scholarly journals Characteristics of convective boundary layer and associated entrainment zone as observed by a ground-based polarization lidar

2020 ◽  
Author(s):  
Fuchao Liu ◽  
Fan Yi ◽  
Zhenping Yin ◽  
Yunpeng Zhang ◽  
Yun He ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Growth Rate ◽  
Convective Boundary Layer ◽  
Statistical Characteristics ◽  
Convective Boundary ◽  
Transform Method ◽  
Entrainment Zone ◽  
Positive Growth Rate ◽  
Aerosol Backscatter ◽  
Variance Profile

Abstract. A tilted polarization lidar (TPL) with a pointing angle of 30° off zenith has been developed for continuous monitoring of the atmosphere with 10-s time and 6.5-m height resolution. From lidar-derived aerosol backscatter, instantaneous ABL depths are retrieved by logarithm gradient method (LGM) and Harr wavelet transform method (HWT), while hourly-mean ABL depths by variance method. A new FWHM method utilizing the full width at half maximum (FWHM) of the variance profile of aerosol backscatter ratio (ABR) fluctuations is proposed to determine the entrainment zone thickness (EZT). Both typical winter and summer clear-day observational cases are presented. It is concluded the convective boundary layer (CBL) evolution can be described by four stages. At the formation stage, the hourly-mean CBL depth grew slowly with a positive growth rate of  0.3 km/h. At the quasi-stationary stage, the hourly-mean CBL depth varied little and the corresponding growth rate changed sign with absolute value of  150 m, while the latter had respective percentages of 2.0 % and 31 % of EZT falling into the same corresponding subranges. Common statistical characteristics also existed for both cases. The growth stage always had the largest mean and stddev of EZT and the quasi-stationary stage usually the smallest stddev of EZT. For all four stages, most EZT values fell into the 50–150 m subrange; the overall percentages of EZT falling into the 50–150 m subrange between 0900 and 1900 LT were 84 % and 67 % for the winter and summer cases, respectively.

scholarly journals Measurement report: characteristics of clear-day convective boundary layer and associated entrainment zone as observed by a ground-based polarization lidar over Wuhan (30.5° N, 114.4° E)

2021 ◽  
Vol 21 (4) ◽  
pp. 2981-2998
Author(s):  
Fuchao Liu ◽  
Fan Yi ◽  
Zhenping Yin ◽  
Yunpeng Zhang ◽  
Yun He ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Growth Stage ◽  
Late Spring ◽  
Atmospheric Measurements ◽  
Convective Boundary ◽  
Late Autumn ◽  
Early Autumn ◽  
Entrainment Zone ◽  
Aerosol Backscatter

Abstract. Knowledge of the convective boundary layer (CBL) and associated entrainment zone (EZ) is important for understanding land–atmosphere interactions and assessing the living conditions in the biosphere. A tilted 532 nm polarization lidar (30∘ off zenith) has been used for the routine atmospheric measurements with 10 s time and 6.5 m height resolution over Wuhan (30.5∘ N, 114.4∘ E). From lidar-retrieved aerosol backscatter, instantaneous atmospheric boundary layer (ABL) depths are obtained using the logarithm gradient method and Harr wavelet transform method, while hourly mean ABL depths are obtained using the variance method. A new approach utilizing the full width at half maximum of the variance profile of aerosol backscatter ratio fluctuations is proposed to determine the entrainment zone thickness (EZT). Four typical clear-day observational cases in different seasons are presented. The CBL evolution is described and studied in four developing stages (formation, growth, quasi-stationary and decay); the instantaneous CBL depths exhibited different fluctuation magnitudes in the four stages and fluctuations at the growth stage were generally larger. The EZT is investigated for the same statistical time interval of 09:00–19:00 LT. It is found that the winter and late autumn cases had an overall smaller mean (mean) and standard deviation (SD) of EZT data compared to those of the late spring and early autumn cases. This statistical conclusion was also true for each of the four developing stages. In addition, compared to those of the late spring and early autumn cases, the winter and late autumn cases had larger percentages of EZT falling into the subranges of 0–50 m but smaller percentages of EZT falling into the subranges of > 150 m. It seems that both the EZT statistics (mean and SD) and percentage of larger EZT values provide measures of entrainment intensity. Common statistical characteristics also existed. All four cases showed moderate variations of the mean of the EZT from stage to stage. The growth stage always had the largest mean and SD of the EZT and the quasi-stationary stage usually the smallest SD of the EZT. For all four stages, most EZT values fell into the 50–150 m subrange; the overall percentage of the EZT falling into the 50–150 m subrange between 09:00 and 19:00 LT was > 67 % for all four cases. We believe that the lidar-derived characteristics of the clear-day CBL and associated EZ can contribute to improving our understanding of the structures and variations of the CBL as well as providing a quantitatively observational basis for EZ parameterization in numerical models.

scholarly journals Bulk Models of the Sheared Convective Boundary Layer: Evaluation through Large Eddy Simulations

2007 ◽  
Vol 64 (3) ◽  
pp. 786-807 ◽  
Author(s):  
Robert Conzemius ◽  
Evgeni Fedorovich
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Entrainment Rate ◽  
Convective Boundary ◽  
Surface Shear ◽  
Large Eddy ◽  
Entrainment Zone ◽  
Sheared Convective Boundary Layer ◽  
Bulk Model ◽  
Approach Zero

Abstract A set of first-order model (FOM) equations, describing the sheared convective boundary layer (CBL) evolution, is derived. The model output is compared with predictions of the zero-order bulk model (ZOM) for the same CBL type. Large eddy simulation (LES) data are employed to test both models. The results show an advantage of the FOM over the ZOM in the prediction of entrainment, but in many CBL cases, the predictions by the two models are fairly close. Despite its relative simplicity, the ZOM is able to quantify the effects of shear production and dissipation in an integral sense—as long as the constants describing the integral dissipation of shear- and buoyancy-produced turbulence kinetic energy (TKE) are prescribed appropriately and the shear is weak enough that the denominator of the ZOM entrainment equation does not approach zero, causing a numerical instability in the solutions. Overall, the FOM better predicts the entrainment rate due to its ability to avoid this instability. Also, the FOM in a more physically consistent manner reproduces the sheared CBL entrainment zone, whose depth is controlled by a balance among shear generation, buoyancy consumption, and dissipation of TKE. Such balance is manifested by nearly constant values of Richardson numbers observed in the entrainment zone of simulated sheared CBLs. Conducted model tests support the conclusion that the surface shear generation of TKE and its corresponding dissipation, as well as the nonstationary terms, can be omitted from the integral TKE balance equation.

scholarly journals Lagrangian Particle Dispersion Modeling of the Fumigation Process Using Large-Eddy Simulation

2005 ◽  
Vol 62 (6) ◽  
pp. 1932-1946 ◽  
Author(s):  
Si-Wan Kim ◽  
Chin-Hoh Moeng ◽  
Jeffrey C. Weil ◽  
Mary C. Barth
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Convective Boundary Layer ◽  
Particle Dispersion ◽  
Lagrangian Particle ◽  
Convective Boundary ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Entrainment Zone ◽  
The Mean

Abstract A Lagrangian particle dispersion model (LPDM) is used to study fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer. Probability density functions of particle location with height and time are calculated from particle trajectories driven by the sum of the resolved-scale velocity from a large-eddy simulation (LES) model and the stochastic subgrid-scale (SGS) velocity. The crosswind-integrated concentration (CWIC) fields show good agreement with water tank experimental data. A comparison of the LPDM output with an Eulerian diffusion model output based on the same LES flow shows qualitative agreement with each other except that a greater overshoot maximum of the ground-level concentration occurs in the Eulerian model. The dimensionless CWICs near the surface for sources located above the entrainment zone collapse to a nearly universal curve provided that the profiles are time shifted, where the shift depends on the source heights. The dimensionless CWICs for sources located within the entrainment zone show a different behavior. Thus, fumigation from sources above the entrainment zone and within the entrainment zone should be treated separately. An examination of the application of Taylor’s translation hypothesis to the fumigation process showed the importance of using the mean boundary layer wind speed as a function of time rather than the initial mean boundary layer wind speed, because the mean boundary layer wind speed decreases as the simulation proceeds. The LPDM using LES is capable of accurately simulating fumigation of particles into the convective boundary layer. This technique provides more computationally efficient simulations than Eulerian models.

scholarly journals Bulk Scaling Model of Entrainment Zone Thickness in a Convective Boundary Layer, with a Shear Effect Promoted by Velocity Difference

Atmosphere ◽  
2020 ◽  
Vol 11 (1) ◽  
pp. 63
Author(s):  
Anran Li ◽  
Wenfeng Gao ◽  
Tao Liu
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Potential Temperature ◽  
Velocity Shear ◽  
Shear Effect ◽  
Velocity Difference ◽  
Convective Boundary ◽  
Scaling Model ◽  
Eddy Simulation ◽  
Entrainment Zone

Studying the thickness of the convective boundary layer (CBL) is helpful for understanding atmospheric structure and the diffusion of air pollutants. When there is velocity shear in CBL, the flow field structure is very different from that of shear-free CBL, which makes the thickness model of the entrainment zone deviate. A large-eddy simulation (LES) approach is carried out for a horizontally homogeneous, atmospheric CBL, with a shear effect promoted by velocity difference to explore the bulk scaling model of the entrainment zone thickness. The post-processed data indicate that the existing bulk scaling models cannot synthetically represent the effects of shear and buoyancy on entrainment, resulting in reduced accuracy or limited applicability. Based on the fraction of turbulent kinetic energy (TKE) used for entrainment, a different form of the characteristic velocity scale, which includes the shear effect, is proposed, and a modified bulk scaling model that uses a potential temperature gradient to replace the potential temperature jump across the entrainment zone is constructed with the numerical results. The new model is found to provide an improved prediction of the entrainment zone thickness in a sheared CBL.

scholarly journals Characterization of wind-shear effects on entrainment in a convective boundary layer

2018 ◽  
Vol 858 ◽  
pp. 145-183 ◽  
Author(s):  
Armin Haghshenas ◽  
Juan Pedro Mellado
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Convective Instability ◽  
Wind Shear ◽  
Scaling Laws ◽  
Strong Wind ◽  
Free Atmosphere ◽  
Convective Boundary ◽  
Entrainment Zone ◽  
Shear Effects

Direct numerical simulations are used to characterize wind-shear effects on entrainment in a barotropic convective boundary layer (CBL) that grows into a linearly stratified atmosphere. We consider weakly to strongly unstable conditions $-z_{enc}/L_{Ob}\gtrsim 4$, where $z_{enc}$ is the encroachment CBL depth and $L_{Ob}$ is the Obukhov length. Dimensional analysis allows us to characterize such a sheared CBL by a normalized CBL depth, a Froude number and a Reynolds number. The first two non-dimensional quantities embed the dependence of the system on time, on the surface buoyancy flux, and on the buoyancy stratification and wind velocity in the free atmosphere. We show that the dependence of entrainment-zone properties on these two non-dimensional quantities can be expressed in terms of just one independent variable, the ratio between a shear scale $(\unicode[STIX]{x0394}z_{i})_{s}\equiv \sqrt{1/3}\unicode[STIX]{x0394}u/N_{0}$ and a convective scale $(\unicode[STIX]{x0394}z_{i})_{c}\equiv 0.25z_{enc}$, where $\unicode[STIX]{x0394}u$ is the velocity increment across the entrainment zone, and $N_{0}$ is the buoyancy frequency of the free atmosphere. Here $(\unicode[STIX]{x0394}z_{i})_{s}$ and $(\unicode[STIX]{x0394}z_{i})_{c}$ represent the entrainment-zone thickness in the limits of weak convective instability (strong wind) and strong convective instability (weak wind), respectively. We derive scaling laws for the CBL depth, the entrainment-zone thickness, the mean entrainment velocity and the entrainment-flux ratio as functions of $(\unicode[STIX]{x0394}z_{i})_{s}/(\unicode[STIX]{x0394}z_{i})_{c}$. These scaling laws can also be expressed as functions of only a Richardson number $(N_{0}z_{enc}/\unicode[STIX]{x0394}u)^{2}$, but not in terms of only the stability parameter $-z_{enc}/L_{Ob}$.

scholarly journals Elastic-backscatter-lidar-based characterization of the convective boundary layer and investigation of related statistics

2010 ◽  
Vol 28 (3) ◽  
pp. 825-847 ◽  
Author(s):  
S. Pal ◽  
A. Behrendt ◽  
V. Wulfmeyer
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Haar Wavelet ◽  
Density Fluctuations ◽  
Inertial Subrange ◽  
Backscatter Coefficient ◽  
Integral Scale ◽  
Convective Boundary ◽  
Lidar Measurements ◽  
Entrainment Zone

Abstract. We applied a ground-based vertically-pointing aerosol lidar to investigate the evolution of the instantaneous atmospheric boundary layer depth, its growth rate, associated entrainment processes, and turbulence characteristics. We used lidar measurements with range resolution of 3 m and time resolution of up to 0.033 s obtained in the course of a sunny day (26 June 2004) over an urban valley (central Stuttgart, 48°47' N, 9°12' E, 240 m above sea level). The lidar system uses a wavelength of 1064 nm and has a power-aperture product of 2.1 W m2. Three techniques are examined for determining the instantaneous convective boundary layer (CBL) depth from the high-resolution lidar measurements: the logarithm gradient method, the inflection point method, and the Haar wavelet transform method. The Haar wavelet-based approach is found to be the most robust technique for the automated detection of the CBL depth. Two different regimes of the CBL are discussed in detail: a quasi-stationary CBL in the afternoon and a CBL with rapid growth during morning transition in the presence of dust layers atop. Two different growth rates were found: 3–5 m/min for the growing CBL in the morning and 0.5–2 m/min during the quasi-steady regime. The mean entrainment zone thickness for the quasi-steady CBL was found to be ~75 m while the CBL top during the entire day varied between 0.7 km and 2.3 km. A fast Fourier-transform-based spectral analysis of the instantaneous CBL depth time series gave a spectral exponent value of 1.50±0.04, confirming non-stationary CBL behavior in the morning while for the other regime a value of 1.00±0.06 was obtained indicating a quasi-stationary state of the CBL. Assuming that the spatio-temporal variation of the particle backscatter cross-section of the aerosols in the scattering volume is due to number density fluctuations (negligible hygroscopic growth), the particle backscatter coefficient profiles can be used to investigate boundary layer turbulence since the aerosols act as tracers. We demonstrate that with our lidar measurements, vertical profiles of variance, skewness, and kurtosis of the fluctuations of the particle backscatter coefficient can be determined. The variance spectra at different altitudes inside the quasi-steady CBL showed an f−5/3 dependency. The integral scale varied from 40 to 90 s (depending on height), which was significantly larger than the temporal resolution of the lidar data. Thus, the major part of the inertial subrange was detected and turbulent fluctuations could be resolved. For the quasi-stationary case, negative values of skewness were found inside the CBL while positive values were observed in the entrainment zone near the top of the CBL. For the case of the rapidly growing CBL, the skewness profile showed both positive and negative values even inside the CBL.

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