Effects of shear in the convective boundary layer: analysis of the turbulent kinetic energy budget

Effects of convective and mechanical turbulence at the entrainment zone are studied through the use of systematic Large-Eddy Simulation (LES) experiments. Five LES experiments with different shear characteristics in the quasi-steady barotropic boundary layer were conducted by increasing the value of the constant geostrophic wind by 5 m s^-1 until the geostrophic wind was equal to 20 m s^-1. The main result of this sensitivity analysis is that the convective boundary layer deepens with increasing wind speed due to the enhancement of the entrainment heat flux by the presence of shear. Regarding the evolution of the turbulence kinetic energy (TKE) budget for the studied cases, the following conclusions are drawn: (i) dissipation increases with shear, (ii) the transport and pressure terms decrease with increasing shear and can become a destruction term at the entrainment zone, and (iii) the time tendency of TKE remains small in all analyzed cases. Convective and local scaling arguments are applied to parameterize the TKE budget terms. Depending on the physical properties of each TKE budget contribution, two types of scaling parameters have been identified. For the processes influenced by mixed-layer properties, boundary layer depth and convective velocity have been used as scaling variables. On the contrary, if the physical processes are restricted to the entrainment zone, the inversion layer depth, the modulus of the horizontal velocity jump and the momentum fluxes at the inversion appear to be the natural choices for scaling these processes. A good fit of the TKE budget terms is obtained with the scaling, especially for shear contribution.     

A Model Study of the Strong and Weak Wind,Stably Stratified Nocturnal Boundary Layer: Influence of Gentle Slopes

With the exception of intermittency and waves, a brief review of the observed and modeled mean structure of the nocturnal boundary layer (NBL) is presented. The effect of gentle slopes on strong and weak wind NBL was investigated here using a one-dimensional model, with a simple correction term to account for the slope effects, identical to the one used by Brost and Wyngaard (1978). The study indicates that the wind profiles, temperature profiles and surface layer turbulence characteristics are extremely sensitive to the imposed geostrophic wind when small slopes are present especially for light winds. This is due to the complex interaction between the buoyancy driven slope flow and the imposed geostrophic wind that in turn influence the shear generation of turbulence. Finally, the current issues in the modeling of weak wind boundary layer are discussed.     

Stable-boundary-layer regimes from the perspective of the low-level jet

This paper reviews results from two field studies of the nocturnal stable atmospheric boundary layer (SBL) over the Great Plains of the United States. Data from a scanning remote-sensing system, a High-Resolution Doppler Lidar (HRDL), provided measurements of mean and turbulent wind components at high spatial and temporal resolution through the lowest 500–1000 m of the atmosphere. This data set has allowed the characteristics of the low-level jet (LLJ) maximum (speed, height, direction) to be documented through entire nights. LLJs form after sunset and produce strong shear in the layer below the LLJ maximum or nose, which is a source of turbulence and mixing in the SBL. Simultaneous HRDL measurements of turbulence quantities related to turbulence kinetic energy (TKE) has allowed the turbulence in the subjet layer to be related to LLJ properties. Turbulence structure was found to be a function of the bulk stability of the subjet layer. For the strong-LLJ (> 15 m s⁻¹), weakly stable cases the strength of the turbulence is proportional to the strength of the LLJ. For these cases with nearly continuous turbulence in the subjet layer, low-level jet scaling, in which lengths are scaled by the LLJ height and velocity variables are scaled by the LLJ speed, was found to be appropriate. For the weak-wind (< 5 m s⁻¹ in the lowest 200 m), very stable boundary layer (vSBL), the boundary layer was found to be very shallow (sometimes < 10 m deep), and turbulent fluxes between the earth’s surface and the atmosphere were found to be essentially shut down. For more intermediate wind speeds and stabilities, the SBL shows varying degrees of intermittency due to various mechanisms, including shearinstability and other gravity waves, density currents, and other mesoscale disturbances.     

Two-dimensional simulations of katabatic layers observed during the GIMEX experiment

The hydrostatic model SALSA is used to simulate a particular event observed during the Greenland Ice Margin EXperiment “GIMEX” (on July 12th, 1991). The time evolution of the large-scale flow was incorporated in the model through time dependent boundary conditions which were updated using the closest upwind sounding. A turbulent scheme for the stable boundary layer and an appropriate parametrization of the surface fluxes implemented in the same model, are used for this study. The simulation results are discussed and compared to the available observations. The computed turbulent fluxes are correctly estimated. The model predicts a mixing zone of about 1500 m high which is in good agreement with tundra site observations. Over the ice cap, the katabatic layer is correctly simulated by the model. Its height of 80–300 m is well estimated. The comparison between the simulation and observations taken at ice cap sites is reasonably valid. The ablation computed along the ice cap corresponds well to the values reconstructed of observations at sites 4 and 9. Finally, a sensibility study to a specified westward geostrophic wind (2 ms⁻¹) shows that the consideration of this latter improves the simulated tundra wind evolution.     

Mathematical Modeling of Diffusion and Transport of Pollutants in the Atmospheric Boundary Layer

— The process of dispersion of air pollutants, in a broad sense, can be considered as the net outcome of various mechanisms involved in the transport of air pollutants from the source to the receptor. The major mechanisms are: (1) advection of pollutants by mean air motion, (2) mixing of pollutants by atmospheric turbulence and (3) mass diffusion due to concentration gradients. In addition, the physical and chemical nature of the effluent, the location of the stack and the nature of the terrain downwind from the stack, effect the dispersion of the pollutants. Various physical and mathematical aspects related to the transport and diffusion of air pollutants in the atmospheric boundary layer are discussed here. Further, some aspects of dispersion in a weak wind stable boundary layer are described. Finally, the current issues in the modeling of weak wind boundary layer are illustrated.     

Summertime Characteristics of the Atmospheric Boundary Layer and Relationships to Ozone Levels over the Eastern United States

— This paper examines the spatial and temporal distributions of the mixing height, ventilation coefficient (defined as the product of mixing height and surface wind speed), and cloud cover over the eastern United States during the summer of 1995, using the high-resolution meteorological data generated by MM5 (Version 1), a mesoscale model widely used in air quality studies. The ability of MM5 to simulate the key temporal and spatial features embedded in the time series of observations of temperature, wind speed, and moisture is assessed using spectral decomposition methods. Also, mixing heights estimated from the MM5 outputs are compared with those derived from observations at a few locations where data with high temporal resolution are available in the Northeast. In addition, the uncertainties associated with the estimation of the evolution of the boundary layer during the morning time are examined. The results indicate that nighttime mixing heights averaged <200?m, rising to 1 km by 10 EST, and to about 2.5?km in the afternoon. Ventilation coefficients followed a similar diurnal pattern, increasing from 500?m²/s at night?to 15,000?m²/s in the afternoon; the increase due to the growing mixing height and increasing surface wind speeds. Spatial variability of these parameters was relatively small (coefficient of variation=0.25) at?night and in the afternoon when conditions were quasi-stationary, but increased (to 0.5) during morning?and evening hours when mixing heights and wind speeds were changing rapidly. Analyses of surface ozone observations from about 400 sites throughout the eastern United States indicate that days with numerous stations reporting surface ozone concentrations in excess of 80 ppb (i.e., “high ozone” days) generally had less daytime cloud cover, lower surface wind speeds, higher mixing heights, and lower ventilation coefficients than did comparable “low ozone” days. Such meteorological features are consistent with a synoptic anticyclone centered over the mid-south region (Kentucky, Tennessee). Low ozone days were characterized by more disturbed weather conditions (low pressure systems, fronts, greater cloud cover, and precipitation events). Ozone observations at two elevated platforms (～400?m agl) in Garner, NC, and Chicago, IL, indicated that ozone concentrations aloft were about 40% larger on “high ozone” days than on “low ozone” days. On average, high levels of ozone persist aloft for about 2 to 3 days. Strong vertical mixing in the daytime can bring this pool of upper-level ozone downward to augment surface ozone production. Since ozone can be transported downwind several hundred kilometers from its source region over this time scale, depending on upper-level winds, effective ozone control strategies must take into consideration spatial scales ranging from local to regional, and time scales of the order of several days.     

Observation of Double-mixing Line Structure in the Convective Boundary Layer During the Summer Monsoon Season

v--vThe special aerological observations carried out as a part of Land Surface Processes Experiment (LASPEX) were used to investigate the thermodynamic structure of the convective boundary layer during the summer monsoon of 1997. The analysis suggested that the convective boundary layer top was found at 700 hPa which was associated with Š_e minimum and Š_es maximum values. Double-mixing line structure was noticed in the conserved variable diagrams which was possibly attributed to the radiative warming/evaporation of falling precipitation.     

Planetary Boundary Layer Flows in the Atmosphere and the Ocean at the Equator

—The boundary layer flows created by the frictional dissipation of the wind speed at the surface in the atmosphere and by surface wind stress in the ocean at the equator and in the equatorial region, are obtained by taking the influence of the surface friction on the zonal velocity as being balanced by vertical transport for the long-term mean flow and by a corresponding time variation for time-dependent flow fields. Solutions are expressed in terms of the velocities in zonal and vertical directions and the divergence of the horizontal current in the two media. It is found that under the ever present easterly flow in the lower atmosphere, the boundary layer flow in the atmosphere is convergence and ascending motion in the lower troposphere, and divergence at the surface and uplift in ocean, and in reverse directions for the westerly flow. Similar results are obtained for time-dependent wind fields and they give way to the steady asymptotic solutions when the period of the variation exceeds 10 months.     

Boundary layer characteristics and turbulent exchange mechanisms in highly complex terrain

The Mesoscale Alpine Programme’s Riviera project investigated the turbulence structure and related exchange processes in an Alpine valley by combining a detailed experimental campaign with high-resolution numerical modelling. The present contribution reviews published material on the Riviera Valley’s boundary layer structure and discusses new material on the near-surface turbulence structure. The general conclusion of the project is that despite the large spatial variability of turbulence characteristics and the crucial influence of topography at all scales, the physical processes can accurately be understood and modelled. Nevertheless, many of the “text book characteristics” like the interaction between the valley and slope wind systems or the erosion of the nocturnal valley inversion need reconsideration, at least for small non-ideal valleys like the Riviera Valley. The project has identified new areas of research such as post-processing methods for turbulence variables in complex terrain and new approaches for the surface energy balance when advection is non-negligible. The exchange of moisture and heat between the valley atmosphere and the free troposphere is dominated by local “secondary” circulations due to the curvature of the valley axis. Because many curved valleys exist, and operational models still have rather poor resolution, parameterization of these processes may be required.     

On the Behavior of the Stable Boundary Layer and the Role of Initial Conditions

Previous studies of the stable atmospheric boundary layer using techniques of nonlinear dynamical systems (MCNIDER et al., 1995) have shown that the equations support multiple solutions in certain parameter spaces. When geostrophic speed is used as a bifurcation parameter, two stable equilibria are found—a warm solution corresponding to the high-wind regime where the surface layer of the atmosphere stays coupled to the outer layer, and a cold solution corresponding to the low-wind, decoupled case. Between the stable equilibria is an unstable region where multiple solutions exist. The bifurcation diagram is a classic S shape with the foldback region showing the multiple solutions. These studies were carried out using a simple two-layer model of the atmosphere with a fairly complete surface energy budget. This allowed the dynamical analysis to be carried out on a coupled set of four ordinary differential equations. The present paper extends this work by examining additional bifurcation parameters and, more importantly, analyzing a set of partial differential equations with full vertical dependence. Simple mathematical representations of classical problems in dynamical analysis often exhibit interesting behavior, such as multiple solutions, that is not retained in the behavior of more complete representations. In the present case the S-shaped bifurcation diagram remains with only slight variations from the two-layer model. For the parameter space in the foldback region, the evolution of the boundary layer may be dramatically affected by the initial conditions at sunset. An eigenvalue analysis carried out to determine whether the system might support pure limit-cycle behavior showed that purely complex eigenvalues are not found. Thus, any cyclic behavior is likely to be transient.     

Early spring turbulent mixing in an ice-covered Arctic fjord during transition to melting

Observations are presented of currents, hydrography and turbulence in a jet-type tidally forced fjord in Svalbard. The fjord was ice covered at the time of the experiment in early spring 2004. Turbulence measurements were conducted by both moored instruments within the uppermost 5 m below the ice and a microstructure profiler covering 3–60 m at 75 m depth. Tidal choking at the mouth of the fjord induces a tidal jet advecting relatively warmer water past the measurement site and dominating the variability in hydrography. While there was no strong correlation with the observed hydrography or mixing and the phase of the semidiurnal tidal cycle, the mean structure in dissipation of turbulent kinetic energy, work done under the ice and the mixing in the water column correlated with the current when conditionally sampled for tidal jet events. Observed levels of dissipation of turbulent kinetic energy per unit mass, 1.1×10⁻⁷ W kg⁻¹, and eddy diffusivity, 7.3×10⁻⁴ m² s⁻¹, were comparable to direct measurements at other coastal sites and shelves with rough topography and strong forcing. During spring tides, an average upward heat flux of 5 W m⁻² in the under-ice boundary layer was observed. Instantaneous (1 h averaged) large heat flux events were correlated with periods of large inflow, hence elevated heat fluxes were associated with the tidal jet and its heat content. Vertical heat fluxes are derived from shear-probe measurements by employing a novel model for eddy diffusivity [Shih et al., 2005. Parameterization of turbulent fluxes and scales using homogeneous sheared stably stratified turbulence simulations. Journal of Fluid Mechanics 525, 193–214]. When compared to the direct heat flux measurements using the eddy correlation method at 5 m below the ice, the upper 4–6 m averaged heat flux estimates from the microstructure profiler agreed with the direct measurements to within 10%. During the experiment water column was stably, but weakly, stratified. Destabilizing buoyancy fluxes recorded close to the ice were absent at 5 m below the ice, and overall, turbulence production was dominated by shear. A scaling for dissipation employing production by both stress and buoyancy [Lombardo and Gregg, 1989. Similarity scaling of viscous and thermal dissipation in a convecting boundary layer. Journal of Geophysical Research 94, 6273–6284] was found to be appropriate for the under-ice boundary layer.     

Influence of the Planetary Boundary Layer Physics on Medium-range Prediction of Monsoon over India

—The present study emphasizes the importance of proper representation of boundary layer physics in a general circulation model. The Turbulent Kinetic Energy (TKE) closure scheme incorpo rates important processes of the Planetary Boundary Layer (PBL) compared to a simplistic first-order closure model. Hence the model which has the TKE closure scheme is capable of simulating important weather systems associated with summer monsoon, such as monsoon depressions and lows that form over the Indian subcontinent quite well compared to the first-order closure model. The present study indicates better performance of the global model with the TKE scheme in the prediction of the monsoon circulation, including the tracks of the depressions over the Indian subcontinent. Medium-range weather prediction has also improved with the use of the TKE closure. However further studies are necessary to improve the forecast, with emphasis on boundary layer processes.     

Current and turbulence measurements at the FINO1 offshore wind energy site: analysis using 5-beam ADCPs

We report concurrent measurements of ocean currents and turbulence at two sites in the North Sea, one site at upwind of the FINO1 platform and the other 200-m downwind of the Alpha Ventus wind farm. At each site, mean currents, Reynolds stresses, turbulence intensity and production of turbulent kinetic energy are obtained from two bottom-mounted 5-beam Nortek Signature1000s, high-frequency Doppler current profiler, at a water depth of approximately 30 m. Measurements from the two sites are compared to statistically identify the effects of wind farm and waves on ocean current variability and the turbulent structure in the water column. Profiles of Reynolds stresses are found to be sensible to both environmental forcing and the wind farm wake-induced distortions in both boundary layers near the surface and the seabed. Production of turbulent kinetic energy and turbulence intensity exhibit approximately similar, but less pronounced, patterns in the presence of farm wake effects.     

Summary of a 4-Year Fog Field Study in Northern Nanjing, Part 1: Fog Boundary Layer

Comprehensive fog field observations were conducted during the winters of 2006–2009 at the Nanjing University of Information Science and Technology to study the macro and micro-physical structures and the physical–chemical processes of dense fogs in the area. The observations included features of the fog boundary layer, characteristics of fog water, the particle spectrum, the chemical composition of atmospheric aerosols, radiation and heat components, turbulence, meteorological elements (air temperature, pressure, wind speed, wind direction), and environmental monitoring. The fogs observed were divided into four types: radiation fog, advection–radiation fog, advection fog, and precipitation fog, according to the mechanisms and primary factors of the fog processes. Fog boundary-layer structures of different types and their corresponding characteristics were then studied. Fog boundary-layer features, temperature structures, wind fields, and fog maintenance are discussed. The results show that radiation fog had remarkable diurnal variation and formed mostly at sunset or midnight, and lifted after sunrise or at noon, and that advection–radiation fog and advection fog were of very long duration. Extremely dense fogs occurred only in radiation-related cases. Inversion in radiation fog was short-lived, disappearing 1 or 2 hours after sunrise or at noon, faster than that in advection–radiation fog. When wind direction reversed from easterly to westerly or from southerly to northerly, the fog became an extremely dense fog. Low-level jet at times impeded fog development, whereas at other times it encouraged fog continuance. The deep inversion was merely an essential condition for a thick fog layer; sufficient vapor supply was advantageous to the formation and maintenance of a deep fog layer.     

Impact of Annular Solar Eclipse of 15 January 2010 on the Atmospheric Boundary Layer Characteristics Over Thumba: A Case Study

On 15 January 2010, Thumba (8.5°N, 76.9°E) witnessed one of the longest known noontime annular solar eclipses (ASEs) spanning a period of about 7 min, centered at 1314 hours local time. In this research article, we present a case study on the behaviour of the atmospheric boundary layer characteristics and its vertical structure in response to this rare celestial event by making use of a suite of different in-situ instruments. During the peak period of the ASE, the incoming solar irradiance was dimmed by about 87% of its normal values, resulting in a significant reduction in the magnitudes of turbulent kinetic energy and surface-layer turbulent fluxes of heat and momentum. The intensity and vertical thickness of the sea/land breeze circulation cell over the study domain also weakened. However, the mixed layer heights determined from balloon-borne GPS Radiosonde did not show any appreciable changes. Analysis of vertical profiles of thermodynamic parameters in association with the wind direction during ASE indicated the formation of a double mixed layer between 700 and 1500 m and is attributed to horizontal advection of a different airmass at those altitudes.     

Turbulence structure in the upper ocean: a comparative study of observations and modeling

Observations of turbulent dissipation rates measured by two independent instruments are compared with numerical model runs to investigate the injection of turbulence generated by sea surface gravity waves. The near-surface observations are made by a moored autonomous instrument, fixed at approximately 8 m below the sea surface. The instrument is equipped with shear probes, a high-resolution pressure sensor, and an inertial motion package to measure time series of dissipation rate and nondirectional surface wave energy spectrum. A free-falling profiler is used additionally to collect vertical microstructure profiles in the upper ocean. For the model simulations, we use a one-dimensional mixed layer model based on a k–ε type second moment turbulence closure, which is modified to include the effects of wave breaking and Langmuir cells. The dissipation rates obtained using the modified k–ε model are elevated near the sea surface and in the upper water column, consistent with the measurements, mainly as a result of wave breaking at the surface, and energy drawn from wave field to the mean flow by Stokes drift. The agreement between observed and simulated turbulent quantities is fairly good, especially when the Stokes production is taken into account.     

Relationship between thermally forced surface wind and sea surface temperature gradient

An important part of the influence of the oceans on the atmosphere is through direct radiation, sensible heat flux and release of latent heat of evaporation, whereby all of these processes are directly related to the surface temperature of the oceans. A main effect of the atmosphere on the oceans is through momentum exchange at the air-ocean interface, and this process is directly related to the surface wind stress. The sea surface temperature (SST) and the surface wind stress are the two important components in the air-ocean system. If SST is given, a thermally forced boundary layer atmospheric circulation can be simulated. On the other hand, if the surface wind stress is given, the wind-driven ocean waves and ocean currents can be computed.The relationship between SST and surface wind is a coupling of the atmosphere and the oceans. It changes a one-way effect (ocean mechanically driven by atmosphere, or atmosphere thermally forced by oceans) into two-way air-sea interactions. Through this coupling the SST distribution, being an output from an ocean model, leads to the thermally forced surface winds, which feeds back into the ocean model as an additional forcing.Based on Kuo's planetary boundary layer model a linear algebraic equation is established to link the SST gradient with the thermally forced surface wind. The surface wind blows across the isotherms from cold to warm region with some deflection angle to the right (left) in the Northern (Southern) Hemisphere. Results from this study show that the atmospheric stratification reduces both the speed and the deflection angle of the thermally forced wind, however, the Coriolis' effect increases the wind speed in stable atmosphere (Ri>10^–4) and increases the deflection angle.     

Numerical Simulation of the Response of the Ocean Surface Layer to Precipitation

Numerical experiments were conducted to investigate the ocean's response to the precipitation. A squall line observed in TOGA COARE was simulated. The simulation reproduced some of the observed ocean responses to the precipitation, such as the formation of a fresh water layer, surface cooling and the variation of upper layer turbulent mixing. The precipitation-induced fresh layer can cause the vertical turbulent diffusivities to decrease from the surface to a depth of about 11–13 meters within a few hours. After the rainfall, the turbulence increases near the surface of the ocean due to the combined effect of increased shear and wind forcing, but decreases with depth due to the development of a stable layer. The main reason for the turbulence variation is the decrease in the vertical turbulence flux below the surface fresh layer because of increased static stability. Sensitivity experiments reveal that the sea-surface temperature increases faster after rainfall due to the formation of a shallow fresh water layer near the surface.     

Direct evaluation of refractive-index structure functions from large-eddy simulation output for atmospheric convective boundary layers

Fluctuations of the refractive index associated with atmospheric turbulence affect the propagation of electromagnetic and acoustic waves in the atmosphere. In the reported study, the effects of turbulence on wave propagation in the atmospheric convective boundary layer (CBL) are considered in terms of the second-order refractive index structure function and related to its structure-function parameter C _n ². Two structure-function evaluation methods are compared. The direct evaluation method involves calculating the refractive index at each point in the simulation domain with subsequent calculation of the structure function. The second method is based on a parameterized linear relationship between the refractive-index structure function and temperature/humidity structure functions. For each evaluation method, vertical profiles of C _n ² computed for separations along the three coordinate directions collapse together over a significant portion of the CBL. Near-surface divergence of C _n ² values along the horizontal directions was noted and attributed to the influence of surface wind shear on the turbulent fluctuations of temperature and humidity. The behavior of C _n ² near the surface was shown to agree favorably with similarity-theory predictions.