A two-regime model for the probability density function of the temperaturestructure parameter in the convective boundary layer

The experimentally observed probability distribution of the acoustic echo-s ignal intensity associated with the temperature structure parameter C-T(2) was found to be markedly non-lognormal in the convective boundary layer. A two-regime model of thermal turbulence is suggested that combines a small-s cale turbulence description with the concept of coherent structures. The pr obability density function of the logarithm of the temperature structure pa rameter C-T(2) is assumed to be approximated by a combination of two normal distributions taken with corresponding weights. It can be interpreted as t he existence of two regimes of thermal turbulence, namely, light background turbulence and strong turbulence within convective plumes. This decomposit ion of the probability density function into `plume' and `background' parts provides an objective procedure for separating the statistics of these two regimes as well as to determine the probability of convective plume occurr ence. It is shown that the conventionally measured overall mean C-T(2), ave raged over a long enough time interval, is determined mainly by the probabi lity of plume occurrence, while the mean C-T(2) value within plumes changes slightly. The decrease of the total mean C-T(2) with height is caused main ly by the decrease of the probability of the plume occurrence with height.