Description: Midlatitude poleward vapor transport mainly occurs in meandering filaments of intense vapor transport, spanning thousands of kilometers long and hundreds of kilometers wide and drifting eastward. The vapor filaments are known as atmospheric rivers (ARs). They can cause extreme wind gusts, intense precipitation, and flooding along densely populated coastal regions. Many recent studies about ARs focused on the statistical analyses of ARs, but a process-level understanding of ARs remains elusive. Here we show that ARs are streams of vapor kinetic energy (VKE) and derive a prognostic equation for VKE to understand what contributes to AR evolution. We find that ARs grow mainly because of potential energy conversion to kinetic energy, decay largely owing to precipitation and turbulence, and the eastward propagation is primarily due to horizontal advection. Our VKE framework complements the integrated vapor transport framework, which is popular for identifying ARs but lacks a prognostic equation for process understanding.

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