This study analyzes the secondary eycwall formation (SEF) process in an idealized cloud-resolving simu lation of a tropical cyclone.In particular, the unbalanced boundary layer response to asymmetric inflow forcing induced by outer rainbands (ORBs) is examined in order to understand the mechanisms driving the sustained convection outside the primarv eyewall during the early phase of SEF.The enhancement of convection in the SEF region follows the formation and inward contraction of an ORB.The azimuthal distribution of the enhanced convection is highly asymmetric but regular, generally along a half circle starting from the downwind portion of the ORB.It turns out that the descending radial inflow in the middle and downwind portions of the ORB initiates/maintains a strong inflow in the boundary layer.The latter is able to penetrate into the inner-core region, sharpens the gradient of radial velocity, and reinforces convergence.Consequently, warm and moist air is continuously lifted up at the leading edge of the strong inflow to support deep convection.Moreover, the inflow from the ORB creates strong supergradient winds that are ejected outward downwind, thereby enhancing convergence and convection on the other side of the storm.The results provide new insight into the key processes responsible for convection enhancement during the early phase of SEFin thrce dimensions and suggest the limitations of axisymmetric studies.There arc also implications rcgarding the impact of the asymmetric boundary layer flow under a translating storm on SEF.