Three-dimensional numerical study of turbulence in an entraining mixed layer

The mean structure calculated by a three-dimensional numerical model of a heated planetary boundary layer, in simulation of DAY 33 of the Australian Wangara data, has been previously described. The present study supplements it by describing properties of the calculated turbulence.A major finding is the importance of entrainment upon turbulence statistics relating to specific humidity, relative to those for potential temperature. The variances, skewness and spectra of velocity, temperature and humidity are presented, as are budget equations for kinetic energy, temperature and humidity variances and heat/moisture fluxes. These are interpreted with regard to the relative importance of the surface flux vs the flux due to entrainment at the top of the mixed layer, and in regard to the structure which would occur if the entrainment were to vanish.The Rotte-type closure assumption is tested for the correlation between the pressure fluctuation and the vertical gradient of vertical velocity, potential temperature, or specific humidity, and found to be qualitatively correct except near the top of the mixed layer.NCAR is sponsored by the National Science Foundation (U.S.A.).     

Modelling the depth of the stable boundary-layer

A simple prognostic model of the depth of the stable boundary layer is developed which includes both the possibilities of growth due to entrainment and decreasing depth associated with turbulence decay. The model is designed to avoid requirement of surface fluxes and instead uses information on profiles of mean wind and temperature. Resulting coefficients for the model are estimated by comparisons with existing studies in the literature and comparison with Wangara data.On leave from the Dept of Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, U.S.A.     

Similarity studies of entrainment in convective mixed layers

A similarity study of entrainment at the top of convectively driven mixed layer is presented. The similarity framework is used for a comparison between various parameterized models of mixed-layer growth rate and between closely related models for the ratio of heat fluxes at ground and inversion levels. These various models are also tested, in dimensionless form, against data from laboratory, field experiments and numerical higher-order-modeling of the convective layer. It is concluded that a rather accurate prediction of mixed-layer growth can be achieved with the simple constant flux ratio model, but that more refined studies of entrainment are required to account for the decrease of the heat flux ratio with increasing convective intensity.List of acronyms CRPE Centre de Recherches en Physique de l'Environnement, Issy-les-Moulineaux, France - EERM Etablissement d'Etudes et de Recherches Météorologiques, Boulogne, France - INRA Institut National de la Recherche Agronomique, Versailles, France - LAMP Laboratoire Associé de Météorologie Physique, Clermont-Ferrand, France - NCAR National Center for Atmospheric Research, Boulder, U.S.A. - NOAA National Oceanic and Atmospheric Administration, Environmental Research Laboratories, Boulder, U.S.A.     

Second order modeling of turbulent transport in the surface mixed layer

A model for the simultaneous transport of heat and water vapor is presented. In an effort to resolve the structure of the entrainment region at the inversion base, models are constructed so as to satisfy realizability as far as possible. Density anomaly and water vapor mixture fraction (specific humidity) are taken as the basic variables. Algebraic expressions for the third moments are derived from first principles, and contain no adjustable constants. Separate equations are carried for the dissipation of each variance, constructed to give rational behavior of all time scale ratios. New forms for relaxation and cross-dissipation terms are constructed in such a way as to guarantee realizability. We describe how realizability was used as a tool to construct these models. We present preliminary results without mean velocity gradients for a dry surface mixed layer leaving the land and starting over water, producing a stable internal humidity boundary layer, but with large fluxes of sensible heat and water vapor (local advection).Prepared for presentation at the 29th OHOLO Conference on Boundary Layer Structure — Modelling and Application to Air Pollution and Wind Energy, Zichron Ya'acov, Israel, March 25–28, 1984. Supported in part by the U.S. Office of Naval Research under the following programs: Physical Oceanography (Code 422PO), Power (Code 473); in part by the U.S. National Science Foundation under grant no. ATM 79-22006; and in part by the U.S. Air Force Geophysics Laboratory.     

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.     

Bivariate conditional sampling of buoyancy flux during an intense cold-air outbreak

Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top.The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux.Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.ST Systems Corporation, Lanham, MD, 20706, U.S.A.     

Evaluation of the profile and the resistance method for estimation of surface fluxes of momentum,sensible and latent heat

Summary Hourly lysimetric and micrometeorological data taken over a grass surface at the Meteorological Research Unit, Cardington U.K. have been analysed. A temperature difference and measurements of wind speed at only one height, combined with an independently estimated effective roughness length allowed sensible heat and momentum fluxes determination by the profile method on an hourly basis. The estimates are compared with direct measurements of sensible heat and friction velocity obtained by the eddy correlation method. The sensible and latent heat fluxes are also modelled by the resistance method. Equations based on the Monin—Obukhov similarity theory are used to account for stability effects through various forms of parameterization Aerodynamic and surface resistances, necessary for the Penman—Monteith equation are calculated from routinely measured meteorological data. The profile method for estimation of sensible heat flux and friction velocity is found to work excellently on the discussed daytime experimental data which correspond mainly to near neutral or slightly unstable conditions.Surface latent and sensible heat fluxes can also be described very well by the resistance method. A slightly better estimate of the sensible heat flux is achieved when stability corrections are taken into account. On the contrary Penman-Monteith equation for estimating latent heat flux is insensitive to adjustments for atmospheric stability.The comparison of the various methods leads to the establishment of empirical relationships which correlate various quantities such as soil heat flux, resistances, evapotranspiration etc. to routinely measured meteorological data.With 8 Figures     

Water vapour flux profiles in the convective boundary layer

Summary Water vapour flux profiles in the atmospheric boundary layer have been derived from measurements of water vapour density fluctuations by a ground-based Differential Absorption Lidar (DIAL) and of vertical wind fluctuations by a ground-based Doppler lidar. The data were collected during the field experiment LITFASS-2003 in May/June 2003 in the area of Lindenberg, Germany. The eddy-correlation method was applied, and error estimates of ±50 W/m² for latent heat flux were found. Since the sampling error dominates the overall measurement accuracy, time intervals between 60 and 120 min were required for a reliable flux calculation, depending on wind speed. Rather large errors may occur with low wind speed because the diurnal cycle restricts the useful interval length. In the lower height range, these measurements are compared with DIAL/radar-RASS fluxes. The agreement is good when comparing covariance and error values. The lidar flux profiles are well complemented by tower measurements at 50 and 90 m above ground and by area-averaged near surface fluxes from a network of micrometeorological stations. Water vapour flux profiles in the convective boundary layer exhibit different structures mainly depending on the magnitude of the entrainment flux. In situations with dry air above the boundary layer a positive entrainment flux is observed which can even exceed the surface flux. Flux profiles which linearly increase from the surface to the top of the boundary layer are observed as well as profiles which decrease in the lower part and increase in the upper part of the boundary layer. In situations with humid air above the boundary layer the entrainment flux is about zero in the upper part of the boundary layer and the profiles in most cases show a linear decrease.     

On buoyancy transfer across an entrainment interface

A striking feature observed during the turbulent entrainment across a density interface at moderately-large Richardson numbers is the presence of an intermediate layer sandwiched between the well-mixed and interfacial layers. In this note, a possible mechanism for its origin, and its role in buoyancy transfer during the entrainment is discussed. Estimates are made for the thickness of the intermediate layer and are compared with experimental results.Also Environmental Engineering Science, California Institute of Technology, Pasadena, CA 91125, U.S.A.     

International turbulence comparison experiment (ITCE-81)

The main results of the International Turbulence Comparison Experiment (Tsimlyansk, U.S.S.R., June–July 1981) are presented. Groups from G.D.R., Poland, U.S.S.R., and Czechoslovakia took part in the experiment, while Bulgarian researchers were present as observers. A comparison of in situ measurements (by acoustic anemometers, thermoanemometers, and resistance thermometers) among themselves and with remote soundings (by sodars) was made. Simultaneous measurements of turbulent fluctuation characteristics and of wind velocity and temperature profiles were performed by different instruments. The results and temperature profiles were performed by different instruments. The results of these measurements were used to estimate the comparative accuracy of various models proposed for the evaluation of turbulent fluxes from profile data.     

基于激光雷达遥感和参数化模式研究城市混合层的发展机制

利用北京大学的微脉冲激光雷达(MPL)观测的偏南气流条件下的混合层高度和夹卷层厚度探测资料,研究简单天气条件下城市混合层的发展机制并与GB94的参数化方案相互映证.通过激光雷达遥感的混合层高度和夹卷层厚度计算了混合层顶的夹卷率A,得到其平衡夹卷阶段的值为0.24.在不考虑机械混合前提下反演了地面感热通量,结果表明遥感的反演值与梯度法的计算值有系统性偏差,但总体上仍旧有较好的相关.偏差量的大小反映了影响混合层发展的机械湍流的参数B,进一步通过GB91模式的模拟确定该参数的最佳值约为3.5.在此基础上讨论了混     

Atmospheric boundary-layer structure observed during a haze event due to forest-fire smoke

During a haze event in Baltimore, U.S.A. from July 6 to 8, 2002, smoke from forest fires in the Québec region (Canada), degraded air quality and impacted upon local climate, decreasing solar radiation and air temperature. The smoke particles in and above the atmospheric boundary layer (ABL) served as a tracer and provided a unique opportunity to investigate the ABL structure, especially entrainment. Elastic backscatter lidar measurements taken during the haze event distinctly reveal the downward sweeps (or wisps) of smoke-laden air from the free atmosphere into the ABL. Visualisations of mechanisms such as dry convection, the entrainment process, detrainment, coherent entrainment structures, and mixing inside the ABL, are presented. Thermals overshooting at the ABL top are shown to create disturbances in the form of gravity waves in the free atmosphere aloft, as evidenced by a corresponding ripple structure at the bottom of the smoke layer. Lidar data, aerosol ground-based measurements and supporting meteorological data are used to link free atmosphere, mixed-layer and ground-level aerosols. During the peak period of the haze event (July 7, 2002), the correlation between time series of elastic backscatter lidar data within the mixed layer and the scattering coefficient from a nephelometer at ground level was found to be high (R=0.96 for z =324 m, and R=0.89 for z=504 m). Ground-level aerosol concentration was at a maximum about 2 h after the smoke layer intersected with the growing ABL, confirming that the wisps do not initially reach the ground.     

Atmosphere-ocean interaction in mid-latitude storms

Summary Surface fluxes of heat, latent heat, and momentum, and entrainment fluxes and vertical motion at the top of the boundary layer have been calculated for limited regions of several mid-latitude ocean storms. Results have been combined to describe distributions of boundary layer processes which are characteristic of such storms. Surface heat fluxes have important effects in the region west of cold or occluded fronts and are relatively unimportant within a band of about 200 km width east of fronts. Entrainment in pre-frontal regions is driven largely by vertical shear at the top of the boundary layer, while in post-frontal regions it is driven largely by surface heat flux. Boundary layers are well defined in regions more than roughly 200 km east or west of fronts; but closer to fronts boundary layers are not well defined due to the combined effects of entrainment, condensation, and vertical motion associated with the distribution of surface stress.With 12 Figures     

Laboratory Experiments on Convective Entrainment Using a Saline Water Tank

Entrainment fluxes in a shear-free convective boundary layer have been measured with a saline water tank set-up. The experiments were targeted towards measuring the entrainment behaviour for medium to high Richardson numbers and use a two-layer design, i.e. two stacked non-stratified (neutral) layers with different densities. With laser induced fluorescence (LIF), the entrainment flux of a fluorescent dye is measured for bulk Richardson numbers in the range 30–260. It is proposed that a carefully chosen combination of top-down and bottom-up processes improves the accuracy of LIF-based entrainment observations. The observed entrainment fluxes are about an order of magnitude lower than reported for thermal water tanks: the derived buoyancy entrainment ratio, $A$ , is found to be $A \approx 0.02$ , which is to be compared with $A\approx 0.25$ for a thermal convection tank (Deardorff et al., J Fluid Mech 100:41–64, 1980). An extensive discussion is devoted to the influence of the Reynolds and Prandtl numbers in laboratory experiments on entrainment.     

Methods to Estimate Surface Fluxes of Momentum and Heat from Routine Weather Observations for Dispersion Applications under Stable Stratification

We examine the efficacy of two methods commonly used to estimate the vertical turbulent fluxes of momentum and sensible heat from routinely observed mean quantities in the surface layer under stable stratification. The single-level method uses mean wind speed and temperature measurements at a single height, whereas the two-level method uses mean wind speed measurements at a single height and mean temperature measurements at two heights. These methods are used in popular meteorological processors such as the U.S. Environmental Protection Agency approved AERMET and CALMET to generate inputs for dispersion simulations. We use data from a flux station of the U.K. Met Office at Cardington for comparison. It is found that the single-level method does not describe the flux variation in the weakly stable regime at all, because of its assumption that the temperature scale, i.e. the ratio of the kinematic sensible heat flux to the friction velocity, is constant, which is plausible only under strongly stable conditions. On the other hand, the two-level method provides a physically realistic variation of the fluxes with stability, but the required temperature measurements at two levels are usually not available on a routine basis. If measurements of the standard deviation of temperature are also available, in addition to the mean temperature at a single level, then they can be usefully employed in a third (single-level) method, with the consequence that the computed fluxes are very similar to those obtained from the two-level method. An improvement to the original single-level method is considered, and flux calculations under low wind conditions are also discussed.     

A comparison of turbulence measurements by different instruments; Tsimlyansk field experiment 1970

In the summer of 1970 an international intercomparison of turbulence measurement sensors was carried out at the Tsimlyansk field station of the Institute of Atmospheric Physics (IFA) Academy of Sciences of the U.S.S.R. Improved sonic anemometer sensors of the IFA were compared with the Kaijo sonic used by the Canadian group and with the fluxatron from CSIRO, Australia.Results from the experiment indicate that spectral and cospectral shapes are basically similar between the sonic anemometers. The spectra and cospectra of the fluxatron in the high-frequency region showed an attenuation which can be explained in terms of the response characteristics of the fluxatron. The maintenance of absolute calibration of the sensors in the field was found to be difficult, but when properly corrected for this variation, the measurements were found to agree within 13%, with a standard error of 0.2. A single point observation of turbulent fluxes at a few meters above the surface and averaged over a period of the order of one hour was shown to be representative of turbulent fluxes at the site.     

Current problems in the stratocumulus-topped atmospheric boundary layer

Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.     

Impact of westerly wind bursts on the warm pool of the TOGA-COARE domain in an OGCM

 A primitive equation model is used to investigate the warm pool equilibrium of the tropical Pacific ocean. Attention is focused on the upper ocean. The oceanic response is described using an isothermal approach applied to warm waters contained in the TOGA-COARE domain. The heat balance shows that all the terms, atmospheric surface fluxes, advection and diffusion, operate in the heat bugdet with different time scales. Over long periods, diffusive heat fluxes transfer heat received from the atmosphere out of the warm pool trough the top of the main thermocline. Over short periods, the impact of westerly wind bursts modifies this balance: atmospheric heating is reversed, diffusion is enhanced and advective heat transports out of the warm pool operate through zonal and vertical contributions. We were able to relate the two latter processes to zonal jets and Ekman pumping, respectively. Conversely, the meridional contribution always represents a source of heat, mainly due to the tropical wind convergence. The modelling results clearly show that except during strong wind events, entrainment cooling is not an important component of the budget. The inability to remove heat is due to the salt stratification which needs to be first reduced or even destroyed by westerly wind bursts to activate heat entrainment into deeper layers. Finally, we suggest that the near zero estimate for the surface heat flux entering the warm pool may be extended to longer periods including seaosnal to interannual time scale. Received: 16 December 1996 / Accepted: 8 July 1997     

Fluxes and Gradients in the Convective Surface Layer and the Possible Role of Boundary-Layer Depth and Entrainment Flux

We study the relation between fluxes and gradients in the very unstable surface layer by comparing recent proposals in the literature with the well-known Businger–Dyer functions. The recent proposals include results from large-eddy simulation (LES), which account for entrainment effects and effects of the boundary-layer depth. A comparison of the relationships is made with experimental data. The LES-based gradient functions show the impact of entrainment in the surface layer, but the scatter in the field data is too large to confirm this. Therefore this result is preliminary and future tests against new observations are recommended. It appears that the Businger–Dyer relationship behaves differently to the alternatives, and that it deviates from observations for large stability.     

Comparison of the Bowen ratio-energy balance and stability-corrected aerodynamic methods for measurement of evapotranspiration

Summary The Bowen ratio-energy balance (BREB) and the stability-corrected aerodynamic method were used to estimate turbulent fluxes of sensible and latent heat at an irrigated alfalfa site in a semi-arid valley in northern Utah, U.S.A., during August and September of 1991. Despite inclusion of a generalized stability factor, the aerodynamic method underestimated the daytime (sunrise-sunset) sensible and latent heat fluxes by approximately 30% in comparison with the BREB method. The sum of the aerodynamic estimates of sensible and latent heat seldom balanced the energy avaiable from net radiation and change in storage. Wind speed was low during the experiment (averaging 1.6 m s^–1), and so a second analysis was run for data from daytime, non-rainy, turbulent conditions (wind > 1.5 m s^–1). This showed that sensible and latent heat were still underestimated by approximately 30% in comparison with the BREB approach. This suggests that underestimation of sensible and latent heat fluxes by the aerodynamic method was not related to the wind speed conditions during the experiment. These results show that the stability-corrected aerodynamic model did not agree with the Bowen ratio method in this experiment. It appears unlikely that the discrepancies resulted from measurement errors. Perhaps the theoretical foundation of the similarity parameters (stability functions) in the aerodynamic model are not sufficiently generalized. The discrepancies found here confirm the necessity of calibration checks on the validity of aerodynamic estimates of the turbulent fluxes.With 7 Figures