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)

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.

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

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.

New Insights into Wind Shear Effects on Entrainment in Convective Boundary Layers Using Conditional Analysis

Conventional analysis has shown that strong wind shear enhances the entrainment buoyancy flux in the convective boundary layer. By conditioning the entrainment zone into regions of turbulent (i.e., strongly vortical) and nonturbulent (i.e., weakly vortical) flow, some unexpected aspects of this process are revealed. It is found that turbulent regions contribute the most to the entrainment buoyancy flux, but that as wind shear increases, the magnitude of the buoyancy flux in turbulent regions remains approximately constant, or even decreases, despite substantially stronger buoyancy fluctuations. The reason is that the correlation between buoyancy and vertical velocity fluctuations decreases with increasing wind shear, to the extent that it compensates the stronger buoyancy fluctuations. In free convection, this correlation is high because the vertical velocity is mainly determined by the buoyancy force acting in the same direction. Under strong shear conditions, buoyancy is no longer the only external source of vertical velocity fluctuations and their correlation consequently decreases. Hence, shear enhancement of the buoyancy flux in the entrainment zone is primarily due to an increase of the turbulent area fraction, rather than a change of flux inside the turbulent regions.

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

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.

Nonsingular Zero-Order Bulk Models of Sheared Convective Boundary Layers

Two zero-order bulk models (ZOMs) are developed for the velocity, buoyancy, and moisture of a cloud-free barotropic convective boundary layer (CBL) that grows into a linearly stratified atmosphere. The models differ in the entrainment closure assumption: in the first one, termed the “energetics-based model,” the negative and positive areas of the buoyancy flux are assumed to match between the model and the actual CBL; in the second one, termed the “geometric-based model,” the modeled CBL depth is assumed to match different definitions of the actual CBL depth. Parameterizations for these properties derived from direct numerical simulation (DNS) are employed as entrainment closure equations. These parameterizations, and hence the resulting models, are free from the potential singularity at finite wind strength that has been a major limitation in previous bulk models. The proposed ZOMs are verified using the DNS data. Model results show that the CBL depths obtained from the energetics-based model and previous ZOMs correspond to the height that marks the transition from the lower to the upper entrainment-zone sublayer; this reference height is few hundred meters above the height of the minimum buoyancy flux. It is also argued that ZOMs, despite their simplicity compared to higher-order models, can accurately represent CBL bulk properties when the relevant features of the actual entrainment zone are considered in the entrainment closures. The vertical structure of the actual entrainment zone, if required, can be constructed a posteriori using the available relationships between the predicted zero-order CBL depth and various definitions of the actual CBL depth.

Observations and hypotheses related to low to middle free tropospheric aerosol, water vapor and altocumulus cloud layers within convective weather regimes: a SEAC⁴RS case study

The NASA Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) project included goals related to aerosol particle life cycle in convective regimes. Using the University of Wisconsin High Spectral Resolution Lidar system at Huntsville, Alabama, USA, and the NASA DC-8 research aircraft, we investigate the altitude dependence of aerosol, water vapor and Altocumulus (Ac) properties in the free troposphere from a canonical 12 August 2013 convective storm case as a segue to a presentation of a mission-wide analysis. It stands to reason that any moisture detrainment from convection must have an associated aerosol layer. Modes of covariability between aerosol, water vapor and Ac are examined relative to the boundary layer entrainment zone, 0 ∘C level, and anvil, a region known to contain Ac clouds and a complex aerosol layering structure (Reid et al., 2017). Multiple aerosol layers in regions warmer than 0 ∘C were observed within the planetary boundary layer entrainment zone. At 0 ∘C there is a proclivity for aerosol and water vapor detrainment from storms, in association with melting level Ac shelves. Finally, at temperatures colder than 0 ∘C, weak aerosol layers were identified above Cumulus congestus tops (∼0 and ∼-20 ∘C). Stronger aerosol signals return in association with anvil outflow. In situ data suggest that detraining particles undergo aqueous-phase or heterogeneous chemical or microphysical transformations, while at the same time larger particles are being scavenged at higher altitudes leading to enhanced nucleation. We conclude by discussing hypotheses regarding links to aerosol emissions and potential indirect effects on Ac clouds.

Exploring aerosol–cloud interaction using VOCALS-REx aircraft measurements

In situ aircraft measurements obtained during the VAMOS (Variability of the American Monsoons) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field campaign are analyzed to study the aerosol–cloud interactions in the stratocumulus clouds over the southeastern Pacific Ocean (SEP), with a focus on three understudied topics (separation of aerosol effects from dynamic effects, dispersion effects, and turbulent entrainment-mixing processes). Our analysis suggests that an increase in aerosol concentration tends to simultaneously increase both cloud droplet number concentration (Nd) and relative dispersion (ε), while an increase in vertical velocity (w) often increases Nd but decreases ε. After constraining the differences of cloud dynamics, the positive correlation between ε and Nd becomes stronger, implying that perturbations of w could weaken the aerosol influence on ε and hence result in an underestimation of dispersion effect. A comparative analysis of the difference of cloud microphysical properties between the entrainment and non-entrainment zones suggests that the entrainment-mixing mechanism is predominantly extremely inhomogeneous in the stratocumulus that capped by a sharp inversion, whereby the variation in liquid water content (25 %) is similar to that of Nd (29 %) and the droplet size remains approximately constant. In entrainment zone, drier air entrained from the top induces fewer cloud droplets with respect to total in-cloud particles (0.56±0.22) than the case in the non-entrainment zone (0.73±0.13) by promoting cloud droplet evaporation. This study is helpful in reducing uncertainties in dispersion effects and entrainment mixing for stratocumulus, and the results of this study may benefit cloud parameterizations in global climate models to more accurately assess aerosol indirect effects.

Observations and hypotheses related to low to middle free tropospheric aerosol, water vapor and altocumulus cloud layers within convective weather regimes: A SEAC4RS case study

The NASA Studies of Emissions & Atmospheric Composition, Clouds & Climate Coupling by Regional Surveys (SEAC4RS) project included goals related to aerosol particle lifecycle in convective regimes. Using the University of Wisconsin High Spectral Resolution Lidar system at Huntsville, Alabama USA and the NASA DC-8 research aircraft, we investigate the altitude dependence of aerosol, water vapor and Altocumulus (Ac) properties in the free troposphere from a canonical August 12, 2013 convective storm case as a segue to a presentation of a mission wide analysis. It stands to reason that any moisture detrainment from convection must have an associated aerosol layer. Modes of covariability between aerosol, water vapor and Ac are examined relative to the boundary layer entrainment zone, 0 °C level, and anvil, a region known to contain Ac clouds and a complex aerosol layering structure (Reid et al., 2017). Multiple aerosol layers in regions warmer than 0 °C were observed within the PBL entrainment zone. At 0 °C there is a proclivity for aerosol and water vapor detrainment from storms, in association with melting level Ac shelves. Finally, at temperatures colder than 0 °C, weak aerosol layers were identified above Cumulus congestus tops (~0 °C and ~ 20 °C). Stronger aerosol signals return in association with anvil outflow. In situ data suggest that detraining particles undergo aqueous phase or heterogeneous chemical or microphysical transformations, while at the same time larger particles are being scavenged at higher altitudes leading to enhanced nucleation. We conclude by discussing hypotheses regarding links to aerosol emissions and potential indirect effects on Ac clouds.