The effect of free-stream turbulence on the dynamics of laminar separation bubble (LSB) over a circular cylinder is investigated via experiments at three turbulence intensities: $T_u=0.06\%, 0.51\%$, and $0.76\%$. Sectional drag and lift force coefficients are calculated from pressure measurements along the circumference of the cylinder at its midspan. A new definition is proposed to identify the onset and end of the critical regime, based on the local minimum in root-mean square of the drag coefficient ($C_d^{\prime }$). The drag crisis shifts to a lower $\text{Re}$ with increase in $T_u$, resulting in an increase in the mean drag coefficient (${\overline{C}}_d$) at the onset of the critical regime with $T_u$. A scaled parameter, $\beta$, is proposed for comparing the critical regimes across different $T_u$. A methodology is devised to identify various flow states and their respective probability at each $\beta$. Based on the number of states observed, the critical regime is divided into subregimes. In the first subregime, the flow intermittently switches between four states, irrespective of $T_u$. For $T_u=0.06\%$, the appearance and disappearance of LSB is found to be synchronized with the formation and shedding of the vortex from the same side. The nondimensional vortex shedding frequency in this subregime is relatively low ($\text{St}=0.12$). Upon increase in $\beta$, an asymmetric, one-bubble state becomes stable. For $T_u=0.51\%$, this asymmetry is intermittent and limited to a smaller subregime, while it nearly vanishes for $T_u=0.76\%$. At the end of the critical regime, the flow is in a stable two-bubble state for all $T_u$.

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• Intermittency
• Transition to turbulence
• Vortex shedding