Characterizing the Turbulent Structure of the Convective Boundary Layer Over the SGP Using ARM/ASR Observations and LES Model Output

To gain a better understanding of the entrainment zone of the Convective Boundary Layer, we use a combination of high resolution observations and realistic (non-idealized) Large Eddy Simulations (LES), over the ARM - Southern Great Planes site. To drive the LES, the ARM variational analysis is used, which means that LES and observations are directly comparable. For 12 selected days, the overall characteristics (such as boundary layer depth, mean temperature and humidity) are in good agreement between LES and observations. Details of the methodology of driving the LES turn out to be crucial for the correct development of boundary layer over time.

To study the structure of the boundary layer and especially the entrainment zone, we focus on the higher order moments of humidity, temperature and momentum. An apparent discrepancy between model and observations is the low kurtosis in the humidity distribution of the observed entrainment zone, while the model results are close to a normal distribution. We are able to reconcile the observational and model results by discussing sampling strategies, ergodicity, and the behavior of dry tongues penetrating into the boundary layer.

Finally, the LES simulations are used to discuss the differences in scaling between the quasi-stationary bounday layer on the one hand, and the morning and evening transitions on the other hand.