A Scale-Dependent Dynamic Model for Scalar Transport in Large-Eddy Simulations of the Atmospheric Boundary Layer

An important challenge in large-eddy simulationsof the atmospheric boundarylayer is the specification of the subgrid-scale(SGS) model coefficient(s)and, in particular, how to account for factorssuch as position in the flow,grid/filter scale and atmospheric stability.A dynamic SGS model (thatassumes scale invariance of the coefficients)is implemented in simulationsof a neutral boundary layer with a constantand uniform surface flux of apassive scalar. Results from our simulationsshow evidence that the lumpedcoefficient in the eddy-diffusion modelcomputed with the dynamic proceduredepends on scale. This scale dependence isstronger near the surface, and itis more important for the scalar than for thevelocity field (Smagorinskycoefficient) due to the stronger anisotropicbehaviour of scalars. Based onthese results, a new scale-dependent dynamicmodel is developed for theeddy-diffusion lumped coefficient. The newmodel, which is similar to theone proposed earlierfor the Smagorinsky coefficient,is fully dynamic, thus not requiring anyparameter specification or tuning.Simulations with the scale-dependent dynamicmodel yield the expected trendsof the coefficients as functions of positionand filter/grid scale.Furthermore, in the surface layer the newmodel gives improved predictionsof mean profiles and turbulence spectra ascompared with the traditionalscale-invariant dynamic model.     

Geometric Alignments of the Subgrid-Scale Force in the Atmospheric Boundary Layer

In recent years field experiments have been undertaken in the lower atmosphere to perform a priori tests of subgrid-scale (SGS) models for large-eddy simulations (LES). The experimental arrangements and data collected have facilitated studies of variables such as the filtered strain rate, SGS stress and dissipation, and the eddy viscosity coefficient. However, the experimental set-ups did not permit analysis of the divergence of the SGS stress (the SGS force vector), which is the term that enters directly in the LES momentum balance equations. Data from a field experiment (SGS2002) in the west desert of Utah, allows the calculation of the SGS force due to the unique 4 × 4 sonic anemometer array. The vector alignment of the SGS force is investigated under a range of atmospheric stabilities. The eddy viscosity model is likely aligned with the measured SGS force under near-neutral and unstable conditions, while its performance is unsatisfactory under stable conditions.     

The Effect of Chessboard Variability of the Surface Fluxes on the Aggregated Turbulence Fields in a Convective Atmospheric Surface Layer

To what degree the variability of surface features can be identified in the turbulent signals observed in the atmospheric boundary layer is still an unresolved problem. This was investigated by conducting an analytical experiment for a one-dimensional 'chessboard'-type surface-flux distribution on the basis of local free convection scaling. The results showed that, due to their nonlinear dependency on the surface fluxes, the dimensionless gradients of the mean quantities and the dimensionless standard deviations are altered by the surface-flux variability. Furthermore, passive scalars, such as humidity, are considerably more sensitive to surface variability than the main active scalar, temperature. However, the response of the gradients of the mean quantities is fairly negligible in the range of variability studied herein as compared to that of the standard deviations, which were found to be more sensitive to the surface-flux variability. In addition, the phase difference between the active and the passive scalar flux distribution strongly affects the passive scalar turbulence. This dissimilarity between passive and active scalars, or between passive scalars when their source distributions are different, brings into question the use of variance methods for the measurement of a scalar flux, such as evaporation, over variable surfaces. The classical Bowen ratio method, which depends on the validity of the Reynolds analogy for the vertical gradients of the mean quantities, was shown to be relatively more robust. However, under conditions of strong surface variability, it can also be expected to fail.     

Convective scaling of the average dissipation rate of temperature variance in the atmospheric surface layer

The flux of sensible heat from the land surface is related to the average rate of dissipation of temperature fluctuations in the atmospheric surface layer through the temperature variance budget equation. In many cases it is desirable to estimate the heat flux from measurement or inference of the dissipation rate. Here we study how the dissipation rate scales with atmospheric stability, using three inertial range methods to calculate the dissipation rate: power spectra, second order structure functions, and third order structure functions. Experimental data are analyzed from a pair of field experiments, during which turbulent fluctuations of velocity and temperature were measured over a broad range of neutral and unstable atmospheric flows. It is shown that the temperature dissipation rate scales with a single convective power law continuously from near-neutral to strongly unstable stratification. The dissipation scaling is found to nearly match production in the near-neutral region, but to be consistently lower than production in the more convective regimes. The convective scaling is shown to offer a simplified means of computing sensible heat flux from the dissipation rate of temperature variance.Also at Johns Hopkins University, Baltimore, MarylandAlso at Los Alamos National Laboratory, Los Alamos, New Mexico.     

Inertial-dissipation methods and turbulent fluxes at the air-ocean interface

The use of high frequency atmospheric turbulence properties (inertial subrange spectra, structure function parameters or dissipation rates) to infer surface fluxes of momentum, sensible heat and latent heat is more practical for most ocean going platforms than direct covariance measurement. The relationships required to deduce the fluxes from such data are examined in detail in this paper and several ambiguities and uncertainties are identified. It is noted that, over water, data on water vapor properties (the dimensionless functions for the mean profile, the structure function parameter and the variance transport term) are extremely sparse and the influence of sea spray is largely unknown. Special attention is given to flux estimation on the basis of the structure function formalism. Existing knowledge about the relevant similarity functions is summarized and discussed in light of the ambiguities identified above.     

Impact of a Saturation-Dependent Dissipation Source Function on Operational Hindcasts of Wind-Waves in the Australian Region

The impact of a modified parametric form of the dissipation source function S ds on the skill of an operational spectral wind-wave model is investigated. Numerical experiments are made with a version of the WAM model presently used operationally by the Australian Bureau of Meteorology to produce sea-state forecasts within the Australian region. Performances of wave hindcasts obtained using several alternative forms of this newly proposed dissipation source function are compared to those produced with forms of S ds commonly used operationally. A new ad hoc technique to compare modelled and observed wave spectra is introduced to assess the quality of calculated one-dimensional frequency spectra.

Our results indicate that wave model performance in terms of integral spectral parameters, such as the significant wave height H s , may benefit from improved parameterisations of the dissipation source term S ds . On the other hand, we also found that model performance was relatively poor in terms of predictions of the one-dimensional frequency spectrum, regardless of the chosen form of S ds . These results suggest that further refinements of the dissipation source term are strongly dependent on improved parametrisations of the nonlinear interactions source term S nl .     

Determination of turbulent momentum and heat fluxes by spectral methods

Methods are studied which permit one to evaluate turbulent fluxes from the results of spectral measurements in turbulent laboratory flows and an unstable atmospheric surface layer. The well known dissipation method of flux measurements, which uses spectral data related to the inertial range, is reanalyzed. New theoretical ideas and the latest experimental data are used to specify this method in cases of moderately and very strongly unstable thermal stratifications.Moreover, it is also explained how to estimate momentum and heat fluxes from data in the low frequency parts of the velocity and temperature spectra in the low frequency ranges beyond the lower limit of the inertial range. This permits one to estimate fluxes using rather simple and cheap instruments (e.g., Pilot-tubes and thermocouples in laboratory flows or cup anemometers and crude resistance thermometers in meteorological studies). The equations for flux determination are based in such cases on the recent models by Kader (1987, 1988) and Kader and Yaglom (1990, 1991) of spectral shapes at mesoscale wave numbers; these models agree quite satisfactorily with many (though not all) data of direct spectral measurements. It is shown that estimated momentum and heat fluxes in the laboratory and in an unstably stratified atmospheric surface layer obtained by the method suggested in this paper agree satisfactorily with direct flux measurements.     

A Surface-Layer Study of the Transport and Dissipation of Turbulent Kinetic Energy and the Variances of Temperature,Humidity and CO$$_$$

We discuss scalar similarities and dissimilarities based on analysis of the dissipation terms in the variance budget equations, considering the turbulent kinetic energy and the variances of temperature, specific humidity and specific CO\(_2\) content. For this purpose, 124 high-frequency sampled segments are selected from the Boundary Layer Late Afternoon and Sunset Turbulence experiment. The consequences of dissipation similarity in the variance transport are also discussed and quantified. The results show that, for the convective atmospheric surface layer, the non-dimensional dissipation terms can be expressed in the framework of Monin–Obukhov similarity theory and are independent of whether the variable is temperature or moisture. The scalar similarity in the dissipation term implies that the characteristic scales of the atmospheric surface layer can be estimated from the respective rate of variance dissipation, the characteristic scale of temperature, and the dissipation rate of temperature variance.     

An Optimal Inverse Method Using Doppler Lidar Measurements to Estimate the Surface Sensible Heat Flux

Inverse methods are widely used in various fields of atmospheric science. However, such methods are not commonly used within the boundary-layer community, where robust observations of surface fluxes are a particular concern. We present a new technique for deriving surface sensible heat fluxes from boundary-layer turbulence observations using an inverse method. Doppler lidar observations of vertical velocity variance are combined with two well-known mixed-layer scaling forward models for a convective boundary layer (CBL). The inverse method is validated using large-eddy simulations of a CBL with increasing wind speed. The majority of the estimated heat fluxes agree within error with the proscribed heat flux, across all wind speeds tested. The method is then applied to Doppler lidar data from the Chilbolton Observatory, UK. Heat fluxes are compared with those from a mast-mounted sonic anemometer. Errors in estimated heat fluxes are on average 18 %, an improvement on previous techniques. However, a significant negative bias is observed (on average $-63\,\%$ ) that is more pronounced in the morning. Results are improved for the fully-developed CBL later in the day, which suggests that the bias is largely related to the choice of forward model, which is kept deliberately simple for this study. Overall, the inverse method provided reasonable flux estimates for the simple case of a CBL. Results shown here demonstrate that this method has promise in utilizing ground-based remote sensing to derive surface fluxes. Extension of the method is relatively straight-forward, and could include more complex forward models, or other measurements.     

Atmospheric radiative equilibria. Part II: bimodal solutions for atmospheric optical properties

 A simple theoretical model of atmospheric radiative equilibrium is solved analytically to help understand the energetics of maintaining Earth's tropical and subtropical climate. The model climate is constrained by energy balance between shortwave (SW) and longwave (LW) radiative fluxes. Given a complete set of SW and LW optical properties in each atmospheric layer, the model yields a unique equilibrium-temperature profile. In contrast, if the atmospheric temperature profile and SW properties are prescribed, the model yields essentially two distinct LW transmissivity profiles. This bimodality is due to a nonlinear competition between the ascending and descending energy fluxes, as well as to their local conversion to sensible heat in the atmosphere. Idealized slab models that are often used to describe the greenhouse effect are shown to be a special case of our model when this nonlinearity is suppressed. In this special case, only one solution for LW transmissivity is possible. Our model's bimodality in LW transmissivity for given SW fluxes and temperature profile may help explain certain features of Earth's climate: at low latitudes the temperature profiles are fairly homogeneous, while the humidity profiles exhibit a bimodal distribution; one mode is associated with regions of moist-and-ascending, the other with dry-and-subsiding air. The model's analytical results show good agreement with the European Centre for Medium-Range Weather Forecasts' reanalysis data. Sensitivity analysis of the temperature profile with respect to LW transmissivity changes leads to an assessment of the low-latitude climate's sensitivity to the “runaway greenhouse” effect. Received: 7 December 1999 / Accepted: 19 February 2001     

Estimation of momentum and heat fluxes in the constant flux layer from temperature measurements

Global estimates of momentum and heat fluxes are required for the application of any general atmospheric and oceanic circulation model. A new technique for the estimation of these fluxes in a constant flux boundary layer is developed. The new approach is a modification of the dissipation technique but the only required measurements are the mean and fluctuating temperatures at two levels within the constant flux layer. All other flux estimation techniques require measurement of both temperature and velocity. Data are presented to compare flux estimation results with the conventional eddy-correlation technique. Also discussed are the limitations of the procedure and restrictions on its applicability.     

Impact of Different Aerosols on the Evolution of the Atmospheric Boundary Layer

The present work analyzes the effect of aerosols on the evolution of the atmospheric boundary layer (ABL) over Shangdianzi in Beijing.A one-dimensional ABL model and a radiative transfer scheme are incorporated to develop the structure of the ABL.The diurnal variation of the atmospheric radiative budget,atmospheric heating rate,sensible and latent heat fluxes,surface and the 2 m air temperatures as well as the ABL height,and its perturbations due to the aerosols with different single-scattering albedo (SSA) are studied by comparing the aerosol-laden atmosphere to the clean atmosphere.The results show that the absorbing aerosols cause less reduction in surface evaporation relative to that by scatting aerosols,and both surface temperature and 2 m temperature decrease from the clean atmosphere to the aerosol-laden atmosphere.The greater the aerosol absorption,the more stable the surface layer.After 12:00 am,the 2 m temperature increases for strong absorption aerosols.In the meantime,there is a slight decrease in the 2 m temperature for purely scattering aerosols due to radiative cooling.The purely scattering aerosols decrease the ABL temperature and enhance the capping inversion,further reducing the ABL height.     

On the Development of a Dynamic Non-linear Closure for Large-Eddy Simulation of the Atmospheric Boundary Layer

A dynamic procedure is developed to compute the model coefficients in the recently introduced modulated gradient models for both momentum and scalar fluxes. The magnitudes of the subgrid-scale (SGS) stress and the SGS flux are estimated using the local equilibrium hypothesis, and their structures (relative magnitude of each of the components) are given by the normalized gradient terms, which are derived from the Taylor expansion of the exact SGS stress/flux. Previously, the two model coefficients have been specified on the basis of theoretical arguments. Here, we develop a dynamic SGS procedure, wherein the model coefficients are computed dynamically according to the statistics of the resolved turbulence, rather than provided a priori or ad hoc. Results show that the two dynamically calculated coefficients have median values that are approximately constant throughout the turbulent atmospheric boundary layer (ABL), and their fluctuations follow a near log-normal distribution. These findings are consistent with the fact that, unlike eddy-viscosity/diffusivity models, modulated gradient models have been found to yield satisfactory results even with constant model coefficients. Results from large-eddy simulations of a neutral ABL and a stable ABL using the new closure show good agreement with reference results, including well-established theoretical predictions. For instance, the closure delivers the expected surface-layer similarity profiles and power-law scaling of the power spectra of velocity and scalar fluctuations. Further, the Lagrangian version of the model is tested in the neutral ABL case, and gives satisfactory results.     

极轨卫星高层通道在GRAPES同化系统中的应用

针对GRAPES(Global/Regional Assimilation and Prediction System)模式三维变分系统高层背景场温湿廓线外推方案的局限性,提出以气候垂直廓线重新构造高层温湿垂直结构,以减小外推方案的偏差。首先采用一维变分同化系统,展开模拟实验:分析目前模式中使用的外推方案误差及其对反演结果的影响,利用高层大气气候廓线构造垂直结构并分析同化偏差。最后,运用GRAPES全球分析预报系统进行同化实验并分析改进程度。结果显示:模拟研究表明采用高层背景场温湿廓线外推方案与实际观测相比最大偏差在1 h Pa附近可达数十度以上,不仅影响平流层,而且对对流层也有影响;用气候温度数据修正GRAPES高层温度数据,可以减少50%以上的偏差,证明了用气候值高层数据优化现行GRAPES模式中同化系统高层插值方案的可行性。全球GRAPES三维变分同化试验结果显示,改进方案不仅显著的改善平流层分析质量,对对流层中高层也有改进。     

Radiative Effects on Temperature in the Stable Surface Layer

The interaction between longwave radiation and temperature fluctuations plays a role in the dissipation of temperature variance. This interaction is most easily described by spectral models of atmospheric turbulence and a spectral radiative dissipation function which gives the intensity of the damping at each radiative wavelength and wavenumber k. We have used a Corrsin–Pao closure for the spectral budgets of turbulent kinetic energy and temperature to study the coupling of radiation to turbulence. The spectral radiative dissipation function and a related integral have been fitted by analytical approximations with the correct asymptotic behavior. This resulted in a simple analytical formula for the dimensionless temperature spectrum as a function of Monin-Obukhov stability, and a new dimensionless parameter describing the relative importance of radiation in the temperature spectral budget. The radiative effects both on the temperature spectrum and on the dimensionless temperature variance can then be calculated. Based on typical values of the radiative dimensionless parameters for the surface layer, we conclude that radiative dissipation is probably negligible there.     

Large-Eddy Simulation of the Stable Atmospheric Boundary Layer using Dynamic Models with Different Averaging Schemes

Large-eddy simulation (LES) of a stable atmospheric boundary layer is performed using recently developed dynamic subgrid-scale (SGS) models. These models not only calculate the Smagorinsky coefficient and SGS Prandtl number dynamically based on the smallest resolved motions in the flow, they also allow for scale dependence of those coefficients. This dynamic calculation requires statistical averaging for numerical stability. Here, we evaluate three commonly used averaging schemes in stable atmospheric boundary-layer simulations: averaging over horizontal planes, over adjacent grid points, and following fluid particle trajectories. Particular attention is focused on assessing the effect of the different averaging methods on resolved flow statistics and SGS model coefficients. Our results indicate that averaging schemes that allow the coefficients to fluctuate locally give results that are in better agreement with boundary-layer similarity theory and previous LES studies. Even among models that are local, the averaging method is found to affect model coefficient probability density function distributions and turbulent spectra of the resolved velocity and temperature fields. Overall, averaging along fluid pathlines is found to produce the best combination of self consistent model coefficients, first- and second-order flow statistics and insensitivity to grid resolution.     

A box model of the Arctic natural variability

We consider a box model of the Arctic system to examine its natural variability pertaining to the decadal Arctic Oscillation (AO) and the multidecadal Low-Frequency Oscillation (LFO). We distinguish the hierarchical order of the winter over the summer open-areas with only the former perturbing the sea-level pressure to effect coupled balances. From such balances, we discern two feedback loops on the winter open-area: a positive ice-flux feedback that elevates its overall variance and a negative buoyancy feedback that suppresses its low-frequency variance to render a decadal AO peak when subjected to white atmospheric noise. This negative buoyancy feedback may also reproduce observed phasing among LFO signals forced by the AMV (Atlantic Multidecadal Variability), thus resolving some outstanding questions. For the summer open-area, its variance is induced mainly by the winter forcings and insensitive to the base state. Its decadal signal merely reflects the preconditioning winter open-area, but its LFO variance is induced additionally and in comparable measure by the winter SAT (surface air temperature) through the latter’s effect on the melt duration and the first-year ice thickness. As such, the summer open-area signal is dominantly multidecadal, which moreover is several times its winter counterpart, consistent with the observed disparity. Although the model is extremely crude, its explicit solution allows quantitative comparison with observations and the generally positive outcome suggests that the model has isolated the essential physics of the Arctic natural variability of our concern.     

RegCM4对中国东部区域气候模拟的辐射收支分析

利用卫星和再分析数据,评估了区域气候模式Reg CM4对中国东部地区辐射收支的基本模拟能力,重点关注地表净短波(SNS)、地表净长波(SNL)、大气顶净短波(TNS)、大气顶净长波(TNL)4个辐射分量。结果表明:1)短波辐射的误差值在夏季较大,而长波辐射的误差值在冬季较大。但各辐射分量模拟误差的空间分布在冬、夏季都有较好的一致性。2)对于地表辐射通量,SNS表现为正偏差(向下净短波偏多),在各分量中误差最大,区域平均误差值近50 W/m2;SNL表现为负偏差(向上净长波偏多);对于大气顶辐射通量,TNS和TNL分别表现为"北负南正"的误差分布和整体正偏差。3)利用空间相关和散点线性回归方法对4个辐射分量的模拟误差进行归因分析,发现在云量、地表反照率、地表温度三个直接影响因子中,云量模拟误差的贡献最大,中国东部地区云量模拟显著偏少。     

A regional climate model simulation over West Africa: parameterization tests and analysis of land-surface fields

The West African Monsoon has been simulated with the regional climate model PROMES, coupled to the land-surface model ORCHIDEE and nested in ECMWF analysis, within AMMA-EU project. Three different runs are presented to address the influence of changes in two parameterizations (moist convection and radiation) on the simulated West African Monsoon. Another aim of the study is to get an insight into the relationship of simulated precipitation and 2-m temperature with land-surface fluxes. To this effect, data from the AMMA land-surface model intercomparison project (ALMIP) have been used. In ALMIP, offline simulations have been made using the same land-surface model than in the coupled simulation presented here, which makes ALMIP data particularly relevant for the present study, as it enables us to analyse the simulated soil and land-surface fields. The simulation of the monsoon depends clearly on the two analysed parameterizations. The inclusion of shallow convection parametrization affects the intensity of the simulated monsoon precipitation and modifies some dynamical aspects of the monsoon. The use of a fractional cloud-cover parameterization and a more complex radiation scheme is important for better reproducing the amplitude of the latitudinal displacement of the precipitation band. This is associated to an improved simulation of the surface temperature field and the easterly jets. However, the parameterization changes do not affect the timing of the main rainy and break periods of the monsoon. A better representation of downward solar radiation is associated with a smaller bias in the surface heat fluxes. The comparison with ALMIP land-surface and soil fields shows that precipitation and temperature biases in the regional climate model simulation are associated to certain biases in land-surface fluxes. The biases in soil moisture seem to be driven by atmospheric biases as they are strongly affected by the parameterization changes in atmospheric processes.