On the measurement of second moments of turbulent wind velocity with a single Doppler radar over non-homogeneous terrain

The measurement of turbulent wind quantities with a single Doppler radar requires a horizontal homogeneity assumption. When the terrain is not horizontally homogeneous, then the measurement of the various moments is contaminated by the gradients of these moments. However, by scanning the radar at three different elevation angles, the contamination from these gradients can be removed from the second-moment quantities of interest.     

单站暴雨成因分析和多普勒雷达资料的应用

通过单站多普勒雷达资料的应用，对哈尔滨市一次短时暴雨进行成因分析，利用单站多普勒雷达资料的显示，结合卫星云图和高空地面的形势配置，从动力、水汽和热力条件，以及触发机制等几个方面对这次短时暴雨的发生进行了临近预报分析。     

单多普勒雷达SVVP风场反演及应用

针对单多普勒雷达风场反演VVP方法中存在的线性方程组的病态矩阵问题,使用改进的SVVP方法,通过多种模拟风场信号对其进行检验和分析,并给出两个反演实例:2009年江苏雨雪天气和安徽冰雹个例。模拟和实例表明SVVP方法反演的风场结构合理,对于反演强对流天气比较有效。     

多普勒雷达与L波段雷达垂直风廓线对比分析北大核心

利用2012-2017年阜阳多普勒雷达与L波段雷达测风数据进行对比分析,统计两者的相关性和测量误差,进一步了解多普勒雷达风廓线产品的准确性和可信度。结果表明:两者测风结果一致性较好,风向和风速相关系数分别为0.97和0.94,标准差分别为19.5°和2.65 m·s^(-1)。多普勒雷达风速总体上在同一高度比L波段雷达风速偏小,两者风速相对偏差平均为24.48%;风速标准差随高度增高呈增大趋势,在降水期间对比差值小于非降水;风向标准差在7 km以下呈递减趋势,8 km以上有小幅增加趋势;风速相关系数随高度增加呈增大趋势,除低空偏低以外,其他高度相关系数均较高。     

Measurement of wind parameters and heat flux with the sensitron Doppler SODAR

The authors present the results of a comparison of wind parameters and heat flux inferred from Doppler SODAR (Sensitron/Sweden) with direct measurements using an acoustic anemometer (Kaijo-Denki, DAT 300) and a platinum wire thermometer. Rather important are the results of a calibration method for c _t ² from measurements of temperature standard deviation, and of an underestimation of the wind speed by Doppler SODAR. An operational means to calculate the flux of sensible heat on the basis of SODAR measurements is studied.     

Vertical transport of turbulent kinetic energy in the surface layer over a paddy field

Turbulent kinetic energy and its vertical flux were measured at two heights over a paddy field. The vertical transport of turbulent kinetic energy was always downward right above the paddy field and was frequently downward at higher levels within a few metres above the crop. Contributions to the downward transport arise mainly from the turbulent kinetic energy of horizontal wind velocity components. It is shown from the analysis of probability distributions that appreciable transport takes place intermittently in a few large downward bursts and that these downdrafts are efficient for downward energy transport.In the budget of turbulent kinetic energy, the flux divergence term and the energy dissipation term are the main loss terms under unstable conditions. These terms increase in magnitude with increasing instability. Buoyant production is insufficient to balance these losses. The imbalance term involving the pressure-work term is probably one of the main energy sources in unstable conditions.     

On the nature of turbulent kinetic energy in a steep and narrow Alpine valley

This contribution investigates the nature of turbulent kinetic energy (TKE) in a steep and narrow Alpine valley under fair-weather summertime conditions. The Riviera Valley in southern Switzerland was chosen for a detailed case study, in which the evaluation of aircraft data (obtained from the MAP-Riviera field campaign) is combined with the application of high-resolution (350-m horizontal grid spacing) large-eddy simulations using the numerical model ARPS. The simulations verify what has already been observed on the basis of measurements: TKE profiles scale surprisingly well if the convective velocity scale w _* is obtained from the sun-exposed eastern slope rather than from the surface directly beneath the profiles considered. ARPS is then used to evaluate the TKE-budget equation, showing that, despite sunny conditions, wind shear is the dominant production mechanism. Therefore, the surface heat flux (and thus w _*) on the eastern slope does not determine the TKE evolution directly but rather, as we believe, indirectly via the interaction of thermally-driven cross-valley and along-valley flows. Excellent correlation between w _* and the up-valley wind speed solidifies this hypothesis.     

单多普勒雷达对一次龙卷过程的观测和分析

本文利用泰州S波段多普勒雷达观测资料和探空、地面资料对2013年7月7日发生在江苏高邮的一次龙卷过程进行分析讨论。此次龙卷过程由超级单体风暴引发,环境分析显示高邮地区低层位于急流辐合区,高层位于急流辐散区,有利于对流发展。龙卷发生前具有强对流不稳定度和中等风切变。雷达回波资料分析显示超级单体在成熟阶段出现明显的钩状回波,有界回波区以及悬垂回波的特征。旋转速度最强时,有龙卷产生,之后超级单体进入消亡过程。底层强垂直风切变和垂直速度不均匀分布,有利于激发龙卷天气的发生或者促进龙卷天气的维持发展。     

单多普勒雷达径向风同化的改进

基于现有的单多普勒雷达径向速度资料直接同化和间接同化方法,提出了一种改进同化方法,通过引入反演方程组作为观测算子,将分析单元内径向风的空间变化信息作为同化变量。同时,根据模式的分辨率和精度要求调整分析单元大小,从而能够有效地弥补数据稀释化所带来的高分辨率数据遗失问题。相对于直接同化只同化雷达观测径向风,改进方法不仅包含径向风信息,而且包含径向风的空间变化信息;相对于传统的间接同化,改进方法不直接使用强约束求解析风场,避免了将反演风场完全等同于"观测风场",能够较大程度地克服反演误差对同化效果的影响。通过理想实验和观测系统模拟试验表明:改进方法对于传统的直接同化和间接同化具有一定的兼容性,应用灵活,具有积极的改进意义。     

Turbulence intensity and turbulent kinetic energy parameters over a heterogeneous terrain of Loess Plateau

A deep understanding of turbulence structure is important for investigating the characteristics of the atmospheric boundary layer, especially over heterogeneous terrain. In the present study, turbulence intensity and turbulent kinetic energy (TKE) parameters are analyzed for different conditions with respect to stability, wind direction and wind speed over a valley region of the Loess Plateau of China during December 2003 and January 2004. The purpose of the study is to examine whether the observed turbulence intensity and TKE parameters satisfy Monin-Obukhov similarity theory (MOST), and analyze the wind shear effect on, and thermal buoyancy function of, the TKE, despite the terrain heterogeneity. The results demonstrate that the normalized intensity of turbulence follows MOST for all stability in the horizontal and vertical directions, as well as the normalized TKE in the horizontal direction. The shear effect of the wind speed in the Loess Plateau region is strong in winter and could enhance turbulence for all stability conditions. During daytime, the buoyancy and shear effect together constitute the generation of TKE under unstable conditions. At night, the contribution of buoyancy to TKE is relatively small, and mechanical shearing is the main production form of turbulence.     

Turbulence Reynolds number and the turbulent kinetic energy balance in the atmospheric surface layer

The relation between the turbulence Reynolds numberR and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale and the Kolmogorov length scale .     

Measurements of the rate of dissipation of turbulent kinetic energy, ɛ, over the ocean

In a series of cruises during the last three years, the Naval Postgraduate School Environmental Physics Group has made more than 1000 shipboard measurements of the rate of dissipation of turbulent kinetic energy, , using inertial subrange (high frequency) techniques. Utilizing the bulk-aerodynamic method to obtain the relevant Monin-Obukhov surface layer scaling parameters, the overwater dimensionless dissipation function 321-01, has been examined with unprecedented statistical certainty. The results agree well with those of Wyngaard and Coté (1971) for the stable case but they agree more closely with the parameterization of McBean and Elliott (1975) for unstable conditions. Drag coefficients computed from the data are in good agreement with the curve given by Garratt (1977).     

An improved method for determining the degree of snow crystal riming by vertical Doppler radar

An improved method is described to quantitatively determine the degree of snow crystal riming in stratiform precipitation using vertically pointing Doppler radar. Using this method, an automatic determination of the degree of riming at any height above the melting layer is possible. The quantitative statement of snow crystal riming with a limited uncertainty is obtained by a strict separation of stratiform and convective precipitation. For stratiform precipitation, the uncertainty of the degree of riming determined by this remote sensing technique typically remains below one unit on a six step riming scale with somewhat larger uncertainties for very weak riming. A criterion is given for when the situation is stratiform enough to use the proposed method for determining the degree of snow crystal riming. This method is usefull in all studies where direct crystal observation is not possible and the degree of riming is needed with good time resolution over a longer time period and in different altitudes.     

On the downward transport of turbulent kinetic energy in the surface layer over plant canopies

Downward fluxes of turbulent kinetic energy have been frequently observed in the air layer just above plant canopies. In order to investigate the mechanism for such downward transport, analysis of observational data is attempted. Height-dependency of turbulent kinetic energy flux and turbulence statistics including higher order moments is represented as a function of a non-dimensional height z/H, where z is an observational height and H an average height of plant canopies. Downward fluxes and non-Gaussianity of wind velocity fluctuations are predominant just above plant canopies and decrease with increasing height. The downward flux is closely related to the high intensity of turbulence and the non-Gaussianity of wind velocity fluctuations, especially with a positive skewness in the longitudinal wind and a negative skewness in the vertical wind. The analysis method of conditional sampling and averaging is applied to the present observations. The results show that the predominance of the intermittent inrush phase over the intermittent ejection phase leads to the above-mentioned non-Gaussianity. Finally, a simple explanation is given in order to interpret the turbulent flow structure in the air layer near the plant canopies, which is associated with the downward energy transport process.     

Structure of the vertical velocity field analyzed with a Doppler Sodar

Fifteen hours of Sodar echoes, collected during the 4th Environmental CEC Campaign at Turbigo, Italy, in September 1979, have been analyzed. We discuss the vertical profiles and the time evolution of the second and third statistical moments of the vertical component of wind velocity, measured at a rate of one height-profile every 6 s for a height range of 1000 m and a vertical resolution of 29 m. We also analyze the power spectra of the vertical velocity.     

一次宏下击暴流的雷达回波特征分析

利用常规探测资料、区域自动气象站资料和烟台新一代天气雷达资料,对2007年7月28日发生在烟台招远、龙口的一次下击暴流引起的灾害性大风天气进行分析。结果得出：此次下击暴流是在副热带高压边缘“上干下湿”不稳定层结中,由于高空冷空气侵入,不稳定能量得到释放,从而发展起来的多单体团风暴和弓型回波复合体所产生的,其主要特征是强反射率因子核从中高层开始发展,迅速下降到达地面,形成辐散大风,风暴前沿有阵风锋出现。     

A two-step variational method for three-dimensional wind retrieval from single Doppler radar

Summary A two-step variational method is developed to retrieve the three-dimension wind field from single-Doppler radar observation for operational applications. The first step produces a proxy background wind field, and the second step retrieves the detailed wind structures in data dense areas on a high-resolution grid. The background field is called proxy because it is obtained in the first step by a spectral analysis of the same radial-wind observations that will be used in the second step and thus the background is not independent of the observations. In the second step, the wind retrieval is constrained by the proxy background and the advection equation of liquid water content in the cost function. Tested with a dataset collected during Meiyu front heavy rainfall on 22 July 2002 in China, the two-step approach is shown to yield the best performance in comparison with a single-step approach that uses either only the proxy background field or the retrieval without the proxy background.     

特大暴风雪动力结构的雷达探测研究

利用牡丹江多普勒天气雷达观测资料对2007年3月4—5日黑龙江省东部地区一次罕见特大暴风雪的雷达回波动力结构特征进行分析。结果表明：高低空急流、风切变、冷暖平流在特大暴风雪过程中特征明显;偏南气流和冷性的东北气流构成的中尺度辐合切变线和西南急流的位置变化对降雪天气的演变过程有重要指示意义;雷达回波反射率因子在15～27dBZ之间,回波面积广,顶高在6km左右,冷暖平流明显,径向速度大,出现＂牛眼＂、速度模糊等现象,可作为暴风雪临近预报的着眼点     

Comparison of the observation capability of an X-band phased-array radar with an X-band Doppler radar and S-band operational radar

An X-band phased-array meteorological radar （XPAR） was developed in China and will be installed in an airplane to observe precipitation systems for research purposes.In order to examine the observational capability of the XPAR and to test the operating mode and calibration before installation in the airplane,a mobile X-band Doppler radar （XDR） and XPAR were installed at the same site to observe convective precipitation events.Nearby S-band operational radar （SA） data were also collected to examine the reflectivity bias of XPAR.An algorithm for quantitative analysis of reflectivity and velocity differences and radar sensitivity of XPAR is presented.The reflectivity and velocity biases of XPAR are examined with SA and XDR.Reflectivity sensitivities,the horizontal and vertical structures of reflectivity by the three radars are compared and analyzed.The results indicated that while the XPRA with different operating modes can capture the main characteristic of 3D structures of precipitation,and the averaged reflectivity differences between XPAR and XDR,and XDR and SA,were 0.4 dB and 6.6 dB on 13 July and-4.5 dB and 5.1 dB on 2 August 2012,respectively.The minimum observed reflectivities at a range of 50 km for XPAR,XDR and SA were about 15.4 dBZ,13.5 dBZ and-3.5 dBZ,respectively.The bias of velocity between XPAR and XDR was negligible.This study provides a possible method for the quantitative comparison of the XPAR data,as well as the sensitivity of reflectivity,calibration,gain and bias introduced by pulse compression.     

Measurements of the turbulent heat flux over the sea

The turbulent heat flux was measured with two instruments simultaneously over the Baltic Sea by means of the eddy-correlation method. In one observational period, a small but noticeable divergence in heat flux was found, which may be explained by the advection of colder air. The parameterization of heat flux by the bulk method leads to a value for C _Hof 1 × 10^–3.