The Rossby number compares inertial acceleration with Coriolis acceleration. For speed , horizontal length , and Coriolis parameter , it is . Small Rossby number supports geostrophic balance and the quasi-geostrophic approximation.
For a convective star, the activity convention compares its rotation period with a convective turnover time. It differs by fixed factors from the fluid Rossby number estimated from speed, length and angular velocity. Small denotes strong rotational influence on convection. Magnetic activity usually rises as this ratio decreases in the unsaturated regime, but levels off in a saturated rapid-rotation regime.
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The Rossby number (Ro) is a dimensionless number that measures the relative importance of inertial forces compared to Coriolis forces in a rotating system, such as the Earth's atmosphere or ocean. It is named after the American meteorologist Carl-Gustav Rossby. The Rossby number is typically defined as: \[ Ro = \frac{U}{fL} \] Where: - \(U\) is a characteristic velocity scale of the flow (e.g., wind speed).