The phenomenon of prefilming airblast atomization is investigated by means of shadowgraphy. The focus is on the liquid accumulation, which is attached to the trailing edge in the wake region of the prefilmer. The prefilmer is planar to facilitate the measurements, and the trailing edge thickness is 230 µm. The investigated liquid is a kerosene surrogate (Shellsol D70). The operating parameters are varied with a bulk gas velocity ranging from 20 to 100 m/s and a surface volumetric liquid flow rate ranging from 20 to 120 mm2/s. In order to statistically investigate the liquid accumulation, it is virtually split into several atomizing cells. Each cell consists of a single ligament and a surrounding liquid part. Different characteristic lengths of the atomization cells are extracted by a in-house post-process tool. The distribution of these length scales are well described by a gamma function, whose scale parameter $m$ is proportional to a Weber number based on the trailing edge thickness (We_te). However, it is observed that the parameter $m$ also varies with the liquid flow rate, independently of We_te. A two-step mechanism is propose ... mehrd for the dynamics of the liquid accumulation, leading to the expression of a new length scale, which combines We_te and the liquid flow rate. It is observed that the prediction of this new length scale is in good agreement with the experiment. Hence, this new characteristic length is a good candidate for predicting the length scale of the accumulation in more general configurations.