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

Abstract

Turbulent humidity fluctuations in the convective boundary layer (CBL) under clear-sky conditions were investigated by deriving moments up to fourth-order. Highresolution humidity measurements were collected with a water vapour differential absorption lidar system during the HD(CP) 2 Observational Prototype Experiment (HOPE). Two cases, both representing a well-developed CBL around local noon, are discussed. While the first case (from the intensive observation period (IOP) 5 on 20 April 2013) compares well with what is considered typical CBL behaviour, the second case (from IOP 6 on 24 April 2013) shows a number of non-typical characteristics. Both cases show similar capping inversions and wind shear across the CBL top. However, a major difference between both cases is the advection of a humid layer above the CBL top during IOP 6. While the variance profile of IOP 5 shows a maximum at the interfacial layer, two variance peaks are observed near the CBL top for IOP 6. A marked difference can also be seen in the third-order moment and skewness profiles: while both are negative (positive) below (above) the CBL top for IOP 5, the structure is more complex for IOP 6. Kurtosis is about three for IOP 5, whereas for IOP 6, the distribution is slightly platykurtic. We believe that the entrainment of an elevated moist layer into the CBL is responsible for the unusual findings for IOP 6, which suggests that it is important to consider the structure of residual humidity layers entrained into the CBL.

Profiles of second- to fourth-order moments of turbulent temperature fluctuations in the convective boundary layer: first measurements with rotational Raman lidar

“…It is planned to extend the investigation of CBL characteristics in future studies also by combining the UHOH RRL data with humidity and wind observations from water vapor DIAL Wagner et al, 2013;Muppa et al, 2015) and Doppler lidar. Furthermore, also the scanning capability of the UHOH RRL will be used in the future to collect data closer to the ground and even the surface layer in order to investigate heterogeneities over different terrain.…”

“…Furthermore, the RASS profiles have typical resolutions of a few minutes which is too large to resolve the inertial subrange. In addition to radar, lidar techniques have also been used for turbulence studies: elastic backscatter lidar (Pal et al, , 2013, ozone differential absorption lidar (ozone DIAL) (Senff et al, 1996), Doppler lidar (e.g., Lenschow et al, 2000Lenschow et al, , 2012Wulfmeyer and Janjic, 2005;O'Connor et al, 2010;Träumner et al, 2015), water vapor differential absorption lidar (WV DIAL) (e.g., Senff et al, 1994;Kiemle et al, 1997;Wulfmeyer, 1999a;Lenschow et al, 2000;Muppa et al, 2015), and water vapor Raman lidar (e.g., Wulfmeyer et al, 2010;Turner et al, 2014a, b) have been employed or a combination of these techniques (e.g., Giez et al, 1999;Wulfmeyer, 1999b;Kiemle et al, 2007Kiemle et al, , 2011Behrendt et al, 2011a;Kalthoff et al, 2013). However, so far, profiling of turbulent temperature fluctuations with active remote sensing was missing.…”

The HD(CP)<sup>2</sup> Observational Prototype Experiment (HOPE) – an overview

“…Consequently, both systems are capable of resolving turbulent fluctuations in the convective boundary layer from the surface to the entrainment zone. Derived products include statistical moments of moisture and temperature turbulent fluctuations Muppa et al, 2016;Wulfmeyer et al, 2015), profiles of stability variables such as buoyancy , and the boundary layer depth, aerosol backscatter fields, and cloud boundaries. The self-calibrating DIAL technique has excellent absolute accuracy (Bhawar et al, 2011) and has been acknowledged as water vapour reference standard of WMO.…”

3-D water vapor field in the atmospheric boundary layer observed with scanning differential absorption lidar

“…The University of Hohenheim (UHOH) WVDIAL is a ground-based mobile instrument which has already demonstrated vertical measurements in the ABL with high resolution and accuracy (Bhawar et al, 2011;Muppa et al, 2016;Wulfmeyer et al, 2016). Here, we present different types of scanning measurements of this system and discuss the measurement uncertainties.…”