Entrainment results including advection and case studies from the Flatland boundary layer experiments

Boundary layer top entrainment measurements from the 1996 Flatland boundary layer experiment (Flatland96) were presented by Angevine et al. [1998a]. At that time, no reliable estimates of temperature advection, which could affect the heat budget approach used, were available. This paper presents the entrainment results recalculated with advection estimated from the National Centers for Environmental Prediction mesoscale Eta numerical model. Estimates of the contribution to entrainment by mechanical turbulence (shear) and of the shear-free entrainment ratio are also calculated. A number of case study examples are also presented to elucidate further the complex interactions that govern entrainment. Accounting for advection in the budgets results in a slightly smaller estimate of the entrainment ratio A(R); the mean A(R) is 0.47 + 0.11, and the median is 0.40. These results are still somewhat larger than long-accepted values and comparable to results from some other studies in comparable conditions. The experiments took place in the area near the Flatland Atmospheric Observatory near Champaign-Urbana, Illinois, in June-August 1996. The site is interesting because it is extraordinarily flat, has uniform land use, and is situated in a prime agricultural area.