Validating Variance Similarity Functions in the Entrainment Zone Using Observations and LES Simulations

In previous work, the water vapor variance in the entrainment zone was proposed to be proportional to the convective velocity scale, gradient water vapor mixing ratio and the Brunt-Vaisala frequency in the interfacial layer, while the variance of the vertical wind at in the entrainment zone was defined in terms of the convective velocity scale. The variances in the entrainment zone have been hypothesized to depend on two distinct functions, which also depend on the Richardson number. In the study, we test these hypotheses both observationally and numerically using a Large eddy simulation (LES) modeling.

Simultaneous measurements of the Eddy correlation surface flux, wind shear profiles from wind profilers, and variance profile measurements of vertical motions and water vapor by Doppler and Raman lidars, respectively, provide a unique opportunity to thoroughly examine the functions used in defining the variances and validate them. These observations were made over the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. We have identified about 30 cases from 2016 during which the convective boundary layer (CBL) is quasi-stationary and well mixed for at least 2 hours. Therefore, utilizing this unique combination of observations, the study focuses on extensively testing the hypotheses that the second-order moments are indeed proportional to the functions which also depend on Richardson number. Furthermore, we simulated the CBL using an LES model for the selected cases at the SGP site and derived the variances to test the similarity functions. The coefficients that are used in defining the functions will also be determined observationally and compared against with the values suggested by LES studies.