Observational and numerical evidence of depressed convective boundary layer heights near a mountain base

Recent field and numerical studies show evidence of the existence of a convective boundary layer height depression near a mountain base. This depression can have implications for air pollutant transport and concentrations in complex terrain. To investigate the mechanisms underlying this phenomenon, idealized simulations with a mesoscale numerical model are performed and combined with available observations. The idealized simulations with a single mountain ridge of various dimensions suggest that the depression evolves in time, is most pronounced in the late afternoon, and becomes larger as slope steepness increases. Observations and modeling results show that the atmosphere is heated more intensely near the mountain base than far away from the mountain base, not only inside the boundary layer but also above. The enhanced heating aloft affects boundary layer growth near the mountain base and is associated with the boundary layer height depression. An analysis of the different terms in the temperature tendency equation indicates that vertical and horizontal advection of warm air, associated with the thermally driven circulation along the mountain slope, play a role in this enhanced heating aloft.