Answer
$r\left(0.5 s_0\right)$ is the wind speed at half the height above ground of maximum wind speed.
Work Step by Step
$s_0$ represents the height above ground where the maximum eyewall wind speed occurs.
$v_0=r(s_0)$ is that maximum wind speed.
$r\left(0.5 s_0\right)$ is the wind speed at a half the height above ground of maximum wind speed.
