Wave generation on the air-sea interface

The sea state and the air flow above the sea during active wave generation is discussed. From energy balance considerations, a relationship between the wind duration and the phase speed of the waves at the peak of the energy spectrum is derived and compared with previous experimental results. It is shown that fluid viscosity plays a negligible role in the transfer of momentum from the air to the sea. Consequently the drag coefficient for the air-sea interface is related only to the apparent roughness of the sea surface.     

热带气旋过程中海-气界面热量交换

为探索热带气旋与海洋相互作用,采用国家海洋局南海分局Marex(马瑞克斯)数据浮标实测资料,计算了1986年南海的7个热带气旋海气界面热量交换值.结果表明:热带气旋海气界面热量交换强烈,主要贡献来自潜热通量;热带气旋环流内水温、气温均是下降趋势,气温下降更为明显;夏季热带气旋环流内,感热通量会出现负值,海面有效反射辐射通量出现减弱现象;秋季热带气旋环流内,感热通量和海面有效反射辐射通量显著加强;在热带气旋环流内,海面吸收的短波辐射通量均出现减弱现象;热带气旋环流内受到冷空气影响时,感热变得相当重要,热带气旋表现为对海洋的响应为主.     

The parameterisation of the energy exchange across the air-sea interface

A 4-layer model is presented to determine the energy exchange in the surface layer of the atmosphere above the sea, which includes the exchange in the turbulent layer (with and without the influence of stratification), the buffer and molecular layer. The results are in good agreement with experimental data, which can also be said about the dependence of the drag coefficient and the Stanton and Dalton numbers on the friction velocity and the stability, the stability dependence being smaller than in earlier investigations without a multilayer model.     

Measurement of Prandtl Number as a Function of Richardson Number Avoiding Self-Correlation

The empirical dependence of turbulence Prandtl number (Pr) on gradient Richardson number (Ri) is presented, derived so as to avoid the effects of self-correlation from common variables. Linear power relationships between the underlying variables that constitute both Pr and Ri are derived empirically from flux and profile observations. Pr and Ri are then reconstructed from these power laws, to indicate their interdependence whilst avoiding self-correlation. Data are selected according to the stability range prior to regression, and the process is iterated from neutral to higher stability until error analysis indicates the method is no longer valid. A Butterworth function is fitted to the resulting Pr ⁻¹(Ri) regression to give an empirical summary of the analysis. The form suggests that asymptotically Pr ⁻¹ decreases as Ri ^3/2. Scatter in the data increases above Ri ~ 1, however, indicating additional constraints to Pr are not captured by Ri alone in this high stability regime. The Butterworth function is analytic for all Ri > 0, and may be included in suitable boundary-layer parameterisation schemes where the turbulent diffusivity for heat is derived from the turbulent diffusivity for momentum.     

2016年4—10月我国主要暴雨天气过程简述

利用杭瑞高速洞庭湖大桥测风塔上的三维超声风速仪观测资料以及岳阳气象站逐小时降水资料,分析了2016年7月3—5日湖南岳阳一次大暴雨过程的三个不同时段近地层湍流特征。结果表明：(1)强降水来临前180—230 min和结束前10—50 min的近地层物理量场出现异常,且强降水来临前的物理量场异常出现时间较结束前早。风速、湍流强度、湍流动能在大暴雨过程中出现异常偏大,强降水来临前230 min,风速出现最大值4.875 m·s^-1;强降水结束前50 min,湍流强度出现最大值,水平纵向、水平横向、垂直向的湍流强度分别为1.255、1.173、0.195。(2)强降水来临前与结束前,湍流功率谱密度出现异常偏大。大暴雨过程中的湍流功率谱密度减小,湍流功率谱密度最大值的频率左移。(3)强降水来临前的湍涡尺度最大,强降水期间的湍涡尺度次之,强降水结束后的湍涡尺度在三者中最小,低频大涡为大暴雨过程中的湍流活动提供了能量。

     

A note on the analogy between momentum transfer across a rough solid surface and the air-sea interface

The aerodynamic classification of the resistance laws above solid surfaces is based on the use of a so-called Reynolds roughness number Re _s =h _s u _*/, whereh _s is the effective roughness height, -viscosity,u _*-friction velocity. The recent experimental studies reported by Toba and Ebuchi (1991), demonstrated that the observed variability of the sea roughness cannot be explained only on the basis of the classification of aerodynamic conditions of the sea surface proposed by Kitaigorodskii and Volkov (1965) and Kitaigorodskii (1968) even though the latter approach gains some support from recent experimental studies (see for example Geernaertet al. 1986). In this paper, an attempt is made to explain some of the recently observed features of the variability of surface roughness (Toba and Ebuchi, 1991; Donelanet al., 1993). The fluctuating regime of the sea surface roughness is also described. It is shown that the contribution from the dissipation subrange to the variability of the sea surface can be very important and by itself can explain Charnock's (1955) regime.     

理查逊数与中尺度扰动稳定性的关系

本文通过用摄动方法求解对数压力坐标系下原始方程组的线性解,来研究理查逊数与中尺度扰动的稳定性的关系,得出了零级近似和一级近似下对称不稳定以及零级近似几种特殊的斜压不稳定判据。     

On the Scale-dependence of the Gradient Richardson Number in the Residual Layer

We present results of a technique for examining the scale-dependence of the gradient Richardson number, Ri, in the nighttime residual layer. The technique makes use of a series of high-resolution, in situ, vertical profiles of wind speed and potential temperature obtained during CASES-99 in south-eastern Kansas, U.S.A. in October 1999. These profiles extended from the surface, through the nighttime stable boundary layer, and well into the residual layer. Analyses of the vertical gradients of both wind speed, potential temperature and turbulence profiles over a wide range of vertical scale sizes are used to estimate profiles of the local Ri and turbulence structure as a function of scale size. The utility of the technique lies both with the extensive height range of the residual layer as well as with the fact that the sub-metre resolution of the raw profiles enables a metre-by-metre ‘sliding’ average of the scale-dependent Richardson number values over hundreds of metres vertically. The results presented here show that small-scale turbulence is a ubiquitous and omnipresent feature of the residual layer, and that the region is dynamic and highly variable, exhibiting persistent turbulent structure on vertical scales of a few tens of metres or less. Furthermore, these scales are comparable to the scales over which the Ri is less than or equal to the critical value of Ri _c of 0.25, although turbulence is also shown to exist in regions with significantly larger Ri values, an observation at least consistent with the concept of hysteresis in turbulence generation and maintenance. Insofar as the important scale sizes are comparable to or smaller than the resolution of current models, it follows that, in order to resolve the observed details of small Ri values and the concomitant turbulence generation, future models need to be capable of significantly higher resolutions.     

Observational Support for the Stability Dependence of the Bulk Richardson Number Across the Stable Boundary Layer

The bulk Richardson number ( $Ri_{Bh}$ ; defined over the entire stable boundary layer) is commonly utilized in observational and modelling studies for the estimation of the boundary-layer height. Traditionally, $Ri_{Bh}$ is assumed to be a quasi-universal constant. Recently, based on large-eddy simulation and wind-tunnel data, a stability-dependent relationship has been proposed for $Ri_{Bh}$ . In this study, we analyze extensive observational data from several field campaigns and provide further support for this newly proposed relationship.     

里查森数对α中尺度涡旋波不稳定的影响

作者讨论了里查森数对α中尺度涡旋波不稳定的影响.结果表明:α中尺度涡旋波的失稳与里查森数有很大关系;当里查森数不太大时,才存在α中尺度涡旋波的不稳定;里查森数越小,越容易出现斜交型不稳定,且斜交型不稳定扰动的波长越短.此时在α中尺度波段以斜交型不稳定占优;在弱稳定层结下,更有利于出现α中尺度涡旋波的不稳定,而大的风切变仅有利于该不稳定增长率的增大.     

Improving Stable Boundary-Layer Height Estimation Using a Stability-Dependent Critical Bulk Richardson Number

For many decades, attempts have been made to find the universal value of the critical bulk Richardson number ( $Ri_{Bc}$ ; defined over the entire stable boundary layer). By analyzing an extensive large-eddy simulation database and various published wind-tunnel data, we show that $Ri_{Bc}$ is not a constant, rather it strongly depends on bulk atmospheric stability. A (qualitatively) similar dependency, based on the well-known resistance laws, was reported by Melgarejo and Deardorff (J Atmos Sci 31:1324–1333, 1974) about forty years ago. To the best of our knowledge, this result has largely been ignored. Based on data analysis, we find that the stability-dependent $Ri_{Bc}$ estimates boundary-layer height more accurately than the conventional constant $Ri_{Bc}$ approach. Furthermore, our results indicate that the common practice of setting $Ri_{Bc}$ as a constant in numerical modelling studies implicitly constrains the bulk stability of the simulated boundary layer. The proposed stability-dependent $Ri_{Bc}$ does not suffer from such an inappropriate constraint.     

Effect of the Entrainment Flux Ratio on the Relationship between Entrainment Rate and Convective Richardson Number

The parameterization of the dimensionless entrainment rate (w _e /w _*) versus the convective Richardson number (Ri _δθ ) is discussed in the framework of a first-order jump model (FOM). A theoretical estimation for the proportionality coefficient in this parameterization, namely, the total entrainment flux ratio, is derived. This states that the total entrainment flux ratio in FOM can be estimated as the ratio of the entrainment zone thickness to the mixed-layer depth, a relationship that is supported by earlier tank experiments, and suggesting that the total entrainment flux ratio should be treated as a variable. Analyses show that the variability of the total entrainment flux ratio is actually the effect of stratification in the free atmosphere on the entrainment process, which should be taken into account in the parameterization. Further examination of data from tank experiments and large-eddy simulations demonstrate that the different power laws for w _e /w _* versus Ri _δθ can be interpreted as the variability of the total entrainment flux ratio. These results indicate that the dimensionless entrainment rate depends not only on the convective Richardson number but also upon the total entrainment flux ratio.     

Probabilistic comparison between turbulence closure model and Bulk Richardson Number approach for ABL height estimation using copula

In this paper, a numerical study has been carried out to assess changes in Atmospheric Boundary Layer (ABL) height under changing hydro-climatic conditions. ABL height is estimated by using the data from experiments conducted at three different sites- LAnd Surface Process EXperiment (LASPEX-97) in Sabarmati river basin, Gujarat, India, during 1997; Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) in Lannemezan, France, during 2011 and Integrated Global Radiosonde Archive (IGRA) in Tucson at Arizona, USA, during 2011. Here ABL height is estimated, during Intensive Observation Period (IOP), using 1-D turbulence closure model at all three site locations as well as following the Bulk Richardson Number (BRN) Approach. For LASPEX-97 only, the 1-D model with module for sub-surface processes coupled with upper-surface model is used for the estimation and compared with the only upper-surface model. ABL height, estimated by the coupled model, is found to be slightly higher than that by the un-coupled model. A copula-based analysis indicates that ABL height estimated by the 1-D model and same by BRN-based approach are statistically similar within 90% confidence interval (CI) under similar stratification condition. Thus, it validates the 1-D turbulence closure model for ABL height estimation. The sensitivity analysis of height of ABL, derived by 1-D model, against critical parameters reveals that surface condition (soil temperature) is more critical than atmospheric condition (net radiation) in forcing change on height of ABL in summer.     

Chanchu台风(0601)流出层的低理查森数特征分析

对流层高层存在流出层是台风环流结构的一个典型特征,但是关于台风流出层层结及不稳定特征的分析相对较少。本文利用了非静力中尺度WRF模式输出的0601号Chanchu台风的模式模拟资料,以Ri(理查森数) < 1为判据,分析了存在于Chanchu台风流出层的低理查森数,以及它的分布特征与形成原因。分析表明,在Chanchu台风流出层10~16 km高度附近,低理查森数出现频率较大,甚至达到湍流标准,这与Emmanuel(2011) 推测得出的结论一致。通过对比台风发展过程发生、发展、成熟和消亡阶段流出层低理查森数的状况,发现低理查森数的出现频率与Chanchu台风强度有较好的对应关系,即Chanchu台风强度越强,低理查森数出现的频率越高,范围也越大,同时,Chanchu流出层的低理查森数的存在及分布特征受到水平风的垂直切变和稳定度影响。在Chanchu台风强度缓变过程中,低理查森数的分布还存在日变化,这与太阳辐射日变化导致的辐射加热和冷却造成的稳定度变化有关。分析揭示出存在于Chanchu台风流出层的低理查森数层的结构特征,也验证了低理查森数层对Chanchu台风的强度和辐射日变化具有敏感性。     

Richardson Number in a Moist Atmosphere and Its Application in the Analysis of Heavy Rainfall Events

To better describe the stability of a moist atmosphere,a new Richardson number,the Richardson number in a non-uniformly saturated moist atmosphere(Ri~*),is defined in this paper.Ri~* is the same as the Richardson number(Ri) except that the generalized potential temperature introduced by Gao et al.in 2004 is used to calculate the Brunt-Vaisala frequency.Then,with outputs from simulations of two heavy rainfall events,Ri~* is applied to diagnosing instabilities of rainfall areas.The results show that convec...     

Height dependence of diurnal variations of wind velocity and water temperature near the air-sea interface of the tropical Atlantic

From measurements during the Atlantic Trade Wind Experiment (ATEX) 1969, amplitudes and phases of the diurnal harmonic water-temperature variation between the sea surface and 50-m depth and of the semi-diurnal wind variation between 1 and 8 m were obtained. If the vertical diffusion of heat in the ocean is thought to be constant, a coefficient of K= 320 cm² s^–1 in the equation of diffusion fits best the observed data in the mixed layer. However, the measurements point to a decrease of K with depth.The height variation of the semi-diurnal zonal wind wave is caused by the influence of eddy viscosity. Our data are well fitted by results of the equation of diffusion, using the assumption of Lettau (1974) that the transfer coefficient of vertical transport of momentum is not only a function of height but also depends on time because of the semi-diurnal variation of surface stress.     

The Critical Richardson Number and Limits of Applicability of Local Similarity Theory in the Stable Boundary Layer

Measurements of atmospheric turbulence made over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to determine the limits of applicability of Monin–Obukhov similarity theory (in the local scaling formulation) in the stable atmospheric boundary layer. Based on the spectral analysis of wind velocity and air temperature fluctuations, it is shown that, when both the gradient Richardson number, Ri, and the flux Richardson number, Rf, exceed a ‘critical value’ of about 0.20–0.25, the inertial subrange associated with the Richardson–Kolmogorov cascade dies out and vertical turbulent fluxes become small. Some small-scale turbulence survives even in this supercritical regime, but this is non-Kolmogorov turbulence, and it decays rapidly with further increasing stability. Similarity theory is based on the turbulent fluxes in the high-frequency part of the spectra that are associated with energy-containing/flux-carrying eddies. Spectral densities in this high-frequency band diminish as the Richardson–Kolmogorov energy cascade weakens; therefore, the applicability of local Monin–Obukhov similarity theory in stable conditions is limited by the inequalities Ri < Ri _cr and Rf < Rf _cr. However, it is found that Rf _cr  =  0.20–0.25 is a primary threshold for applicability. Applying this prerequisite shows that the data follow classical Monin–Obukhov local z-less predictions after the irrelevant cases (turbulence without the Richardson–Kolmogorov cascade) have been filtered out.     

Classification of Vertical Wind Speed Profiles Observed Above a Sloping Forest at Nighttime Using the Bulk Richardson Number

Wind speed profiles above a forest canopy relate to scalar exchange between the forest canopy and the atmosphere. Many studies have reported that vertical wind speed profiles above a relatively flat forest can be classified by a stability index developed assuming wind flow above a flat plane. However, can such a stability index be used to classify vertical wind speed profiles observed above a sloping forest at nighttime, where drainage flow often occurs? This paper examines the use of the bulk Richardson number to classify wind speed profiles observed above a sloping forest at nighttime. Wind speed profiles above a sloping forest were classified by the bulk Richardson number Ri _B . Use of Ri _B distinguished between drainage flow, shear flow, and transitional flow from drainage flow to shear flow. These results suggest that Ri _B is useful to interpret nighttime CO₂ and energy fluxes above a sloping forest. Through clear observational evidence, we also show that the reference height should be high enough to avoid drainage-flow effects when calculating Ri _B .