Using empirical orthogonal function (EOF) expansion, this paper analyzes Weather Research and Forecasting (WRF) simulative data of the torrential rain occurring in Shanghai on August 25 of 2008 to diagnose the deviation field of kinetic energy. The conversion mechanism from the available potential energy to kinetic energy is discussed for various weather system scales, and meso-β scale rain clusters in particular. The results show that the preceding three EOF expansion modes can be individually called the ambient mode, the torrential rain system mode, and the rain clusters mode. Around the precipitation area, the absolute value of the horizontal gradient in the low-level kinetic energy deviation field is bigger, especially in the meso-β scale rain clusters mode. This indicates that the wind field of this mode is ageostrophic with strong convergence, and its motion is a non-equilibrium inertial-gravity wave (containing a mixed eddy-inertial gravity wave). On a certain isobaric surface, the available potential energy is homological with geopotential deviation in the time-variation. The horizontal gradient of the available potential energy is homological with the spatial gradient of the geopotential deviation. So the variation of geopotential deviation can represent a change in the available potential energy. Before a rain occurrence, the available potential energy converts to kinetic energy for all three modes around the precipitation area. For the rain clusters mode, especially, the change is more obvious. This mode is a meso-β scale system, so this scale system is key in the conversion course from available potential energy to kinetic energy, playing an intermediary role in the eddy field’s conversion mechanism of available potential energy to kinetic energy.