Spatial And Temporal Variability Of Mixed-Layer Depth And Entrainment Zone Thickness

Abstract

Mixed-layer depth and entrainment zone thickness areextracted from two large lidar data sets with arecently developed technique. The entrainment fluxratio (which is often used to model entrainment inatmospheric boundary-layer models) can be calculatedfrom these two quantities. This ratio is generallybelieved to be in the range of 0.1 and 0.4. Aqualitative analysis of time series (MERMOZ II dataset) confirms this range of values under equilibriumconditions (afternoon hours), but also shows that itclearly underestimates the importance of entrainmentduring the morning hours when the mixed layer isgrowing most rapidly. An examination of the spatialdistribution of the entrainment flux ratio (Pacific'93 data set) shows that this parameter is spatiallyhighly variable, even during ‘equilibrium’ hours inthe afternoon. In regions where the boundary layerhas to adjust to new boundary conditions at theground, values much larger than 0.4 can be observed. Although these results can only be interpretedqualitatively, they suggest that currently usedentrainment parameterisations in boundary-layer modelsare not sufficient to capture the entrainment processproperly.