Scale interactions in sustaining persistent torrential rainfall events during the Mei-yu season

Energy budgets based on the temporal scale separation were used to investigate two persistent torrential rainfall events over the Yangtze River Basin during the 2010 Mei-yu season. Interactions between the precipitation-related eddy flows and their background circulations were analyzed quantitatively to show the corresponding physical scenario. During the persistent precipitation, the precipitation-related eddy flows and their background circulations sustained their respective energy through different mechanisms. For background circulations, the baroclinic energy conversion and the energy transport dominated their maintenance. In contrast, for precipitation-related eddy flows, the upper, middle, and lower troposphere showed significantly different energy paths: in the upper troposphere, a remarkable downscaled energy cascade process (ECP) of available potential energy occurred, which favored the maintenance of upper level wind perturbations through the baroclinic energy conversion; in the middle troposphere, the energy transport controlled the evolution of eddy flows; in the lower troposphere, a significant downscaled ECP of kinetic energy maintained its intensity, which directly favored the sustainment of the Mei-yu front and the lower level jet. These downscaled ECPs, which reflected the direct effects of background circulations on the eddy flows, dominated the evolution of precipitation-related eddy flows. In contrast, although upscaled ECPs that indicated the feedback effects of eddy flows on their background circulations were also obvious, they were negligible in the variation of background circulations. The further analysis indicates that whether the background circulations could transfer their energy to the precipitation-related eddy flows was determined by their configurations relative to the eddy flows.