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The dynamics in the thin boundary layers of temperature and velocity is thekey to a deeper understanding of turbulent transport of heat and momentum inthermal convection. The velocity gradient at the hot and cold plates of aRayleigh-B\'{e}nard convection cell forms the two-dimensional skin frictionfield and is related to the formation of thermal plumes in the respectiveboundary layers. Our analysis is based on a direct numerical simulation ofRayleigh-B\'{e}nard convection in a closed cylindrical cell of aspect ratio$\Gamma=1$ and focused on the critical points of the skin friction field. Weidentify triplets of critical points, which are composed of two unstable nodesand a saddle between them, as the characteristic building block of the skinfriction field. Isolated triplets as well as networks of triplets are detected.The majority of the ridges of line-like thermal plumes coincide with theunstable manifolds of the saddles. From a dynamical Lagrangian perspective,thermal plumes are formed together with an attractive hyperbolic LagrangianCoherent Structure of the skin friction field. We also discuss the differencesfrom the skin friction field in turbulent channel flows from the perspective ofthe Poincar\'{e}-Hopf index theorem for two-dimensional vector fields.

Role of critical points of the skin friction field in formation of plumes in thermal convection