Decaying turbulence occurs when the energy source that drives turbulence is cut-off or diminished; therefore the turbulence can no longer be maintained, and it decays through viscous dissipation. Such decay occurs in the atmosphere, for example, during the daytime-to-nighttime transition, when strong, convectively driven turbulence is deprived of its source – the surface heating. The turbulence must then decay to level appropriate to more stable conditions. In this study, we drive a Direct Numerical Simulation (DNS) of turbulence with a surface heat flux characteristic of the afternoon transition. The DNS method allows us to compute all the terms of the TKE balance as well as spectra of the TKE as a function of time. In this paper we describe the evolution of dominant scales of the TKE balance during the decay process. We will also compare characteristics of the simulated decaying boundary layer with observations of dissipation rate, TKE, and other variable measured by a wind profiling radar and sonic anemometer during the afternoon transition in Salt Lake City. The data were collected in October 2000 as part of the first field study carried out under the U.S. Department of Energy’s Vertical Transport and Mixing (VTMX) Program.