  Poleward water vapor transport in the midlatitudes mainly occurs in
meandering filaments of intense water vapor transport, spanning thousands of
kilometers long and hundreds of kilometers wide and drifting eastward. The
water 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 air with enhanced vapor kinetic energy (VKE)
and derive a governing equation for Integrated VKE (IVKE) to understand what
contributes to the evolution of ARs. We find that ARs grow mainly because of
potential energy conversion to kinetic energy, decay largely owing to
condensation and turbulence, and the eastward drift is primarily due to
horizontal advection of VKE. Our VKE framework complements the integrated vapor
transport framework, which is popular for identifying ARs but lacks a
prognostic equation for understanding the physical processes.
