A new hybrid large-eddy simulation / Reynolds-averaged Navier-Stokes simulation (LES/RANS) method is presented in this work. In this approach, the resolved turbulence kinetic energy, ensemble-averaged modeled turbulence kinetic energy and turbulence frequency, and time-resolved turbulence frequency are used to form an estimate of an outer-layer length scale that is nearly Reynolds-number independent. The ratio of this outer-layer scale with an inner-layer length scale (proportional to the wall distance) is used to construct a blending function that facilitates the shift between an unsteady RANS formulation near solid surfaces and an LES formulation away from the wall. The new model is tested through simulations of compressible flat-plate boundary layers over a wide range of Reynolds numbers and Mach 2.85 flow over a smooth compression ramp. The results show that the new model provides results for mean and second-moment statistics that are in good agreement with experiment and are comparable to those obtained using the original model (Edwards, et al., AIAA J., 2008), which required a case-by-case calibration of a model constant.