Characteristics of the early-morning shallow convective boundary layer from Helipod Flights during STINHO-2

Airborne turbulence measurements in the shallow convective boundary layer (SCBL) during the morning transition were performed using the helicopter probe Helipod. The experiment was carried out during the STINHO-2 summer campaign over heterogeneous land surface. While the top z(i) of the morning SCBL was rapidly rising, the Helipod performed straight and level flights. With this flight strategy, vertical profiles of horizontally averaged boundary-layer properties with respect to z(i) were obtained. The time to fly the 5 km straight and level legs was short enough to provide snapshots of the lower atmosphere. During the burning-off of the inversion layer the potential temperature in the residual nocturnal inversion layer remained unchanged. The increase of the SCBL depth, the surface temperature and the surface heat flux in time could be well described by a linear approximation. Due to a short eddy-overturning time a good vertical mixing of the SCBL was expected. However, the heat flux at about 100 m above the surface exhibited larger deviation from linear temporal behaviour. Applying the Deardorff scaling scheme, large scatter around the classic daytime convective boundary layer (CBL) heat flux profile was found. The dimensionless heat flux crossed zero at about 0.6 z(i) and a mean entrainment parameter of 0.83 was derived. Entrainment was rated to be the most important mechanism for SCBL growth. Significant difference to the daytime CBL was found regarding the dimensionless statistical dispersions.