Abstract
Turbulent humidity fluctuations in the convective boundary layer (CBL) under clear-sky conditions were investigated by deriving moments up to fourth-order. High-resolution humidity measurements were collected with a water vapour differential absorption lidar system during the $$\hbox {HD(CP)}^{2}$$ 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.
Highlights
Turbulence is responsible for the vertical transport of heat and moisture in the convective boundary layer (CBL)
Turbulent humidity fluctuations in the CBL including the interfacial layer were investigated by deriving the higher-order moments from University of Hohenheim (UHOH) differential absorption lidar (DIAL) data collected during the HD(CP)2 Observation Prototype Experiment (HOPE) campaign
Two periods around noon under clear-sky conditions were identified and reliable profiles of up to the fourth-order moment were obtained for humidity with unprecedented precision
Summary
Turbulence is responsible for the vertical transport of heat and moisture in the convective boundary layer (CBL). An adequate representation of the thermodynamic structure of the CBL in atmospheric models is critical; moisture transport across the CBL influences the formation of fair-weather cumuli (Zhu and Albrecht 2002) and the initiation of deep convection and flash floods (Weckwerth 2000; Behrendt et al 2011; Corsmeier et al 2011; Stevens and Bony 2013). Several large international experiments aimed at a better understanding of the humidity field in the CBL have been carried out, e.g. the Mesoscale Alpine Program for high mountains during the 1990s (Richard et al 2007); the International H2O Project (IHOP_2002) in 2002 (Weckwerth et al 2004), and the Convective and Orographically-induced Precipitation Study (COPS) in 2007 (Wulfmeyer et al 2011). The HD(CP) Observation Prototype Experiment (HOPE), conducted in spring 2013, was dedicated to providing high resolution datasets of meteorological variables at the subgrid scale that are subject to parametrizations in the high resolution models of HD(CP)
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