Precipitation sources and moisture transport in atmospheric rivers from a Lagrangian perspective

Abstract

[eng] Atmospheric rivers (ARs) are filaments of enhanced moisture in the atmosphere, usually located in subtropical zones and mid-latitudes over oceanic areas. Enormous quantities of water vapor travel through these structures, so that precipitation occurs when they make landfall and the water vapor is forced upwards. Although the rainfall intensity is highly variable, some of them may cause heavy precipitation events, leading to hazardous consequences. All in all, ARs are responsible for the vast majority of the polewards water vapor transport in mid-latitudes, and a better knowledge of their physical properties is needed to understand what controls their behavior and, therefore, the different effects they may have on our daily lives. Among these features that we need to investigate is the water vapor origin. An approximate picture of the regions where ARs gain moisture is known nowadays, thanks to the use of different Lagrangian techniques. However, there is a lack of studies regarding the origin of water vapor that finally forms precipitation over a continental region. The difference is actually important, since rainfall occurs along the AR and during its lifetime: ARs are different from a corridor of water vapor in which moisture is exclusively added and removed at the beginning and at the end, respectively. To address this issue, in this thesis we present a Lagrangian method to correctly compute the sources of precipitation in AR events. The core component of the procedure is the computation of backward air parcel trajectories. The rest of the methodology is described in detail and applied to a pair of ARs that affected the Iberian Peninsula in the 1980s. The results are considered together with other more conventional approaches to study ARs, which makes us gain some insight on how water vapor is transported, and formulates interesting future research questions.