The statement that the Rossby radius acts as the fundamental horizontal scale of rotational fluid motion in geophysics is a foundational definition in geophysical fluid dynamics, making it common knowledge that requires no citation.
The claim states a core definition and fundamental concept from geophysical fluid dynamics regarding the Rossby radius (or Rossby deformation radius) setting the horizontal length scale where rotational effects become comparable to buoyancy or gravity wave effects. None of the retrieved papers evaluate or challenge this textbook definition; instead, they discuss specific observational or numerical findings in planetary atmospheres, oceans, and laboratory models where rotation and scaling are applied. Because it is a definitional and foundational concept in the field, it is classified as common knowledge.