A Multi-scale Dynamic Study Of A Landfalling Atmospheric River And Its Torrential Rain Process

As elongated and transient filaments of strong horizontal water vapor transport,atmospheric rivers(ARs)are of great importance to the regional weather and climate,incurring extreme rainfall and flash flooding in the coastal regions during their landfall.Among them,the AR that hits the west coast of North America is a renowned one,which plays an important role in the extreme rainfall events in California.Using the recently-developed functional analysis apparatus called multiscale window transform(MWT)and the MWT-based framework of multiscale energetics analysis,the AR landed at the west coast of North America and its associated rainstorm in February 2019 are diagnosed,in order for an understanding of the dynamic mechanisms for the formation and maintenance of the AR,and the cause of the ARrelated heavy rain from the point view of multiscale energetics.This study begins with the MWT analysis of the atmospheric fields.The original fields are reconstructed onto three scale windows,namely,the background flow,synoptic-scale and mesoscale windows.On the background flow window,the 300-h Pa subtropical jet remains as it is in the original field.On the synoptic-scale window,there are obvious features of the AR,including a narrow-and-long band of high water vapor content and a low-level jet.On the mesoscale window,the rainfall location is overlapped with ascending motions.Using the MWT-based localized multiscale energetics analysis,the dynamical processes underlying the AR are then investigated.It is found that the formation of the AR is attributed to barotropic and baroclinic instabilities.That means the energy is transferred from the background flow window to the synoptic-scale window.The dominant source of energy for the intensification of the AR is the nonlocal energy advection,by which the available potential energy is transported from the periphery of the extratropical cyclone to the AR.Via buoyancy conversion,the available potential energy is then converted into kinetic energy on the synopticscale window.During the landfall of the AR,the kinetic energy is depleted by the frictional dissipation,resulting in the decay of the event.Presented next is the dynamical diagnosis for the AR-related heavy rain.By the peaks in hourly rainfall,this rainstorm can be divided into two stages(Stages I and II).It is found that latent heating plays an important role in two stages,which can directly heat the atmosphere and produce available potential energy.The available potential energy is then converted into kinetic energy via buoyancy conversion on the meso-scale window.In addition,the energy transfers from the background flow window to the meso-scale window are different in the two stages.In Stage I,available potential energy is transferred from the background flow to the mesoscale processes,whereas in Stage II,the transfer is that of kinetic energy.