Effect of atmospheric stability on the growth of surface gravity waves

In this paper we study the effect of atmospheric stability on the growth of surface gravity waves. To that end we numerically solved the Taylor-Goldstein equation for wind profiles which deviate from a logarithmic form because stratification affects the turbulent momentum transport. Using Charnock's relation for the roughness height z ₀ of the wind profile, it is argued that the growth rate of the wave depends on the dimensionless phase velocity c/u _* (where u _* is the friction velocity) and a measure of the effect of atmospheric stability, namely the dimensionless Obukhov length gL/u _* ², whereas it only depends weakly on gz _t /u _* ² (where z _t is the roughness height of the temperature profile). Remarkably for a given value of u _* /c, the growth rate is larger for a stable stratification (L > 0) than for an unstable one (L < 0). We explain why this is the case. If, on the other hand, one considers the growth rate as a function of c/U ₁₀ (where U ₁₀ is the windspeed at 10 m), the situation reverses for c/U ₁₀ < 1. For practical application in wave prediction models, we propose a new parameterization of the growth rate of the waves which is an improvement of the Snyder et al. (1981) proposal because the effect of stability is taken into account.     

Direct human influence of irrigation on atmospheric water vapour and climate

Human activity increases the atmospheric water vapour content in an indirect way through climate feedbacks. We conclude here that human activity also has a direct influence on the water vapour concentration through irrigation. In idealised simulations we estimate a global mean radiative forcing in the range of 0.03 to +0.1 Wm^–2 due to the increase in water vapour from irrigation. However, because the water cycle is embodied in the climate system, irrigation has a more complex influence on climate. We also simulate a change in the temperature vertical profile and a large surface cooling of up to 0.8 K over irrigated land areas. This is of opposite sign than expected from the radiative forcing alone, and this questions the applicability of the radiative forcing concept for such a climatic perturbation. Further, this study shows stronger links than previously recognised between climate change and freshwater scarcity which are environmental issues of paramount importance for the twenty first century.     

The influence of a water surface on mesoscale dynamics as a function of atmospheric stability

Several numerical experiments have been undertaken with a three-dimensional mesoscale model in order to determine to what extent a water surface such as a lake can influence mesoscale flow patterns.It is shown that the influence of the lake is important when cumulus clouds are present. These clouds, generated by evaporation from the water surface are small but induce significant secondary circulations which disrupt the flow field on the mesoscale.Artificial suppression of cloud activity results in a situation where the lake exerts little influence on the atmospheric environment in comparison to the control experiment where the body of water is absent. Atmospheric stability controls the intensity of perturbations to the mean flow when clouds are present.The study is of interest when modeling a number of complex phenomena simultaneously; the results shown here indicate that under certain stability conditions, a small lake can be ignored as to its dynamic and thermodynamic influence on atmospheric processes, thus leading to a neglect of a number of equations taking into account moisture terms explicitly.     

The influence of sea waves on the wind profile

From wind profile and wave measurements performed during the JONSWAP II experiment, relations between the dimensionless profile slope and the significant wave height are derived. It is shown that the wind profile is distorted by the waves especially in the vicinity of the water surface. The wave influence on the profile seems to be restricted to heights below about three wave heights. Above this level, the dimensionless profile slope is an approximately constant value corresponding to a drag coefficient of about 1.15 × 10^–3.     

Advances in Studies on Nonlinear Atmospheric Waves

A review on the progress in the research of nonlinear atmospheric waves, especially the nonlinear Rossby waves is made in this paper. Many results reported here have been obtained in Peking University.     

The influence of nonstationarity on the turbulent flux–gradient relationship for stable stratification

Extensive eddy-correlation datasets are analyzed to examine the influence of nonstationarity of the mean flow on the flux–gradient relationship near the surface. This nonstationarity is due to wavelike motions, meandering of the wind vector, and numerous unidentified small-scale mesoscale motions. While the data do not reveal an obvious critical gradient Richardson number, the maximum downward heat flux increases approximately linearly with increasing friction velocity for significant stability. The largest of our datasets is chosen to more closely examine the influence of stability, nonstationarity, distortion of the mean wind profile and self-correlation on the flux-gradient relationship. Stability is expressed in terms of z/L, the gradient Richardson number or the bulk Richardson number over the tower layer. The efficiency of the momentum transport systematically increases with increasing nonstationarity and attendant distortion of the mean wind profile. Enhancement of the turbulent momentum flux associated with nonstationarity is examined in terms of the nondimensional shear, Prandtl number and the eddy diffusivity.     

The influence of stratification on heat and momentum turbulent transfer in antarctica

Data from the Antarctic winter at Halley Base have been used in order to evaluate qualitatively and quantitatively how the stratification in the low atmosphere (evaluated with the gradient Richardson number, Ri) influences the eddy transfers of heat and momentum. Vertical profiles of wind and temperature up to 32 m, and turbulent fluxes ( , and ) measured from three ultrasonic thermo-anemometers installed at 5, 17 and 32 m are employed to calculate Ri, the friction velocity (u _*) and the eddy diffusivities for heat (K _h ) and momentum (K _m ). The results show a big dependence of stability onK _m ,K _h andu _*, with a sharp decrease of these turbulent parameters with increasing stability. The ratio of eddy diffusivities (K _h /K _m ) is also analyzed and presents a decreasing tendency as Ri increases, reaching values even less than 1, i.e., there were situations where the turbulent transfer of momentum was greater than that of heat. Possible mechanisms of turbulent mixing are discussed.     

The influence of atmospheric stability on the ‘Leipzig’ boundary-layer structure

The Leipzig wind profile recorded in 1931 has long been considered representative of the wind profile expected in a steady, barotropic flow in the planetary boundary layer over a flat homogeneous surface. As such, the Leipzig data set has been extensively used in the development of computational models for planetary boundary-layer flows. To date, workers have assumed that the effect of stable stratification in the atmosphere over Leipzig was negligible. In the present work, a second-order turbulence closure that explicitly accounts for the effects of stable stratification is used to analyse the Leipzig data. The importance of stable stratification in this near-neutral data set is demonstrated.     

大气层结不稳定能量的简便计算方法

本文运用大气能量学、大气热力学和大气动力学基本原理,提出了适用于载水气块在可逆湿绝热上升过程中,和非载水气块在假绝热上升过程中,大气层结不稳定能量的两个简便计算公式。     

The deep water stratification of ocean general circulation models

Abstract

A central problem in climate and ocean modelling is the accurate simulation of the climatological state of the oceanic density field. A constant vertical diffusivity for heat and salt is frequently employed in ocean general circulation models (OGCMs) and it is usually assigned a value designed to optimize the depth of the pycnocline. One undesired consequence of this choice is a poor representation of the deep water, which is usually insufficiently stratified. In contrast to the uniform diffusivity of many models, some observational studies suggest that the vertical diffusivity is not constant but increases with depth, possibly in inverse proportion to the local buoyancy frequency. Numerical experiments with an OGCM are presented that demonstrate that allowing the vertical diffusivity to increase below the pycnocline substantially increases the stratification of the abyssal water mass of these models without significantly affecting the pycnocline depth, and hence may lead to a better representation of the vertical density structure.     

平流层大气重力波时空分布特征及其可能影响机制

大气重力波是一种普遍存在于大气层中的波动现象,与多种不同尺度天气现象均有密切联系,研究平流层重力波的时空分布特征及其可能影响机制对于全球大气环流、大尺度气候变化和各类中尺度天气系统的研究具有重要意义.利用基于PANGAEA数据中心提供的2002—2015年逐月平流层重力波参数资料和SPARC数据中心提供的1992—1997年逐月纬向风资料,分析了平流层重力波参数的时空分布特征,并讨论了影响平流层重力波变化的可能机制.结果表明,对于重力波参数的纬向平均分布,平流层重力波扰动温度和垂直波长随高度增加而增大,而水平波数和绝对动量通量则相反.在夏半球的中低纬度和冬半球的高纬度存在重力波参数的大值区,在赤道附近全年存在重力波参数的低值区.平流层重力波参数水平分布表现为纬向上的带状分布,强度随季节发生变化.在相同纬度,重力波参数的大值中心出现在大陆,特别是山脉地区.平流层纬向风和重力波参数二者的分布具有一致性,说明背景风对大气的扰动是影响重力波参数的可能机制之一.     

The influence of convergence movement on turbulent transportation in the atmospheric boundary layer

Classical turbulent K closure theory of the atmospheric boundary layer assumes that the vertical turbulent transport flux of any macroscopic quantity is equivalent to that quantity‘s vertical gradient transport flux. But a cross coupling between the thermodynamic processes and the dynamic processes in the atmospheric system is demonstrated based on the Curier-Prigogine principle of cross coupling of linear thermodynamics. The vertical turbulent transportation of energy and substance in the atmospheric boundary layer is related not only to their macroscopic gradient but also to the convergence and the di-vergence movement. The transportation of the convergence or divergence movement is important for the atmospheric boundary layer of the heterogeneous underlying surface and the convection boundary layer.Based on this, the turbulent transportation in the atmospheric boundary layer, the energy budget of the heterogeneous underlying surface and the convection boundary layer, and the boundary layer parameteri-zation of land surface processes over the heterogeneous underlying surface are studied. This research offers clues not only for establishing the atmospheric boundary layer theory about the heterogeneous underlying surface, but also for overcoming the difficulties encountered recently in the application of the atmospheric boundary layer theory.     

大气Rossby波动力学的研究进展

Rossby波是旋转大气和海洋中的一类重要波动,在天气和气候演变中起有重要的作用。该文介绍了北半球中纬度地区的两类重要Rossby波——斜压不稳定波和准定常行星波,在最近20余年来取得的一些重要研究进展。文中涉及的主要研究领域有：斜压不稳定波包动力学、湿斜压过程对斜压不稳定波的作用、北半球风暴轴动力学、斜压波包与高影响天气的预报、准定常行星波的形成机理、准定常行星波的水平传播与能量频散、准定常行星波的垂直传播与能量频散、三维准定常行星波的传播与能量频散。     

The observed generation and breaking of atmospheric internal gravity waves over the ocean

Observations are presented for internal gravity waves and their breaking at a height of 23.5 m over the ocean in surface-based inversions which are formed because of the advection of warm air over cold water. The spectral and cospectral analyses of velocity and temperature fluctuations were made to establish the characteristic features of the waves. Flow visualization photographs of smoke released during the breaking of a wave are also presented. Comparison between the turbulent energies present during and after breaking of a wave indicates enormous mixing and dispersion occurring during breaking.This research was performed under Contract No. EY-76-C-02-0016 with the United States Energy Research and Development Administration.     

Internal atmospheric gravity waves near the coast of Antarctica

Two gravity wave events as observed at Georg von Neumayer Station in Antarctica are described and analyzed. Wind and temperature are recorded at a meteorological tower. Surface pressure time series are available from four sites so that rather exact evaluations of phase speed and wavelength are possible. Radiosonde ascents provide information on the structure of the atmosphere above the boundary layer.The pressure traces of both events are dominated by sinusoidal oscillations with a well defined frequency. Related variations of wind and temperature are small during the first event (16 July 1986) as are those of temperature on 29 September 1986. However, wind oscillations are quite large during this second event. An attempt is made to interpret the data in the light of linear gravity theory. It is found that linear gravity waves of frequency and phase speed as observed were able to propagate throughout the troposphere on 16 July. We conjecture on the basis of linear theory that the wave of 29 September was propagating on the surface inversion.     

Spatial correlation functions of surface-layer atmospheric turbulence in neutral stratification

The longitudinal (i.e., in the direction of the mean wind) spectra and cospectra of wind components and temperature fluctuations in the atmospheric surface layer during neutral conditions were carefully investigated by Kader (1984, 1987) for a broad range of wave numbers which included wavelengths far beyond the large-scale limit of the inertial subrange. At the same time, some direct measurements of spatial correlation functions of the longitudinal wind component and temperature were performed by Zubkovskii and Fedorov (1986) and Zubkovskii and Sushko (1987). Section 2 of the present paper gives a review of the available results on longitudinal spectra and cospectra of wind velocity and temperature fluctuations in neutral stratification and examines the consequences of these results related to the longitudinal autocorrelation and symmetrized cross-correlation functions of surface-layer turbulence. In Section 3 it is shown that the correlation equations of Section 2 agree satisfactorily with some recent measurements of the longitudinal correlation functions in the range of distances from 3 m to 100 m. Some measurements of the lateral correlation functions of atmospheric turbulence are also presented in Section 3. It is shown that these measurements lead to some predictions concerning the never-measured lateral space spectra of surface-layer turbulence.     

Effect of thermal stratification on the growth of the internal boundary layer

The growth of a two-dimensional internal boundary layer (IBL), which develops when a neutral or unstably stratified flow over a uniform terrain encounters a step-change in surface roughness, is numerically investigated by a higher-order turbulence closure theory. It is found that the thickness of the IBL increases as ~ x ⁿ, where x is the downstream distance from the roughness-transition line. For a given set of upstream conditions, the value of the exponent n depends only on the Monin-Obukhov length L, and it is approximately independent of the roughness-change parameter M = In (z01/z02). At large fetches, increases markedly with increasing instability.NRC-NAS Resident Research Associate at AFCRL, 1973–74