The Impact of Longwave Radiation on the Morning Transition: Results from the GABLS3 Large-Eddy Simulation

The evolution of the night-time stable boundary layer (SBL) into the daytime convective boundary layer (CBL) through the morning transition has been less extensively studied in large-eddy simulations than either the developed SBL or the CBL. The GABLS3 large-eddy case, extending from 00 UTC to 09 UTC and accompanied by hourly observations from 200-m tower, provides a good basis for studies of the transition.

Previous work has established that both buoyancy and shear are important in determining the rate of entrainment and the growth of the boundary layer through the transition. Here, we examine the impact of including longwave radiation in the simulations. Radiative cooling of the residual layer and smoothing of the temperature profile around the top of the SBL reduce the strength of the inversion above the developing mixed layer, leading to significantly faster growth of the mixed layer later in the transition. The resulting deeper and drier mixed layer exhibits better agreement with the observations.

We also discuss the impact of longwave radiation on the characteristics of the turbulence and compare the diagnosed entrainment rates with those predicted using some published parametrizations.