Clear Convective Boundary Layer Research Articles

The use of 100 m scale NWP models to understand differences between different measures of mixing height in a morning growing clear convective boundary layer over London

AbstractAn important practical problem is to understand the depth to which pollutants will be mixed in the boundary layer over cities. This is particularly important shortly after sunrise on clear days when the mixed layer is often shallow and there may be large emissions from rush‐hour traffic. The 100 m scale NWP models currently under development have the potential to represent the boundary‐layer structure over cities on neighbourhood scales for air quality and other applications. Here we describe work with Met Office Unified Model (MetUM) configurations with grid lengths of 100, 50 and 25 m to represent the morning growth of a clear convective boundary layer over London compared to observations. All three models explicitly represent the convective overturning although it is not as well resolved at 100 m and this is a particular problem early in the day. It is found that in both the observations and in the models that the depth to which aerosol is mixed up into the model from the surface is lower than the turbulence mixed‐layer depth as measured from the vertical velocity variance. We present an analysis in the 25 m model that shows that the reason for the discrepancy is that the aerosol is transported upwards from the surface by convective plumes whereas further up there is wave motion which does not transport aerosol. The signature for these two layers can be seen both in the 25 m model and in the observations. We discuss the representation of these effects in the coarser‐resolution 100 and 55 m models and also possible mechanisms for the generation of the waves.

The impact of spin‐up and resolution on the representation of a clear convective boundary layer over London in order 100 m grid‐length versions of the Met Office Unified Model

With a number of operational centres looking forward to the possibilities of “city scale” NWP and climate modelling it is important to understand the behaviour of order 100 m models over cities. A key issue is how to handle the representation of partially resolved turbulence in these models. In this article we compare the representation of a clear convective boundary‐layer case in London in 100 and 50 m grid‐length versions of the Unified Model (MetUM) with observations. Comparison of Doppler lidar observations of the vertical velocity shows that convective overturning in the boundary layer is broadly well represented in terms of its depth and magnitude. The role of model resolution was investigated by comparing a 50 m grid‐length model with the 100 m one. It is found that, although going to 50 m grid length does not greatly change many of the bulk properties (mixing height, heat flux profiles, etc.) the spatial structure of the overturning is significantly different. This is confirmed with spectral analysis which shows that the 50 m model resolves significantly more of the energetic eddies, and a length‐scale analysis that shows the 50 and 100 m models produce convective structures 2–3 times larger than observed. We conclude that, for the MetUM, model grid‐lengths of order 100 m may well be sufficient for predicting many bulk and statistical properties of convective boundary layers; however, the details of the spatial structures around convective overturning in these situations are likely to be still under‐resolved. Spin‐up artefacts emanating from the inflow boundary of the model are investigated by comparing with a smaller 100 m grid‐length domain which is more dominated by such effects. These manifest themselves as along‐wind boundary‐layer rolls which produce a less realistic comparison with the lidar observations. A stability analysis is presented in order to better understand the formation of these rolls.

The Effect of Aerosol Radiative Heating on Turbulence Statistics and Spectra in the Atmospheric Convective Boundary Layer: A Large-Eddy Simulation Study

Turbulence statistics and spectra in a radiatively heated convective boundary layer (CBL) under aerosol pollution conditions are less investigated than their counterparts in the clear CBL. In this study, a large-eddy simulation (LES) coupled with an aerosol radiative transfer model is employed to determine the impact of aerosol radiative heating on CBL turbulence statistics. One-dimensional velocity spectra and velocity–temperature cospectra are invoked to characterize the turbulence flow in the CBL with varying aerosol pollution conditions. The results show that aerosol heating makes the profiles of turbulent heat flux curvilinear, while the total (turbulent plus radiative) heat flux profile retains the linear relationship with height throughout the CBL. The horizontal and vertical velocity variances are reduced significantly throughout the radiatively heated CBL with increased aerosol optical depth (AOD). The potential temperature variance is also reduced, especially in the entrainment zone and near the surface. The velocity spectral density tends to be smaller overall, and the peak of the velocity spectra is shifted toward larger wavenumbers as AOD increases. This shift reveals that the energy-containing turbulent eddies become smaller, which is also supported by visual inspection of the vertical velocity pattern over horizontal planes. The modified CBL turbulence scales for velocity and temperature are found to be applicable for normalizing the corresponding profiles, indicating that a correction factor for aerosol radiative heating is needed for capturing the general features of the CBL structure in the presence of aerosol radiative heating.

Some Observational Evidence for Dry Soils Supporting Enhanced Relative Humidity at the Convective Boundary Layer Top

Abstract The tendency of the relative humidity at the top of a clear convective boundary layer (RHtop) is studied as an indicator of cloud formation over a semiarid region within the conceptual framework introduced by Ek and Holtslag. Typically the tendency of RHtop increases if the evaporative fraction at the land surface increases, which supports boundary layer moistening but only when boundary layer growth is limited by atmospheric factors. This regime was supported by Cabauw observations in the original study. Here, new observational evidence that the tendency of RHtop can also increase as the surface becomes more dry, as is consistent with another regime of the conceptual framework, is provided. The observations used are from the African Monsoon Multidisciplinary Analyses (AMMA) intensive observational campaign near Niamey, Niger, 20–25 June 2006. In addition, the authors evaluate whether various versions of the Weather Research and Forecasting single-column model confirm the different regimes of the conceptual framework for a typical day in the AMMA campaign. It appears that the model confirms that dryer soils can support cloud formation.

Estimating the Orientation and Spacing of Midlatitude Linear Convective Boundary Layer Features: Cloud Streets

Abstract Linear features in a clear convective boundary layer (CBL) over the North Atlantic Ocean were studied during a weak cold air outbreak using a down-looking airborne lidar. Sequential lidar profiles were placed together and color coded to provide images of aerosol and molecular scattering from below the aircraft to the ocean surface, over a 36-km segment of a flight track approximately 150 km off the coast of southern Virginia. The aircraft flew on a path approximately perpendicular to the expected orientation of cloud streets if they had formed. The lidar image clearly shows randomly sized convective cells in the CBL, grouping under the crests of a gravity wave in the stable troposphere. It is suggested that the wave develops as energetic convective cells in the CBL penetrate into the stable layer aloft and act as obstructions to the relative flow. An analytic study, published in 1965, demonstrates that vertical disturbances on the top of the CBL adjust to be in resonance with a horizontal gravity wave in the free troposphere. The results of the study along with an interpretation of the lidar images have led to the development of a simple conceptual model that is used to estimate the spacing and orientation of long linear convective features in the midlatitude CBL. In addition, the conceptual model can explain the change in cloud street patterns with increasing fetch, seen in satellite images. Comparisons with observations from this study and five other midlatitude field programs show good agreement. A suggestion for future research is presented.

Boundary Layer Characteristics over Homogeneous and Heterogeneous Surfaces Simulated by MM5 and DALES

AbstractThe multiple-single-column approach is proposed as a new concept to study the boundary layer parameterization scheme in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results are compared with the Dutch Atmospheric Large-Eddy Simulation Model (DALES). Numerical experiments were performed over homogeneous and heterogeneous surfaces under clear convective boundary layer conditions. Identical simulations using MM5 and DALES were performed, which enabled an evaluation of the MM5 boundary layer scheme with DALES results. From the experiment with a homogeneous surface, MM5 shows a slightly shallower, colder, and moister boundary layer than DALES. This result is produced by an underestimation of turbulent mixing near the surface and less-vigorous entrainment of heat and dry air in MM5. In the heterogeneous surface experiment, the domain is divided into dry and wet patches, with the result that both models produce a mesoscale circulation. However, relative to the homogeneous case, larger differences were found between the models in the representation of the boundary layer dynamics. In DALES, the surface heterogeneity influenced the turbulent motions, making the mesoscale circulation much stronger (wmax is 6 times as large) than in MM5. Because of this stronger circulation, the boundary layer height, bulk temperature, and humidity also displayed differences in time and spatial patterns. Because of the land–atmosphere coupling in MM5, the mesoscale circulation strengthened the surface flux heterogeneity. Cold and moist air advection close to the surface from the wet patch to the dry patch increased the sensible heat flux above the dry patch and thus the induced mesoscale flow.

The Use of Millimeter Doppler Radar Echoes to Estimate Vertical Air Velocities in the Fair-Weather Convective Boundary Layer

Abstract Vertical velocity characteristics of the optically clear convective boundary layer (CBL) are examined by means of profiling airborne radar data collected in the central Great Plains during the International H2O Project, May–June 2002 (IHOP 2002). Clear-air echoes are sufficiently strong for the radar, a 95-GHz cloud radar, to detect most of the CBL at a resolution of ∼30 m. Vertical radar transects across the CBL are remarkably dominated by well-defined plumes of higher reflectivity. These echo plumes occupy most of the depth of the CBL in the developing and mature stages of the CBL. Gust probe data indicate that the plumes tend to correspond with ascending motion. Evidence exists in the literature, and arises from this study, that the clear-air scatterers are mostly small insects. The close-range Doppler radar velocities, some 100 m above and below the aircraft, are compared to gust probe vertical velocities after both are corrected for aircraft motion. It is found that the radar vertical velocities have a downward bias of 0.5 ± 0.2 m s−1 on average. This bias is of the same sign as that reported in wind profiler data in the CBL, but it is larger. The difference between aircraft and radar vertical velocities becomes larger in stronger updrafts. This does not happen in cases where the scatterers are hydrometeors: hydrometeors fall out at their terminal velocity, which does not directly depend on updraft speed. The existence of the CBL echo plumes and radar “fine lines,” sustained by low-level air convergence, has long been attributed to a biotic response to updrafts. This response has been assumed to be controlled by air temperature; that is, insects subside when they encounter cold air in the upper CBL. The authors propose that the biotic response is not temperature controlled but, rather, is dependent on the vertical displacement.

Countergradient Fluxes of Conserved Variables in the Clear convective and Stratocumulus-topped Boundary Layer: The role of the Entrainment Flux

Large-eddy simulations of a clear convective boundary layer (CBL) and a stratocumulus- topped boundary layer are studied. Bottom-up and a top-down scalars were included in the simulations, and the principle of linear superposition of variables was applied to reconstruct the fields of any arbitrary conserved variable. This approach allows a systematic analysis of countergradient fluxes as a function of the flux ratio, which is defined as the ratio between the entrainment flux and the surface flux of the conserved quantity. In general, the turbulent flux of an arbitrary conserved quantity is counter to the mean vertical gradient if the heights where the vertical flux and the mean vertical gradient change sign do not coincide. The regime where the flux is countergradient is there- fore bounded by the so-called zero-flux and zero-gradient heights. Because the vertical flux changes sign only if the entrainment flux has an opposite sign to the surface flux, countergradient fluxes are predominantly found for negative flux ratios. In the CBL the flux ratio for the virtual potential tem- perature is, to a good approximation, constant, and equal to −0.2. Only if the moisture contribution to the virtual potential temperature is negligibly small will the flux ratio for the potential temperature be equal to this value. Otherwise, the flux ratio for the potential temperature can have any arbitrary (negative) value, and, as a consequence, the fluxes for the potential temperature and the virtual poten- tial temperature will be countergradient at different heights. As a practical application of the results, vertical profiles of the countergradient correction term for different entrainment-to-surface-flux ratios are discussed.

Large-Eddy Simulation: How Large is Large Enough?

The length scale evolution of various quantities in a clear convective boundary layer (CBL), a stratocumulus-topped boundary layer, and three radiatively cooled (“smoke cloud”) convective boundary layers are studied by means of large-eddy simulations on a large horizontal domain (25.6 × 25.6 km2). In the CBL the virtual potential temperature and the vertical velocity fields are dominated by horizontal scales on the order of the boundary layer depth. In contrast, the potential temperature and the specific humidity fields become gradually dominated by mesoscale fluctuations. However, at the mesoscales their effects on the virtual potential temperature fluctuations nearly compensate. It is found that mesoscale fluctuations are negligibly small only for conserved variables that have an entrainment to surface flux ratio close to −0.25, which is about the flux ratio for the buoyancy. In the CBL the moisture and potential temperature flux ratios can have values that significantly deviate from this number. The geometry of the buoyancy flux was manipulated by cooling the clear convective boundary layer from the top, in addition to a positive buoyancy flux at the surface. For these radiatively cooled cases it is found that both the vertical velocity as well as the virtual potential temperature spectra tend to broaden. The role of the buoyancy flux in their respective prognostic variance equations is discussed. It is argued that in the upper part of the clear CBL, where the mean vertical stratification is stable, vertical velocity variance and virtual potential temperature variance cannot be produced simultaneously. For the stratocumulus case, in which latent heat release effects in the cloud layer play an important role in its dynamics, the field of any quantity, except for the vertical velocity, becomes dominated by mesoscale fluctuations. In general, the location of the spectral peak of any quantity becoming constrained by the domain size should be avoided. The answer to the question of how large the LES horizontal domain size should be in order to include mesoscale fluctuations will, on the one hand, depend on the type of convection to be simulated and the kind of physical question one aims to address, and, on the other hand, the time duration of the simulation. Only if one aims to study the dynamics of a dry CBL that excludes moisture, a rather small domain size suffices. In case one aims to examine either the spatial evolution of the fields of any arbitrary conserved scalar in the CBL, or any quantity in stratocumulus clouds except for the vertical velocity, a larger domain size that allows the development of mesoscale fluctuations will be necessary.

Measurements Of Fine-Scale Structure At The Top Of Marine Stratocumulus

During the Dynamics and Chemistry of the MarineStratocumulus (DYCOMS) experiment in July–August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marinestratocumulus cloud decks that cap the mixed layer off the coast of southernCalifornia. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds – roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 × 10-3 kg m-3.An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3–6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does notcontain isotropic turbulence.

Entrainment Parameterization in Convective Boundary Layers

Abstract Various runs were performed with a large eddy simulation (LES) model to evaluate different types of entrainment parametrizations. For this evaluation, three types of boundary layers were simulated: a clear convective boundary layer (CBL), a boundary layer containing a smoke concentration, and a cloud-topped boundary layer. It is shown that the assumption that the entrainment flux equals the product of the entrainment rate and the jump over a discontinuous inversion is not valid in CBLs simulated by an LES model. A finite inversion thickness (i.e., a first-order jump model) is needed to define an entrainment flux for which this approximation of the flux is valid. This entrainment flux includes not only the buoyancy flux at the inversion, but also the surface heat flux. The parameterization of the buoyancy flux at the inversion is evaluated for different closures, as suggested in the literature (i.e., Eulerian partitioning, process partitioning, and a closure developed by Deardorff), and where need...

Microphysical and Turbulent Structure of Nocturnal Stratocumulus as Observed during ASTEX

Abstract Measurements of the microphysical and turbulence structure of stratocumulus made during the Atlantic Stratocumulus Transition Experiment are presented. The measurements made from a C-130 aircraft, belonging to the Meteorological Research Flight, on the night of 12–13 June 1992 show that the convection in the boundary layer is driven both by longwave radiative cooling at cloud top and by the surface buoyancy flux. The turbulence kinetic energy budget, velocity and temperature variance, and vertical fluxes are calculated to discover how the turbulence structure varies with height. The vertical velocity variance profile is found to resemble that of a clear convective boundary layer. The entrainment velocity and entrainment fluxes are estimated. The results show that the entrainment is very efficient in the case studied. As a result, the buoyancy production of turbulent kinetic energy in the cloud layer is considerably reduced. Horizontally averaged droplet spectra are calculated to study the relativ...

Biogenic nonmethane hydrocarbon emissions estimated from tethered balloon observations

A new technique for estimating surface fluxes of trace gases, the mixed‐layer gradient technique, is used to calculate isoprene and terpene emissions from forests. The technique is applied to tethered balloon measurements made over the Amazon forest and a pine‐oak forest in Alabama at altitudes up to 300 m. The observations were made during the dry season Amazon Boundary Layer Experiment (ABLE 2A) and the Rural Oxidants in the Southern Environment 1990 experiment (ROSE I). Results from large eddy simulations of scalar transport in the clear convective boundary layer are used to infer fluxes from the balloon profiles. Profiles from the Amazon give a mean daytime emission of 3630±1400 μg isoprene m−2 h−1, where the uncertainty represents the standard deviation of the mean of eight flux estimates. Twenty profiles from Alabama give emissions of 4470±3300 μg isoprene m−2 h−1, 1740±1060 μg alpha‐pinene m−2 h−1, and 790±560 μg beta‐pinene m−2 h−1, respectively. These results are in agreement with emissions derived from chemical budgets. The emissions may be overestimated because of uncertainty about how to incorporate the effects of the canopy on the mixed‐layer gradients. The large variability in these emission estimates is probably due to the relatively short sampling times of the balloon profiles, though spatially heterogeneous emissions may also play a role. Fluxes derived using this technique are representative of an upwind footprint of several kilometers and are independent of hydrocarbon oxidation rate and mean advection.

Composite Structure of Plumes in Stratus-topped Boundary Layers

Abstract Knowledge of convective plumes within the clear convective boundary layer (CBL) is quite advanced owing to direct measurements, tank experiments, and large-eddy simulation studies. As a result, modeling of the CBL is relatively successful. Progress for the stratus-topped boundary layer (STBL), however, is slow. This study compares the plume structure of the surface-heated CBL with that of the cloud-top-cooled STBL in the hope of extending our knowledge of the CBL to the STBL. A conditional sampling technique is applied to the STBL flow fields that are generated through large-simulations, so that the structures of typical updrafts and downdrafts may be derived. For the purpose of comparing the surface-heated CBL and the cloud-top-cooled STBL, an idealized STBL is simulated where the turbulence is maintained solely by cloud-top radiative cooling. The principal difference between the CBL and this idealized STBL lies in the origin of the plumes: primary plumes in the CBL are generated at a rigid surf...

Plume Fluxes in Clear and Cloudy Convective Boundary Layers

From results Of large-eddy simulations of the clear convective boundary layer and of a stratus-topped boundary layer, mean properties of “plumes” that consist of “updrafts” and “downdrafts” are determined. The plumes are defined locally by the sign of the vertical velocity or of moisture fluctuation or by a combination of both. As a further alternative, updraft and downdrafts in which the vertical velocity magnitude exceeds certain threshold values are considered. The first two variants divide the motion field into two streams, whereas in the other variants “environmental” air forms a separate stream. The computed mean properties are in general agreement with existing measurements. From the results we compute mean vertical fluxes assuming “top-hat profiles” and compare these with the actual fluxes. It is shown that the most uniform flux approximation is obtained if the plume structure is classified in terms of vertical velocity w. For such “w plumes,” the top-hat profiles approximate about 60% of the actual fluxes if updrafts and downdrafts are distinguished with zero threshold values just according to the sign of the vertical velocity. A higher percentage is obtained with nonzero threshold values.

A Three-Aircraft Intercomparison of Two Types of Air Motion Measurement Systems

Abstract We present procedures to evaluate air motion measurements on two or more aircraft by flying them in formation at a known lateral displacement. The analysis is applied to two formation flights involving three aircraft—the NCAR Electra, Sabreliner and King Air—in a clear convective boundary layer to compare two types of air motion sensing probes mounted on different aircraft. The lateral separation between the Electra in the center, and the other two aircraft was ≈30 m. One sensing system utilized constrained vanes and the other differential pressure measurements across ports on a nose radome to obtain the two airflow angles that are used to calculate the transverse air velocity components. Both systems used a Pitot-static pressure difference for obtaining the longitudinal velocity component. We compare differences in means and variances, spectra and cospectra, and spatial coherences between the same velocity components measured on the different aircraft. The differences are, in most cases, compara...

Plume Budgets in Clear and Cloudy Convective Boundary Layers

Abstract From results Of large-eddy simulations of the clear convective boundary layer and of a stratus-topped boundary layer, mean properties of “plumes” that consist of “updrafts” and “downdrafts” are determined. The plumes are defined locally by the sign of the vertical velocity or of moisture fluctuation or by a combination of both. As a further alternative, updraft and downdrafts in which the vertical velocity magnitude exceeds certain threshold values are considered. The first two variants divide the motion field into two streams, whereas in the other variants “environmental” air forms a separate stream. The computed mean properties are in general agreement with existing measurements. From the results we compute mean vertical fluxes assuming “top-hat profiles” and compare these with the actual fluxes. It is shown that the most uniform flux approximation is obtained if the plume structure is classified in terms of vertical velocity w. For such “w plumes,” the top-hat profiles approximate about 60% of...

Large-Eddy Simulation of a Stratus-Topped Boundary Layer. Part II: Implications for Mixed-Layer Modeling

Abstract Two sets of large-eddy simulation data were used to study some of the assumptions about the cloud-topped boundary layer (CTBL) structure which are used in mixed-layer models. The roles of buoyant production and cloud-top radiative cooling in turbulent kinetic energy generation were examined. The buoyant production in the turbulent kinetic energy (TKE) budgets was partitioned into production and consumption components by grouping the warm-rising and cold-sinking parcels and the warm-sinking and cold-rising parcels, separately. The results indicate that the ratio of the consumption part of the buoyant production to the sum of the shear production and the production part of the buoyant production is 0.15 for the clear convective mixed layer, and 0.22 for the CTBL. Almost all mixed-layer models use the experimentally (from either direct measurement or tank experiment) obtained ratio of the buoyancy flux at the top of the clear convective boundary layer to the flux at the surface for their entrainment...

Tethered‐balloon observations of the structure of small cumulus clouds

AbstractThe turbulence and microphysical structures of small cumulus clouds have been studied in some detail using three turbulence probes and a droplet spectrometer attached to the tethering cable of a large (1300 m3) balloon. the airflow in and around clouds showed some evidence for a ‘P‐shaped’ circulation in the vertical plane and rotation in the horizontal plane. Power spectra of the vertical component of air motion both in and out of cloud and in the mixed layer below cloud base are described and contrasted. Mean turbulence characteristics (i.e. averaged over a few hours) strongly resemble those from the upper regions of the clear convective boundary layer but in the vicinity of clouds the turbulence field is considerably altered. Observations of the cloud droplet field near cloud top and the rate of dissipation of turbulent kinetic energy provide evidence that the entrainment and mixing of dry air proceed essentially inhomogeneously.