Sources and Transport Pathways of Precipitating Waters in Cold-Season Deep North Atlantic Cyclones
Lukas Papritz, Franziska Aemisegger, Heini Wernli
Abstract
Abstract Extratropical cyclones are responsible for a large share of precipitation at midlatitudes and they profoundly impact the characteristics of the water cycle. In this study, we use the ERA5 and a cyclone tracking scheme combined with a Lagrangian diagnostic to identify the sources of moisture precipitating close to the center of 676 deep North Atlantic cyclones in winters 1979–2018. Moisture uptakes occur predominantly in originally cold and dry air heated over the North Atlantic, in particular, over the warm waters of the Gulf Stream, whereas more remote sources from land or the subtropics are less important. Analyzing the dynamical environment of moisture uptakes, we find that moisture precipitating during the cyclone intensification phase originates in the precyclone environment in the cold sectors of preceding cyclones and the cyclone–anticyclone interaction zone. These moisture uptakes are linked to the cyclone’s ascent regions via the so-called feeder airstream, a northeasterly cyclone-relative flow that arises due to the cyclone propagation exceeding the advection by the low-level background flow. During the decay phase, more and more of the moisture originates in the cyclone’s own cold sector. Consequently, the residence time of precipitating waters in cyclones is short (median of ≈2 days) and transport distances are typically less than the distance traveled by the cyclone itself. These findings emphasize the importance of preconditioning by surface fluxes in the precyclone environment for the formation of precipitation in cyclones and suggest an important role for the hand-over of moisture from one cyclone to the next within a storm track.