An Objective Method for Deriving Atmospheric Structure from Airborne Lidar Observations

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.

Section: The Uhoh Water Vapour Dial Systemmentioning

confidence: 99%

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

Muppa

Behrendt

Späth

et al. 2015

“…The third approach identifies the height at which the average between the integral of the two atmospheric layers is the greatest, δ 3 , which resembles Davis et al (2000) and Emeis et al (2008, Sect. 2.2.3),…”

Section: Wake Height-wind Direction Dependencementioning

confidence: 99%

Variations of the Wake Height over the Bolund Escarpment Measured by a Scanning Lidar

Lange

Mann

Angelou

et al. 2015

The wake zone behind the escarpment of the Bolund peninsula in the Roskilde Fjord, Denmark, has been investigated with the help of a continuous-wave Doppler lidar. The instrument measures the line-of-sight wind speed 390 times per second in highly resolved 7-m tall profiles by rapidly changing the focus distance and beam direction. The profiles reveal the detailed and rapidly changing structure of the wake induced by the Bolund escarpment. The wake grows with distance from the escarpment, with the wake height depending strongly on the wind direction, such that the minimum height appears when the flow is perpendicular to the escarpment. The wake increases by 10-70 % when the wind direction deviates ±15 • from perpendicular depending on the distance to the edge and to a lesser degree on the method by which the wake height is determined. This finding is supported by a comparison with in situ measurements acquired on the Bolund peninsula.

“…We did not make an attempt to determine the resulting bias, as this becomes only important if detailed comparisons with profiles of similarity relationships or LES are performed, which was not the subject of this work. Nevertheless, in order to measure z i (t), we applied the Haar wavelet analysis introduced in Davis et al (2000), which was also used in Cohn and Angevine (2000) and in Wulfmeyer and Janjić (2005). Details of Figure 5 depicts the 12-h observation of mixing ratio, with temporal and spatial resolutions of δt = 10 s and δz = 75 m, respectively.…”

Section: Determination Of Boundary-layer Structure and Depthmentioning

confidence: 99%

Can Water Vapour Raman Lidar Resolve Profiles of Turbulent Variables in the Convective Boundary Layer?

Wulfmeyer

Pal

Turner

et al. 2010

111

High-resolution water vapour measurements made by the Atmospheric Radiation Measurement (ARM) Raman lidar operated at the Southern Great Plains Climate Research Facility site near Lamont, Oklahoma, U.S.A. are presented. Using a 2-h measurement period for the convective boundary layer (CBL) on 13 September 2005, with temporal and spatial resolutions of 10 s and 75 m, respectively, spectral and autocovariance analyses of water vapour mixing ratio time series are performed. It is demonstrated that the major part of the inertial subrange was detected and that the integral scale was significantly larger than the time resolution. Consequently, the major part of the turbulent fluctuations was resolved. Different methods to retrieve noise error profiles yield consistent results and compare well with noise profiles estimated using Poisson statistics of the Raman lidar signals. Integral scale, mixing-ratio variance, skewness, and kurtosis profiles were determined including error bars with respect to statistical and sampling errors. The integral scale ranges between 70 and 130 s at the top of the CBL. Within the CBL, up to the third order, noise errors are significantly smaller than sampling errors and the absolute values of turbulent variables, respectively. The mixing-ratio variance profile rises monotonically from ≈0.07 to ≈3.7 g 2 kg −2 in the entrainment zone. The skewness is nearly zero up to 0.6 z/z i , becomes −1 around 0.7-0.8 z/z i , crosses zero at about 0.95 z/z i , and reaches about 1.7 at 1.1 z/z i (here, z is the height and z i is the CBL depth). The noise errors are too large to derive fourth-order moments with sufficient accuracy. Consequently, to the best of our knowledge, the ARM Raman lidar is the first water vapour Raman lidar with demonstrated capability to retrieve profiles of turbulent variables up to the third order during daytime throughout the atmospheric CBL.