Determination of Convective Boundary Layer Entrainment Fluxes, Dissipation Rates, and the Molecular Destruction of Variances: Theoretical Description and a Strategy for Its Confirmation with a Novel Lidar System Synergy

Abstract

Atmospheric variables in the convective boundary layer (CBL), which are critical for turbulence parameterizations in weather and climate models, are assessed. These include entrainment fluxes, higher-order moments of humidity, potential temperature, and vertical wind, as well as dissipation rates. Theoretical relationships between the integral scales, gradients, and higher-order moments of atmospheric variables, fluxes, and dissipation rates are developed mainly focusing on the entrainment layer (EL) at the top of the CBL. These equations form the starting point for tests of and new approaches in CBL turbulence parameterizations. For the investigation of these relationships, an observational approach using a synergy of ground-based water vapor, temperature, and wind lidar systems is proposed. These systems measure instantaneous vertical profiles with high temporal and spatial resolution throughout the CBL including the EL. The resolution of these systems permits the simultaneous measurement of gradients and fluctuations of these atmospheric variables. For accurate analyses of the gradients and the shapes of turbulence profiles, the lidar system performances are very important. It is shown that each lidar profile can be characterized very well with respect to bias and system noise and that the constant bias has negligible effect on the measurement of turbulent fluctuations. It is demonstrated how different gradient relationships can be measured and tested with the proposed lidar synergy within operational measurements or new field campaigns. Particularly, a novel approach is introduced for measuring the rate of destruction of humidity and temperature variances, which is an important component of the variance budget equations.

Turbulent Humidity Fluctuations in the Convective Boundary Layer: Case Studies Using Water Vapour Differential Absorption Lidar Measurements

“…Lenschow et al (2000) introduced a procedure for the estimation of higher-order moments that accounts for random instrumental noise. This method was successfully used not only to investigate higher-order moments of water vapour DIAL but also with water vapour Raman lidar data Turner et al 2014), and with Doppler lidar data for vertical velocity Hogan et al 2009;Lenschow et al 2012;Wulfmeyer et al 2015). More recently, temperature higher-order moments were estimated using rotational Raman lidar data .…”

“…In our study, a 20-point extrapolation was used for the linear extrapolation and fit of the structure function. This is a reasonable approach because the first zero crossing of the autocovariance function τ 0 = 2.5 , where is the integral scale Wulfmeyer et al 2015),…”

“…For improved turbulence parametrization schemes, general scaling relationships between the higher-order moments of temperature and humidity fluctuations in the CBL and their vertical gradients in the interfacial layer need to be investigated (Sorbjan 2005;Wulfmeyer et al 2015). However, present LES studies show a significant uncertainty in their results due to processes at the subgrid scale (Stevens and Lenschow 2001;Zhou et al 2014).…”

“…Remote sensing techniques overcome these limitations and cover larger ranges simultaneously and continuously. Monitoring moisture fluctuations with high resolution for turbulence studies is a challenging task due to the noise limitation of these instruments; nevertheless, new techniques based on lidar are capable of providing data with sufficient precision and accuracy Wulfmeyer et al 2010Wulfmeyer et al , 2015. Couvreux et al (2005) compared the results of airborne water vapour DIAL measurements from the IHOP campaign with LES for a growing CBL.…”

“…Later, this study was further extended to the analysis of skewness profiles (Couvreux et al 2007), showing that dry tongues contribute significantly to the turbulent transport from the top of the CBL into its interior. Wind shear at the interfacial layer also plays an important role in the dynamics and growth of the CBL (Conzemius and Fedorovich 2006;Sorbjan 2006;Fedorovich and Conzemius 2008;Wulfmeyer et al 2015). However, the role of wind shear on the profile of humidity variance and skewness in the CBL has so far not been investigated in detail.…”