Convective boundary layer entrainment: A review and consolidation using Doppler lidar

Lidars are excellent instruments for boundary layer meteorology. Measurements of boundary layer height variations in the order of a few ten meters by detecting aerosols are a typical application for elastic backscatter lidars, whereas turbulence measurements in the boundary layer are the domain of Doppler lidars. The potential of lidar techniques to investigate the entrainment of warmer and drier air from the free atmosphere into the turbulent boundary layer was already discovered in the 1980s, but dropped off in the last years. Using new reliable, fully automated Doppler lidar, a reinvestigation of the entrainment studies was carried out to meet the demand of an improved process understanding and a more reliable parametrization necessary for high resolution models. Combining aerosol backscatter measurements with high resolution wind velocity measurements, the entrainment process can be examined from two perspectives: on the one hand it is possible to visualize the small scale processes carrying air from the free atmosphere into the boundary layer, which results in a growth of the turbulent layer. On the other hand the velocity measurements can be used to characterize the turbulence simultaneously by length and velocity scales. Our measurements are showing different individual entrainment processes, whose occurrence is linked to distinct stability situations in the free atmosphere, entrainment zone thicknesses and entrainment velocities. Studying different conceptual approaches for parametrization, well-established relationships using the scales “boundary layer height” and “convective velocity” were confirmed. Furthermore, it can be shown that the correlation coefficient between entrainment zone thickness respectively entrainment velocity and stability parameters based on the stratification of the free atmosphere can be increased by a factor of up to 1.8 using the integral length scale and the variance of the vertical wind velocity instead of the scales used so far.