Study on sky rivers: Concept, theory, and implications

Presented in this paper is an introduction to the study on sky rivers, especially the concept, theory, and implications. A sky river is in essence a generalization for a rich variety of meteorological systems including tropical cyclones, extratropical frontal cyclones, shear lines and others from the viewpoint of fluid mechanics. All these meteorological systems share a common feature that, there exists a contacting surface between two air masses with contrasting velocities, temperatures, humidity, and pressures. Under the forces caused by large gradients of state variables normal to the contacting surfaces and other forces, concentrated water vapor flows along paths on the contacting surfaces like river channels on earth surface, characterized by cloud and precipitation processes along the paths. A simplified and direct approach to identify the sky rivers is proposed on the basis of their physical interpretation. Results show that the sky rivers globally exist and have close relations with precipitation. In mathematics, the sky rivers are the discontinuity surfaces of the governing equations for atmospheric flows, and they are formulated in this study as the characteristic surfaces where the characteristic values are real and the derivatives normal to the surface do not exist by means of general theory for partial differential equations to provide a description for time-space evolution of the sky rivers. Finally, future study areas regarding the sky rivers are discussed.