areas of high evaporation, with significant hotspots over the Gulf Stream region and its northeastern extension, and continental sources for cyclones in the Labrador Sea. During the early stages of cyclone development, moisture is delivered to the cyclone center, and this changes to more remote sources when intensity is increasing and when precipitation is at its peak. Local evaporation becomes again more dominant as cyclones generate less precipitation and start to decay. Therefore, the source distance is largest during the intensification phase and decreases thereafter. We lastly discuss differences between extratropical cyclones that undergo an extratropical transition.