Role of nocturnal turbulence and advection in the formation of shallow cumulus over land

Shallow cumuli over land are normally studied from a diurnal perspective. However, the thermodynamic vertical profiles of the morning transition may play an important role in setting up favorable conditions for the formation of shallow cumuli. In turn, these profiles are highly dependent on the evolution of the nocturnal boundary characteristics and of their layer aloft.

By analyzing thermodynamic profiles measured by radiosondes launched every three-hours at four different stations, we were able to determine how horizontal advection and turbulent mixing modify the atmospheric stability and the differences in potential temperature and specific humidity at the interface between the atmospheric boundary layer and the layer above it. Two consecutive nights were studied. Both showed very similar boundary layer development, but variations in the layer aloft by a low-level jet advection event during the second night and the intense turbulent mechanical mixing led to the development of two diurnal boundary layers with very different characteristics: a clear boundary layer during the first day and a cloudy boundary layer in the second day understudy.

In order to complete the observational study, we performed a sensitivity analysis using a mixed-layer model to examine the role of the morning initial conditions in the formation of shallow cumuli over land. The complexity and subtlety of the observed situation, namely the interaction of a strongly mixed nocturnal boundary layer and horizontal advection, make this case a suitable for verifying the capacity of mesoscale models to reproduce cloudy boundary layers that are largely dependent on conditions during the previous night