Spatial and temporal variability of mixed-layer depth and entrainment zone thickness

Mixed-layer depth and entrainment zone thickness are extracted from two large lidar data sets with a recently developed technique. The entrainment flux ratio (which is often used to model entrainment in atmospheric boundary-layer models) can be calculated from these two quantities. This ratio is generally believed to be in the range of 0.1 and 0.4. A qualitative analysis of time series (MERMOZ II data set) confirms this range of values under equilibrium conditions (afternoon hours), but also shows that it clearly underestimates the importance of entrainment during the morning hours when the mixed layer is growing most rapidly. An examination of the spatial distribution of the entrainment flux ratio (Pacific '93 data set) shows that this parameter is spatially highly variable, even during `equilibrium' hours in the afternoon. In regions where the boundary layer has to adjust to new boundary conditions at the ground, values much larger than 0.4 can be observed. Although these results can only be interpreted qualitatively, they suggest that currently used entrainment parameterisations in boundary-layer models are not sufficient to capture the entrainment process properly.