Entrainment zone definitions: a large eddy study

Entrainment zone depth is often used as a controlling factor in parameterizations of entrainment rate for the convective boundary layer. There are several different definitions of the entrainment zone in common use: the region of negative buoyancy flux at the top of the BL, the region of high vertical gradient in some scalar quantity, the region between defined limits of the probability distribution of estimates of BL top. The approach used depends in large part on the data set in question, but the different definitions can give very different values for the same environment, complicating the comparison of different data sets and the development of universally applicable parameterizations. Initial results of an LES study of the relationships between different entrainment zone definitions, boundary-layer properties, and entrainment rate for the cloud free, convective boundary layer will be presented. The LES provides a controlled environment in which entrainment zone estimates can be made using all the definitions in common use. In particular we will compare the well established definitions with a recently developed multi-scale wavelet technique (Brooks 2003) that allows the limits of the local transition zone between well-mixed BL and free troposphere to be determined in an objective manner from vertical profiles of scalar quantities.