Characterizing turbulence in the CBL using ARM observations and LES

Poster PDF

Description

A similarity relationship for the water vapor variance in the interfacial layer (IL) at the top of the convective boundary layer (CBL) was proposed to be proportional to the convective velocity scale, and the gradients of the water vapor mixing ratio and the Brunt-Vaisala frequency in the entrainment zone. In the presence of wind shear in the IL, the similarity relationship was hypothesized to also include a dependence on the gradient Richardson number. Similarity relationships such as this are often developed using LES output, but are not well evaluated with observations. Simultaneous measurements of the surface buoyancy flux, wind-shear profiles from a radar wind profiler, water-vapor mixing ratio and temperature measurements and their gradients from a Raman lidar provide a unique opportunity to thoroughly examine the function used in defining the variance and validate it. These observations were made over the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. We identified 19 cases from 2016 during which the CBL was quasi-stationary and well mixed for at least 2 hours in the afternoon. Furthermore, we simulated the CBL using an LES model for these cases and derived the water-vapor variance and other profiles to test the similarity function. Utilizing this unique combination of observations and LES, we demonstrate that the water vapor variance in the IL has no dependence on wind shear. Furthermore, we demonstrate that the predicted variance using the original similarity function matches the observed and LES-modeled variance very well, with linear correlations between the two variances of 0.82 and 0.95 respectively. Finally, we use this ensemble of LES simulations to express the moments of humidity, temperature and velocity normalized by boundary layer depth, and study the relative impact of entrainment and surface processes on the evolution of those moments.