The evolution of a convective planetary boundary layer — A higher-order-closure model study

The evolution of the boundary layer on day 33 of the Wangara experiment in southeast Australia is calculated with a higher-order-closure turbulence model. This model, which includes equations for the mean field as well as the second moments of the turbulent field, is described in detail. The mean profiles of wind, temperature, and humidity, the profiles of heat and humidity fluxes, the Reynolds stress distributions, and the height of the boundary layer are shown between 10 a.m. and 6 p.m. The results agree well with those from Deardorff's 3-D simulation and take relatively little computer time.     

A model for eddy diffusivity in a stable boundary layer

Expressions for the vertical and the lateral diffusivity coefficients were derived from the Local Similarity Theory and the Statistical Diffusion Theory. For such, the spectral density energies for the turbulent velocities were used. The expressions here derived are compared with the diffusivity coefficients for momentum and heat suggested by Sorbjan (from the Minnesota experiments) and Nieuwstadt (from the Cabauw experiments). This comparison allows us to conclude that turbulence is equally efficient in transporting momentum, heat and contaminants in an ideally stable boundary layer.     

A TKE-dissipation model for the atmospheric boundary layer

The dissipation, , of turbulent kinetic energy (TKE) is a key parameter in atmospheric boundary-layer (ABL) models. Besides being a sink for momentum, it is often used together with the TKE to define an internal turbulence time scale for closure relations. A prognostic formulation for the dissipation of TKE is formulated, based on isotropic tensor modeling methods. The formulation is coupled to a level 2.5 second-order closure model and evaluated against measurements taken in horizontally homogeneous conditions, as well as against a tailored length-scale formulation. A formulation suitable for convective as well as neutral and stable ABLs is suggested.On leave from Department of Meteorology, Uppsala University, P.O. Box 516, S-751 20 Uppsala, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The temperature profile and heat transfer law in a neutrally and stably stratified planetary boundary layer

A logarithmic + polynomial approximation is proposed for the vertical temperature profile in a neutrally or stably stratified planetary boundary layer (PBL) in conditions of quasi-stationarity. Using this approximation with the asymptotic logarithmic + linear law of the Monin-Obukhov similarity theory for the near-surface layer and with the Zilitinkevich formula for the PBL thickness allows one to derive an analytical expression for the function C in the heat transfer law, which permits simple parameterization of the thermal interaction between the atmosphere and the underlying medium in terms of external parameters, such as the geostrophic wind velocity and the temperature difference across the PBL.     

边界层温度廓线遥感反演的本地化改进研究

采用MODIS资料和美国发展的MODIS大气温、湿度廓线统计反演算法,估算大气温度、湿度廓线作为初始场,应用101层快速透过率模式(PFAAST)估算了大气透过率,并采用Newton非线性迭代算法反演中国西北荒漠戈壁地区大气温度廓线。结果表明:该方法对边界层高度及以上部分的大气温度反演得比较好,误差基本都在2 K范围内,边界层范围内的温度反演误差较大,反演误差与气溶胶光学厚度增量和地表温度估算误差呈显著正相关关系,与大气水汽混合比的关系较差。文中从敏感性试验和理论分析角度阐述了地表温度和气溶胶光学厚度估算误差对大气温度反演误差的影响,发现不同光谱波段的地表温度权重均随地表温度的增加有不同程度增加,地表温度反演误差增加将增加地表温度权重,提高地表温度估算误差有助于提高地表温度权重的精度;荒漠戈壁地区大气边界层中气溶胶浓度较高,光学厚度较大,使边界层大气透过率降低,进而降低卫星红外遥感波段的地表温度权重和空气温度权重。由于该模式没有很好地考虑边界层中沙尘气溶胶的影响,使卫星反演的大气透过率偏高,以至于高估地表温度权重和大气温度权重,使得反演的表面温度和空气温度偏低。该研究结合太阳光度计获得的光学厚度资料,采用统计方法对气溶胶效应引起的大气透过率误差和表面温度估算误差进行校正,并对物理算法进行本地化改进,实现了边界层温度廓线的反演。     

A bulk model for the atmospheric planetary boundary layer

The integrated momentum and thermodynamic equations through the planetary boundary layer (PBL) are solved numerically to predict the mean changes of wind and potential temperature from which surface fluxes are computed using bulk transfer coefficients of momentum and heat. The second part of the study involves a formulation and testing of a PBL height model based on the turbulent energy budget equation where turbulent fluxes of wind and heat are considered as the source of energy. The model exhibits capability of predicting the PBL height development for both stable and unstable regimes of observed conditions. Results of the model agree favourably with those of Deardorff's (1974a) and Tennekes' (1973) models in convective conditions.Contribution number 396.     

Role of land-surface temperature feedback on model performance for the stable boundary layer

At present a variety of boundary-layer schemes is in use in numerical models and often a large variation of model results is found. This is clear from model intercomparisons, such as organized within the GEWEX Atmospheric Boundary Layer Study (GABLS). In this paper we analyze how the specification of the land-surface temperature affects the results of a boundary-layer scheme, in particular for stable conditions. As such we use a well established column model of the boundary layer and we vary relevant parameters in the turbulence scheme for stable conditions. By doing so, we can reproduce the outcome for a variety of boundary-layer models. This is illustrated with the original set-up of the second GABLS intercomparison study using prescribed geostrophic winds and land-surface temperatures as inspired by (but not identical to) observations of CASES-99 for a period of more than two diurnal cycles. The model runs are repeated using a surface temperature that is calculated with a simple land-surface scheme. In the latter case, it is found that the range of model results in stable conditions is reduced for the sensible heat fluxes, and the profiles of potential temperature and wind speed. However, in the latter case the modelled surface temperatures are rather different than with the original set-up, which also impacts on near-surface air temperature and wind speed. As such it appears that the model results in stable conditions are strongly influenced by non-linear feedbacks in which the magnitude of the geostrophic wind speed and the related land-surface temperature play an important role.     

A study of temperature fluctuations in the atmospheric boundary layer

The spectral density of temperature fluctuations in the boundary layer has been studied in the range 0.2 to 2 Hz. It is shown that the temperature spectrum is strongly variable from one minute to the next and that the spectral slope tends to increase with the standard deviation of temperature fluctuations and with the stability of the medium. It is shown that C _T ² values computed from short time series (30 s) tend to be smaller than the C _T ² values computed from several minutes of record.Formerly at Laboratoire de Météorologie Dynamique du C.N.R.S. (Paris).     

丘陵地区边界层风廓线雷达数据统计特性分析

采用数据获取率来分析和评价风廓线雷达的探测能力, 对通过2012年的风廓线数据进行统计分析。结果表明:数据获取率和信噪比都随季节变化, 夏季探测能力大于冬季。按照数据获取率达到80%的要求, 确定边界层风廓线雷达无降雨天气有效探测高度为3 km, 并确定低模和高模最佳衔接高度为0.6 km, 能够获得更好的数据获取率。在无降雨天气, 信噪比随高度呈现对数函数单调递减的变化规律, 夏季信噪比的衰减程度比冬季大;在降雨天气, 信噪比随高度呈现一次函数的变化规律, 其斜率范围在-10.44~-2.47之间, 而夏季信噪比的衰减程度比冬季小。     

On the evolution of the height and temperature difference across the nocturnal stable boundary layer

Evidence is furnished which indicates that in some cases the height of the stable boundary layer (SBL) and the magnitude of the temperature difference across the SBL may be more appropriately described by an error function erf(t/) rather than the generally accepted square root time dependence. The time constant was observed to have values of one to three hours. The discovery has been found to be site dependent, however, as data from other sites follow the usual square root time evolution.     

A model for vertical diffusion coefficients in a growing urban boundary layer

Concentrations of carbon monoxide and methane, observed during the Los Angeles Reactive Pollution Program (LARPP) have been used to infer the behavior of vertical diffusion coefficients as a function of height and time. Computations were made for a sunny period with little wind, between about 8 a.m. and noon. During this period, the height of the mixed layer grew approximately linearly as function of time. In the surface layer, mixing coefficients increased rapidly with height. From about 50 m to about half the mixing heights, the mixing coefficients were so large that the concentrations were invariant with height. In the top half of the mixed layer, mixing coefficients decreased with height; a simple tentative interpolation formula was developed for this region.     

A model study of the nocturnal boundary layer

In this paper, a second-order model is proposed for the study of the evolution of the nocturnal boundary layer (NBL). The model is tested against the Wangara data on atmospheric boundary layer. The computer results show ihat the model can simulate some important characters observed in the NBL, and that a kind of sudden change may occur in the developing process of NBL.     

A bulk model of the well-mixed boundary layer

A bulk boundary-layer model is developed to predict surface fluxes and conditions in the well-mixed layer between the surface and the lower troposphere. The model includes the effects of all the dominant processes, including advection, in a dry boundary layer. The numerical model is compared with theoretical predictions for the growth of an internal boundary layer, and it is used to simulate the generation of a sea breeze by the diurnal cycle of radiative heating.     

A model of the planetary boundary layer over a snow surface

A model of the planetary boundary layer over a snow surface has been developed. It contains the vertical heat exchange processes due to radiation, conduction, and atmospheric turbulence. Parametrization of the boundary layer is based on similarity functions developed by Hoffert and Sud (1976), which involve a dimensionless variable, ζ, dependent on boundary-layer height and a localized Monin-Obukhov length. The model also contains the atmospheric surface layer and the snowpack itself, where snowmelt and snow evaporation are calculated. The results indicate a strong dependence of surface temperatures, especially at night, on the bursts of turbulence which result from the frictional damping of surface-layer winds during periods of high stability, as described by Businger (1973). The model also shows the cooling and drying effect of the snow on the atmosphere, which may be the mechanism for air mass transformation in sub-Arctic regions.     

夜间大气边界层的高阶矩数值模拟

本文建立了一个二阶矩模式,用此摸式对Wangara资料进行了实例模拟.结果表明,该模式能模拟出夜间边界层发展以及湍流结构的一些主要特征.计算发现在夜间边界层发展过程中存在一种突变现象.     

行星边界层中多极值风速廓线的研究

本文利用325米气象塔及声雷达探测资料分析了夜间平稳天气条件下行星边界层中风速出现多极值的现象。分析指出,多极值风速与边界层中多层逆温有密切的联系,当温度场出现多层逆温并维持一定的强度和时间后,便相应地出现了多层风速极大,风速极大值出现的高度在逆温顶附近或稍高。 文内还用一维非定常模式模拟了多极值风速廓线的形成。     

A theoretical model of multi-regime convection in a stratocumulus-topped boundary layer

The possible effects on stratocumulus circulations caused by drizzle and radiative cooling or heating are investigated theoretically using a simple Nonlinear Dynamical System (NDS). These effects are incorporated implicitly via the background temperature profile, and are expressed as departures from neutral conditions. These neutral conditions are assumed to be dry adiabatic in the surface, sub-cloud and inversion regions, and moist adiabatic in the cloud region.The NDS domain is divided into six distinct regions that represent those commonly observed in the planetary boundary layer (PBL): 1) the surface layer, 2) the sub-cloud layer, 3) the cloud-base layer, 4) the cloud layer, 5) the cloud-top layer, and 6) the capping inversion. The NDS successfully represents the effects of the capping inversion. Circulations are limited in their upward extent by the inversion, and would only penetrate into it when surface forcing rates are quite large.Surprisingly, when there are identical forcing rates but different initial conditions for the dynamic and thermodynamic flelds, the NDS yields two solutions throughout a wide range of cloud-base stabilities. This range covers the transition from a stable to an unstable cloud-base layer (layer 3 above). The first solution is a steady one having a decoupled form, with separate circulations in the sub-cloud region and the cloud region. The second solution is a temporally varying one exhibiting periodic coupling. The circulation in this case starts as a shallow eddy near the surface. This eddy grows into a deeper plume that penetrates into the inversion before finally dying and beginning the process again. The existence of these two fundamentally different solutions for the same forcing rates, or multi-regime convection, suggests that the PBL response to a particular forcing rate may depend critically on the initial conditions of the dynamic and thermodynamic fields. As a consequence, future modeling efforts of the PBL should consider a broad range of initial flelds.     

A model for heat patch in the boundary layer of the geostrophic flow

A hydrodynamic model is considered for a disturbance of wind field structure of the boundary layer in the geostrophic flow running onto a warmer underlying surface. It is shown that the breeze effect causes the lowering (or heightening) of streamlines in a narrow area of sharp changes in curvature of horizontal profile of temperature disturbance. The technique by A.A. Dorodnitsyn for solution of parabolic equation set is illustrated by an example of a streamline and vertical velocity calculation over a wide “warm” river. A scheme is presented explaining the breeze effect on the boundary layer structure.     

夜间城市边界层发展的数值研究

本文使用非定常非线性二维数值模式,研究夜间气流流经城市热岛上空引起的风场、温度场和垂直涡旋扩散参数的调整以及城市热岛环流的发展演变.