  Rotor blades of wind turbines in the atmospheric boundary layer regularly
experience the aerodynamic phenomenon of dynamic stall consisting of a
temporary overshoot of lift and detrimental fatigue loads. Particularly the
formation of dynamic stall under three-dimensional inflow conditions raises
open questions. Aerodynamic behavior of a DU 91-W2-250 wind profile undergoing
light dynamic stall is thus analyzed in a wind tunnel. Effects of a gust with
streamwise and spanwise periodic variation are investigated by comparing total
and local lift generation with flow formation above the airfoil. The observed
stall cycle is divided into five stages of which one reveals lift overshoot of
up to $16~\%$. The aerodynamic response of the airfoil shows a delay of about
$1/8$ period between evolution of local angle of attack and lift giving a
counterclockwise dynamic polar. A proper orthogonal decomposition (POD)
analysis of the flow field contributes to understand aerodynamic consequences
of the three-dimensional gust. Local inflow, total lift as well as certain lift
events are captured by one POD eigenmode, respectively. Obtained results lead
to the conclusion that the flow and particularly the stalled wake of an airfoil
facing a three-dimensional gust are strongly coupled in the spanwise direction.
This yields to flow stabilization, inhibition of stall, and in turn,
counterclockwise dynamic polar along with augmented total lift.
