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

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 $$\hbox {HD(CP)}^{2}$$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.

[1]  Bjorn Stevens,et al.  What Are Climate Models Missing? , 2013, Science.

[2]  Turbulence Characteristics in an Elevated Shear Layer over Owens Valley , 2010 .

[3]  B. Albrecht,et al.  A theoretical and observational analysis on the formation of fair-weather cumuli , 2002 .

[4]  Donald H. Lenschow,et al.  An Objective Method for Deriving Atmospheric Structure from Airborne Lidar Observations , 2000 .

[5]  F. G. Fernald.  Analysis of atmospheric lidar observations: some comments. , 1984, Applied optics.

[6]  P. Di Girolamo,et al.  Observation of convection initiation processes with a suite of state‐of‐the‐art research instruments during COPS IOP 8b , 2010 .

[7]  A. Holtslag,et al.  Interactions between dry‐air entrainment, surface evaporation and convective boundary‐layer development , 2009 .

[8]  J. Deardorff,et al.  Three-dimensional numerical study of turbulence in an entraining mixed layer , 1974 .

[9]  Z. Sorbjan.  Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-eddy Simulations. Part 1: Free Convection , 2005 .

[10]  Volker Wulfmeyer,et al.  Investigation of Turbulent Processes in the Lower Troposphere with Water Vapor DIAL and Radar–RASS , 1999 .

[11]  F. Chow,et al.  The Convective Boundary Layer in the Terra Incognita , 2014 .

[12]  D. Lenschow,et al.  Water Vapor Flux Measurements from Ground-Based Vertically Pointed Water Vapor Differential Absorption and Doppler Lidars , 1999 .

[13]  B. Stevens,et al.  Structure of the Entrainment Zone Capping the Convective Atmospheric Boundary Layer , 1998 .

[14]  A. Ansmann.  Errors in ground-based water-vapor DIAL measurements due to Doppler-broadened Rayleigh backscattering. , 1985, Applied optics.

[15]  V. Wulfmeyer,et al.  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 , 2016 .

[16]  S. Grossmann.  The Spectrum of Turbulence , 2003 .

[17]  E. G. Norton,et al.  Processes driving deep convection over complex terrain: a multi‐scale analysis of observations from COPS IOP 9c , 2011 .

[18]  Volker Wulfmeyer,et al.  Elastic-backscatter-lidar-based characterization of the convective boundary layer and investigation of related statistics , 2010 .

[19]  Tammy M. Weckwerth,et al.  Thermodynamic Variability within the Convective Boundary Layer Due to Horizontal Convective Rolls , 1996 .

[20]  V. Wulfmeyer,et al.  High-power Ti:sapphire laser at 820 nm for scanning ground-based water-vapor differential absorption lidar. , 2013, Applied optics.

[21]  H. Jonker,et al.  Large-Eddy Simulation: How Large is Large Enough? , 2004 .

[22]  Tammy M. Weckwerth,et al.  The Effect of Small-Scale Moisture Variability on Thunderstorm Initiation , 2000 .

[23]  R. Grossman,et al.  Moisture flux and mixing processes in the daytime continental convective boundary layer , 1995 .

[24]  Volker Wulfmeyer,et al.  Measuring Second- through Fourth-Order Moments in Noisy Data , 2000 .

[25]  E. Fedorovich,et al.  Effects of wind shear on the atmospheric convective boundary layer structure and evolution , 2008 .

[26]  Evgeni Fedorovich,et al.  Dynamics of Sheared Convective Boundary Layer Entrainment. Part I: Methodological Background and Large-Eddy Simulations , 2006 .

[27]  John C. Wyngaard,et al.  Top-down and bottom-up diffusion of a scalar in the convective boundary layer , 1984 .

[28]  R. Stull.  An Introduction to Boundary Layer Meteorology , 1988 .

[29]  Volker Wulfmeyer,et al.  Can Water Vapour Raman Lidar Resolve Profiles of Turbulent Variables in the Convective Boundary Layer? , 2010 .

[30]  B. H. Stockton,et al.  Laboratory studies of the entrainment zone of a convectively mixed layer , 1980, Journal of Fluid Mechanics.

[31]  A. Arakawa,et al.  Representation of Boundary Layer Moisture Transport in Cloud-Resolving Models , 2012 .

[32]  Lindsay J. Bennett,et al.  The Convective and Orographically‐induced Precipitation Study (COPS): the scientific strategy, the field phase, and research highlights , 2011 .

[33]  B. Stevens,et al.  Observations, experiments, and large eddy simulation , 2001 .

[34]  Gerhard Ehret,et al.  Estimation of boundary layer humidity fluxes and statistics from airborne differential absorption lidar (DIAL) , 1997 .

[35]  E. Browell,et al.  Airborne and spaceborne lidar measurements of water vapor profiles: a sensitivity analysis. , 1989, Applied optics.

[36]  D. Lenschow,et al.  How long is long enough when measuring fluxes and other turbulence statistics , 1994 .

[37]  L. Mahrt.  Boundary‐layer moisture regimes , 1991 .

[38]  John C. Wyngaard,et al.  Statistics of Conservative Scalars in the Convective Boundary Layer , 1984 .

[39]  Steven E. Koch,et al.  An Overview of the International H2O Project (IHOP_2002) and Some Preliminary Highlights , 2004 .

[40]  V. Wulfmeyer,et al.  Investigations of Humidity Skewness and Variance Profiles in the Convective Boundary Layer and Comparison of the Latter with Large Eddy Simulation Results , 1999 .

[41]  John C. Wyngaard,et al.  Evaluation of turbulent transport and dissipation closures in second-order modeling , 1989 .

[42]  Volker Wulfmeyer,et al.  Profiles of second- to fourth-order moments of turbulent temperature fluctuations in the convective boundary layer: first measurements with rotational Raman lidar , 2014 .

[43]  L. Mahrt.  Mixed Layer Moisture structure , 1976 .

[44]  V. Masson,et al.  Water‐vapour variability within a convective boundary‐layer assessed by large‐eddy simulations and IHOP_2002 observations , 2005 .

[45]  J. Bösenberg,et al.  Ground-based differential absorption lidar for water-vapor profiling: assessment of accuracy, resolution, and meteorological applications. , 1998, Applied optics.

[46]  P. Di Girolamo,et al.  The water vapour intercomparison effort in the framework of the Convective and Orographically‐induced Precipitation Study: airborne‐to‐ground‐based and airborne‐to‐airborne lidar systems , 2011 .

[47]  A. Holtslag,et al.  Scaling Variances of Scalars in a Convective Boundary Layer Under Different Entrainment Regimes , 2006 .

[48]  Andreas Wieser,et al.  Turbulent Structures and Coherence in the Atmospheric Surface Layer , 2014, Boundary-Layer Meteorology.

[49]  Guy N. Pearson,et al.  Vertical velocity variance and skewness in clear and cloud‐topped boundary layers as revealed by Doppler lidar , 2009 .

[50]  Bianca Adler,et al.  KITcube - a mobile observation platform for convection studies deployed during HyMeX , 2013 .

[51]  E. Patton,et al.  The Effect of Mesh Resolution on Convective Boundary Layer Statistics and Structures Generated by Large-Eddy Simulation , 2011 .

[52]  R. Ferrare,et al.  Aircraft Evaluation of Ground-Based Raman Lidar Water Vapor Turbulence Profiles in Convective Mixed Layers , 2014 .

[53]  J. Kaimal,et al.  Turbulence Structure in the Convective Boundary Layer , 1976 .

[54]  Correction scheme for spectral broadening by Rayleigh scattering in differential absorption lidar measurements of water vapor in the troposphere. , 1987, Applied optics.

[55]  V. Masson,et al.  Negative water vapour skewness and dry tongues in the convective boundary layer: observations and large-eddy simulation budget analysis , 2007 .

[56]  A. Savitzky,et al.  Smoothing and Differentiation of Data by Simplified Least Squares Procedures. , 1964 .

[57]  D. Lenschow,et al.  A Comparison of Higher-Order Vertical Velocity Moments in the Convective Boundary Layer from Lidar with In Situ Measurements and Large-Eddy Simulation , 2012, Boundary-Layer Meteorology.

[58]  Volker Wulfmeyer,et al.  Temperature profiling of the atmospheric boundary layer with rotational Raman lidar during the HD(CP) 2 Observational Prototype Experiment , 2014 .

[59]  Z. Sorbjan.  Effects Caused by Varying the Strength of the Capping Inversion Based on a Large Eddy Simulation Model of the Shear-Free Convective Boundary Layer , 1996 .

[60]  Lindsay J. Bennett,et al.  Observations of the Evolution of the Nocturnal and Convective Boundary Layers and the Structure of Open-Celled Convection on 14 June 2002 , 2010 .

[61]  Z. Sorbjan.  Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-Eddy Simulations. Part II: Forced Convection , 2006 .

[62]  G. Zängl,et al.  Quantitative precipitation forecasting in the Alps: The advances achieved by the Mesoscale Alpine Programme , 2007 .