Determination of the convective boundary-layer height with laser remote sensing

Different methods to determine the height of the convective boundary layer from lidar measurements are described and compared. The differences in either aerosol backscatter or in humidity between the boundary layer and the free troposphere are used, and either the variance or the gradient profile of the parameter under study is evaluated. On average the different methods are in very good agreement. Temporal resolution of the gradient methods is very high, on the order of seconds, but often there is an ambiguity in the choice of the “relevant” minimum in the gradient that corresponds to the boundary-layer height. This is avoided by combining the variance and the gradient methods, using the result of the variance analysis as an indicator for the region where the minimum of the gradient is sought. The combined method is useful for automated determination of the boundary-layer height at least under convective conditions. Aerosol backscatter is found to be as good an indicator for boundary-layer air as humidity, so a relatively simple backscatter lidar is sufficient for determination of the boundary-layer height.     

Amendment to “Analytical Solution for the Convectively-Mixed Atmospheric Boundary Layer”: Inclusion of Subsidence

In Ouwersloot and Vilà-Guerau de Arellano (Boundary-Layer Meteorol. doi:10.1007/s10546-013-9816-z, 2013, this issue), the analytical solutions for the boundary-layer height and scalar evolutions are derived for the convective boundary layer, based on the prognostic equations of mixed-layer slab models without taking subsidence into account. Here, we include and quantify the added effect of subsidence if the subsidence velocity scales linearly with height throughout the atmosphere. This enables analytical analyses for a wider range of observational cases. As a demonstration, the sensitivity of the boundary-layer height and the potential temperature jump to subsidence and the free tropospheric stability is graphically presented. The new relations show the importance of the temporal distribution of the surface buoyancy flux in determining the evolution if there is subsidence.     

Measurements and Parametrizations of the Atmospheric Boundary-Layer Height at Dome C,Antarctica

An experimental campaign, Study of the Atmospheric Boundary Layer Environmental at Dome C, was held during 2005 at the French-Italian station of Concordia at Dome C. Ground-based remote sensors, as well as in situ instrumentation, were used during the experimental campaign. The measurements allowed the direct estimation of the polar atmospheric boundary-layer height and the test of several parametrizations for the unstable and stable boundary layers. During the months of January and February, weak convection was observed while, during the polar night, a long-lived stable boundary layer occurred continuously. Under unstable stratification the mixing-layer height was determined using the sodar backscattered echoes and potential temperature profiles. The two estimations are highly correlated, with the mixing height ranging between 30 and 350 m. A simple prognostic one-dimensional model was used to estimate the convective mixing-layer height, with the correlation coefficient between observations and model results being 0.66. The boundary-layer height under stable conditions was estimated from radiosounding profiles as the height where the critical Richardson number is reached; values between 10 and 150 m were found. A visual inspection of potential temperature profiles was also used as further confirmation of the experimental height; the results of the two methods are in good agreement. Six parametrizations from the literature for the stable boundary-layer height were tested. Only the parametrization that considers the long-lived stable boundary layer and takes into account the interaction of the stable layer with the free atmosphere is in agreement with the observations.     

Mean Profiles of Moisture Fluxes in Snow-Filled Boundary Layers

Profiles of moisture fluxes have been examined for convective boundary layers containing clouds and snow, using data derived from aircraft measurements taken on four dates during the 1983/1984 University of Chicago lake-effect snow project. Flux profiles were derived from vertical stacks of aircraft cross-wind flight legs taken at various heights over Lake Michigan near the downwind shore. It was found that, if ice processes are taken into account, profiles of potential temperature and water content were very similar to those presented in past studies of convective boundary layers strongly heated from below. Profiles of total water content and equivalent potential temperature adjusted for ice were nearly invariant with height, except very near the top of the boundary layer, suggesting that internal boundary-layer mixing processes were rapid relative to the rates at which heat and vapour were transported into the boundary layer through entrainment and surface fluxes. Ice was found to play a significant, measurable role in boundary-layer moisture fluxes. It was estimated that 40 to 57% of the upward vapour flux was returned to the surface in the form of snow, converting about 45 to 64% of the surface latent heat flux into sensible heat in the snow-producing process. Assuming advective fluxes are relatively small (thought to be appropriate after the first few tens of km over the lake as suggested by past studies), the boundary layer was found to warm at a rate faster than could be explained by surface heat fluxes and latent heat releases alone, the remainder of the heating presumably coming from radiational processes and entrainment. Discussions of moisture phase change processes throughout the boundary layer and estimates of errors of these flux measurements are presented.     

Characteristics and Numerical Simulations of Extremely Large Atmospheric Boundary-layer Heights over an Arid Region in North-west China

Over arid regions in north-west China, the atmospheric boundary layer can be extremely high during daytime in late spring and summer. For instance, the depth of the observed convective boundary layer can exceed 3,000 m or even be up to 4,000 m at some stations. In order to characterize the atmospheric boundary-layer (ABL) conditions and to understand the mechanisms that produce such an extreme boundary-layer height, an advanced research version of the community weather research and forecasting numerical model (WRF) is employed to simulate observed extreme boundary-layer heights in May 2000. The ability of the WRF model in simulating the atmospheric boundary layer over arid areas is evaluated. Several key parameters that contribute to the extremely deep boundary layer are identified through sensitivity experiments, and it is found that the WRF model is able to capture characteristics of the observed deep atmospheric boundary layer. Results demonstrate the influence of soil moisture and surface albedo on the simulation of the extremely deep boundary layer. In addition, the choice of land-surface model and forecast lead times also plays a role in the accurate numerical simulation of the ABL height.     

Statistics of shallow convection on Mars based on large-eddy simulations. Part 1: shearless conditions

Results of large-eddy simulations of shallow, quasi-steady, shear-less convection in the Martian boundary layer are presented and discussed. In the considered three cases, turbulence is forced by the radiative flux divergence, prescribed as given functions of height, and the strength of the surface heat flux. It is constrained by the temperature inversion at the boundary-layer top. The resulting convective boundary layer exhibits horizontal cellular structures. The presence of radiative heating causes dimensionless statistics of turbulence to depend on the parameter F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer.     

Impact of the Diurnal Cycle of the Atmospheric Boundary Layer on Wind-Turbine Wakes: A Numerical Modelling Study

The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation. Idealized diurnal cycle simulations of the atmospheric boundary layer are performed with the geophysical flow solver EULAG over both homogeneous and heterogeneous terrain. Under homogeneous conditions, the diurnal cycle significantly affects the low-level wind shear and atmospheric turbulence. A strong vertical wind shear and veering with height occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas atmospheric turbulence is much larger in the convective boundary layer and in the evening boundary layer. The increased shear under heterogeneous conditions changes these wind characteristics, counteracting the formation of the night-time Ekman spiral. The convective, stable, evening, and morning regimes of the atmospheric boundary layer over a homogeneous surface as well as the convective and stable regimes over a heterogeneous surface are used to study the flow in a wind-turbine wake. Synchronized turbulent inflow data from the idealized atmospheric boundary-layer simulations with periodic horizontal boundary conditions are applied to the wind-turbine simulations with open streamwise boundary conditions. The resulting wake is strongly influenced by the stability of the atmosphere. In both cases, the flow in the wake recovers more rapidly under convective conditions during the day than under stable conditions at night. The simulated wakes produced for the night-time situation completely differ between heterogeneous and homogeneous surface conditions. The wake characteristics of the transitional periods are influenced by the flow regime prior to the transition. Furthermore, there are different wake deflections over the height of the rotor, which reflect the incoming wind direction.     

Approximate Analytical Solution to Diurnal Atmospheric Boundary-Layer Growth Under Well-Watered Conditions

Simplified numerical models of the atmospheric boundary layer (ABL) are useful both for understanding the underlying dynamics and potentially providing parsimonious modelling approaches for inclusion in larger models. Herein the governing equations of a simplified slab model of the uniformly mixed, purely convective, diurnal ABL are shown to allow immediate solutions for the potential temperature and specific humidity as functions of the ABL height and net radiation when expressed in integral form. By employing a linearized saturation vapour relation, the height of the mixed layer is shown to obey a non-linear ordinary differential equation with quadratic dependence on ABL height. A perturbation solution provides general analytical approximations, of which the leading term is shown to represent the contribution under equilibrium evaporation. These solutions allow the diurnal evolution of the height, potential temperature, and specific humidity (i.e., also vapour pressure deficit) of the mixed layer to be expressed analytically for arbitrary radiative forcing functions.     

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.     

A model for the height of the internal boundary layer over an area with an irregular coastline

A model for the time and space variation of the internal boundary-layer height over a land area with an irregular coastline is presented. It is based on the analytical model of the boundary-layer height proposed by Gryning and Batchvarova (1990) and Batchvarova and Gryning (1991), The model accounts for the temperature jump and the mean vertical air motion at the top of the internal boundary-layer. Four cases from experiments in Nanticoke and Vancouver are used for model validation. The agreement between the calculated and measured internal boundary layer height at the observational sites is fairly good. The input information for the model consist of wind speed and direction, friction velocity and kinematic heat flux in time and space for the area, and the potential temperature gradient and the mean vertical air motion above the internal boundary layer. For the experiments used in the validation the effect of subsidence is relatively important in the afternoon under low wind speed high pressure conditions, lowering the height of the internal boundary layer by up to 10%, and it is negligible in the morning hours. The effect of the mixing height over the sea is found to be negligible.     

Improving Non-local Parameterization of the Convective Boundary Layer

We propose improvements in the “non-local” parameterization scheme of the convective boundary layer. The countergradient terms for components of the momentum fluxes are introduced in a form analogous to those for other scalars. The scheme also includes explicit expressions for entrainment fluxes of momentum, temperature, and humidity. A simplified procedure for calculating the boundary-layer height is proposed, consisting of two steps: the evaluation of the convection level, followed by the assessment of the depth of the interfacial layer.     

Airborne measurements of turbulent trace gas fluxes and analysis of eddy structure in the convective boundary layer over complex terrain

Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NO_x transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O₃ at the surface.The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NO_x loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies.     

Obtaining Potential Virtual Temperature Profiles,Entrainment Fluxes,and Spectra from Mini Unmanned Aerial Vehicle Data

We present a simple but effective small unmanned aerial vehicle design that is able to make high-resolution temperature and humidity measurements of the atmospheric boundary layer. The air model used is an adapted commercial design, and is able to carry all the instrumentation (barometer, temperature and humidity sensor, and datalogger) required for such measurements. It is fitted with an autopilot that controls the plane’s ascent and descent in a spiral to 1800 m above ground. We describe the results obtained on three different days when the plane, called Aerolemma-3, flew continuously throughout the day. Surface measurements of the sensible virtual heat flux made simultaneously allowed the calculation of all standard convective turbulence scales for the boundary layer, as well as a rigorous test of existing models for the entrainment flux at the top of the boundary layer, and for its growth. A novel approach to calculate the entrainment flux from the top-down, bottom-up model of Wynagaard and Brost is used. We also calculated temperature fluctuations by means of a spectral high-pass filter, and calculated their spectra. Although the time series are small, tapering proved ineffective in this case. The spectra from the untapered series displayed a consistent −5/3 behaviour, and from them it was possible to calculate a dimensionless dissipation function, which exhibited the expected similarity behaviour against boundary-layer bulk stability. The simplicity, ease of use and economy of such small aircraft make us optimistic about their usefulness in boundary-layer research.     

The structure of the convective atmospheric boundary layer as revealed by lidar and Doppler radars

Results on the structure of the convective atmospheric boundary layer based on the analyses of data from the instrumented NSSL-KTVY tower, airborne Doppler lidar, and ground-based Doppler radars are presented. The vertically averaged wind over the boundary layer was found to be insensitive to baroclinicity, supporting the hypothesis of Arya and Wyngaard (1975). The computed momentum flux profiles were affected by baroclinicity. Horizontal wind spectra from lidar, radar, and tower data compared well with each other both in shape and magnitude. A consistent peak found near 4 km in all the computed spectra might have been caused by horizontally symmetric cells with horizontal wavelength 4 times the boundary-layer height as shown in Kuettner (1971) for the case of weak wind shear.     

Boundary-layer effects on mountain waves: a new look at some historical studies

Early studies of mountain waves reported various results that have rarely been investigated since. These include: large-amplitude mountain waves above an unstable boundary layer much higher than the mountains; a repeated downwind drift and upwind jump of mountain waves; and larger vertical wind magnitude near sunrise and/or sunset. These are investigated using over 3,000 radiosondes and meso-strato-troposphere (MST) radar. Superadiabatic temperature gradients are found beneath mountain waves, explainable by convection which appears to raise the mountain-wave launching height. Movement of mountain-wave patterns is studied by a new method using height–time vertical wind data. A swaying motion of mountain waves, with period of a few minutes, appears to be equally upwind and downwind, rather than asymmetric at the heights measurable. Also, vertical wind shows no change in mean, variance or extreme values near sunrise and sunset, despite the expected diurnal changes of boundary-layer structure. An explanation for differences between MST radar and other measurements and models of mountain waves is suggested in terms of more than one variety of mountain wave. Type 1 has stable air near the ground; type 2 is above a convective/turbulent boundary layer of significant height as compared to the mountains.     

Numerical simulation of cold easterly circulations over the Canadian Western Plains using a mesoscale boundary-layer model

Arctic outbreaks over the Canadian Western Plains during the late spring period frequently take the form of a cold east-northeasterly flow over a warmer, sloping surface. A mesoscale numerical model is developed in an attempt to simulate such circulations. Following Lavoie (1972) the atmospheric structure of the cold air mass is represented by three layers: a constant flux layer in contact with the earth's surface, a well-mixed planetary boundary layer capped by an inversion, and a deep stratum of overlying stable air. Averaging the set of governing primitive equations through the depth of the mixed layer yields predictive equations for the horizontal wind components, potential temperature, specific humidity, and the height of the inversion. Time-dependent calculations are limited to this layer by parameterizing the interactions between the mixed layer and both the underlying and overlying layers. Precipitation from limited convective clouds, and latent heat within the layer are included in terms of mesoscale variables.A 47.6-km by 47.6-km grid mesh of 1369 points covering the Canadian Prairie Provinces is used to represent the variables. The governing equations are solved numerically with terrain influences, surface roughness, temperature variations, and moisture fluxes allowed to perturb the mixed layer from its initial conditions until resultant mesoscale boundary-layer weather patterns evolve.The mean spring topographic precipitation pattern is successfully reproduced by the simulated late spring upslope flow with limited convective precipitation. Mesoscale planetary boundary-layer weather patterns appear to exert a dominant control over the location and intensity of perturbations in the spring precipitation pattern. The elimination of surface heating significantly reduces the area and intensity of precipitation. A case study based on observed initial conditions showed that the model could reproduce a persistent limited convective precipitation pattern maintained by upslope flow and that a low-level trough exerts a marked influence on the location and the intensity of the precipitation.     

An Auxiliary Tool to Determine the Height of the Boundary Layer

Results from radiosoundings, performed both over land and over sea, show that the ascent rate of a radiosounding balloon, the vertical velocity of the balloon, can be used to determine the height of the boundary layer. In many cases the balloon has a higher ascent rate in the boundary layer and a lower, less variable, ascent rate above. The decrease in ascending velocity appears as a jump at the top of the boundary layer. Two examples of potential temperature profiles for unstable stratification and one profile for stable conditions are shown with the corresponding ascent rates. A comparison between the boundary-layer height determined from potential temperature profiles and from ascent rates is presented for a larger dataset. The different ascent rates of the balloon in the boundary layer and above can be explained by a decrease in drag on the balloon in combination with a lowering of the critical Reynolds number in the boundary layer caused by turbulence. Hence, by simply logging the time from release of a radiosonde, it is possible to obtain additional information that can be used to estimate the height of both the unstable and stable boundary layers.     

The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding

We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.     

A Simple Model Of The Convective Internal Boundary Layer And Its Application To Surface Heat Flux Estimates Within Polynyas

A simple model of the convective (thermal) internalboundary layer has been developed for climatologicalstudies of air-sea-ice interaction, where in situobservations are scarce and first-order estimates ofsurface heat fluxes are required. It is amixed-layer slab model, based on a steady-statesolution of the conservation of potentialtemperature equation, assuming a balance betweenadvection and turbulent heat-flux convergence. Boththe potential temperature and the surface heat fluxare allowed to vary with fetch, so the subsequentboundary-layer modification alters the fluxconvergence and thus the boundary-layer growth rate.For simplicity, microphysical and radiativeprocesses are neglected.The model is validated using several case studies.For a clear-sky cold-air outbreak over a coastalpolynya the observed boundary-layer heights,mixed-layer potential temperatures and surface heatfluxes are all well reproduced. In other cases,where clouds are present, the model still capturesmost of the observed boundary-layer modification,although there are increasing discrepancies withfetch, due to the neglected microphysical andradiative processes. The application of the model toclimatological studies of air-sea interaction withincoastal polynyas is discussed.