A comparison of the radar ray path equations and approximations for use in radar data assimilation

The radar ray path equations are used to determine the physical location of each radar measurement. These equations are necessary for mapping radar data to computational grids for diagnosis, display and numerical weather prediction (NWP). They are also used to determine the forward operators for assimilation of radar data into forecast models. In this paper, a stepwise ray tracing method is developed. The influence of the atmospheric refractive index on the ray path equations at different locations related to an intense cold front is examined against the ray path derived from the new tracing method. It is shown that the radar ray path is not very sensitive to sharp vertical gradients of refractive index caused by the strong temperature inversion and large moisture gradient in this case. In the paper, the errors caused by using the simplified straight ray path equations are also examined. It is found that there will be significant errors in the physical location of radar measurements if the earth’s curvature is not considered, especially at lower elevation angles. A reduced form of the equation for beam height calculation is derived using Taylor series expansion. It is computationally more efficient and also avoids the need to use double precision variables to mitigate the small difference between two large terms in the original form. The accuracy of this reduced form is found to be sufficient for modeling use.     

Sensitivity analysis of the Ångstrom exponent for multimodal aerosol size distributions

This study characterizes the Ångstrom exponent for polydispersed aerosol size distributions. Under the assumption of a lognormal size distribution, the dependence of Ångstrom exponent on the size distribution and the refractive index with varying real and imaginary parts are determined. Further, the influence of coarse mode particles on the Ångstrom exponent is investigated quantitatively. The results show that the nuclei mode has less influence under the simulation conditions considered in this study. It is also shown that the refractive index is an important factor influencing the Ångstrom exponent. The effect of the coarse mode on the Ångstrom exponent computed with different aerosol number concentrations and as a function of a geometric standard deviation and a geometric mean diameter is tested. It is shown that the coarse mode is crucial for determining the Ångstrom exponent.     

Light refraction by mean temperature gradients in the near-earth atmosphere

Visible light ray paths in the atmospheric surface layer are numerically computed by division of 500- to 5000-m ranges into small intervals so that the ray path height and thus refractive index gradient is nearly constant for each step. Meteorological conditions are varied by using different combinations of sensible heat fluxes, surface stresses, and surface roughnesses. Although the effect of water vapor gradients can be substantial, their effect is not included here. The results are confined to heights of less than 5 m because of the restrictive values chosen for the ratio of the eddy diffusivity of heat to that of momentum. Mirages hide lower portions of images and the minimum height seen varies approximately inversely proportionally to the observer height. Image distortion and laser beam displacement or angle of arrival can be used to determine the mean refractive index gradient, which determines the temperature gradient in the absence of large moisture gradients along the propagation path. A simple, non-iterative formula relating laser beam displacement or angle of arrival to the average temperature gradient can be used only if the beam height varies less than 10% throughout the path.     

Pressure and humidity effects on optical refractive-index fluctuations

Although temperature fluctuations dominate the variance of optical refractive index fluctuations it has been shown recently that humidity fluctuations can also be important (e.g., Friche et al., 1975). This paper reports on simultaneous measurements of temperature, humidity and pressure so that the relative importance of all three can be investigated. For the dry site where the measurements were made, the humidity contribution was less than other investigators had found. The major contribution of the pressure fluctuations was through their covariance with temperature, but this term was found to be between 0.03 and 0.4% of the total variance. The results thus confirmed that pressure fluctuations can be neglected in most circumstances. Both the temperature and humidity spectra displayed -5/3 power laws at small scales while the temperature-humidity cospectrum decreased more rapidly than a -5/3 power law. The temperature-pressure cospectrum decreased even more rapidly than the temperature-humidity cospectrum. The temperature-pressure correlation coefficient was found to be about -0.1. The humidity-pressure correlation was typically between ±0.05 and the cospectrum poorly defined.Contribution of the Bedford Institute of Oceanography.     

气溶胶元素碳的测量及其与虚折射率的相关性

大气气溶胶的虚折射率是大气气溶胶的一个重要物理参数,它表示气溶胶对光辐射的吸收特性。大气气溶胶中的元素碳是重要的吸光物质。因此,对大气气溶胶中元素碳的测量及与大气气溶胶虚折射率测量的相关性研究具有重要意义。 本文提供了利用240C元素分析仪测量大气气溶胶中元素碳含量的方法,并对大气气溶胶元素碳含量与大气气溶胶虚折射率的测量结果作了相关性分析。结果表明,两者之间存在明显的正相关。     

Aerosol polarized phase function and single-scattering albedo retrieved from ground-based measurements

Aerosol optical parameters, polarized phase function and single-scattering albdeo, have been retrieved from ground-based sun photometer measurements in Beijing 2003. The measured aerosol optical thickness varies from 0.12 to 0.77 with an average value of 0.39. The measured Ångström coefficient ranges from 0.75 to 1.47 with an average value of 1.21. The retrieved single-scattering albedo at 870 nm is within the 0.76–0.94 range and the average value is 0.85, suggests there are considerable aerosol absorptions in Beijing. The maximum value of retrieved polarized phase function at 870 nm ranges from 0.068 to 0.225 with an average value of 0.16, and it illustrates good correlations with the Ångström coefficient, i.e. the relative size of aerosol particles. Analyses of measurements and theoretical calculations show the polarized phase function is sensitive to aerosol size distribution and complex refractive index, especially the imaginary part of the refractive index which denotes aerosol light absorbing effects. These results suggest that the polarized phase function is an effective and unique aerosol optical parameter and is able to improve the retrieval of aerosol physical properties.     

不同 AMDAR资料同化时间窗对台风“摩羯”预报影响研究

2018年第14号台风“摩羯”对山东造成了大范围暴雨和大风天气,基于WRF（Weather Research and Forecasting）模式及其Hybrid-3DVAR混合同化预报系统,对Hybrid-3DVAR不同集合协方差比例和不同航空气象数据转发（aircraft meteorological data relay,以下简称AMDAR）资料同化时间窗对台风“摩羯”预报的影响进行了数值研究。结果表明：加大集合协方差比例对台风“摩羯”路径预报有较大影响和改进;当全部取来自集合体的流依赖误差协方差时,预报的台风路径最好,降水预报也最接近实况;AMDAR资料同化对于台风路径和降水预报也有正的改进作用,但加大集合协方差比例到100%时对台风路径预报影响更大;不同资料同化时间窗会影响同化的AMDAR资料数量,从而影响台风降水精细化预报;45 min同化时间窗的要素预报误差最小,对台风造成的强降水精细特征预报最接近实况;不同资料同化时间窗主要影响台风降水预报落区分布,对台风路径预报影响相对较小。     

基于功率谱的风廓线雷达回波强度定标方法

风廓线雷达已在我国得到大范围的业务布网应用,现有业务产品主要为风场信息。为了充分发挥风廓线雷达的作用,获取更多的天气过程信息,该文提出仅使用风廓线雷达返回信号功率谱进行数据定标（DCNP）的方法。使用雷达系统噪声功率对返回信号功率谱单位幅度进行标校计算,基于标校后的雷达探测功率谱分布数据计算回波强度功率谱密度分布、回波强度、大气折射率结构常数。利用2017年北京风廓线雷达、2016年南京风廓线雷达和2018年梅州风廓线雷达观测数据,对我国业务运行的3种主要型号风廓线雷达进行算法评估试验。定标方法的计算结果稳定,风廓线雷达不同探测模式之间的一致性较好。使用每个测站定标结果与相邻天气雷达数据进行比较,风廓线雷达回波强度定标结果与天气雷达也有较好的一致性。DCNP方法与基于信噪比（SNR）的强度计算方法进行比较,与SNR方法相比,DCNP方法定标结果更加稳定可靠。     

Monitoring the Sensible Heat Flux over Urban Areas using Large Aperture Scintillometry: Case Study of Marseille City During the Escompte Experiment

Abtract Sensible heat flux estimated by Large Aperture Scintillometry (LAS) has been tested against the more traditional eddy covariance technique over Marseille city centre, a reasonably homogeneous surface. Over the 3 week test period fluxes were found to be similar, yet less noisy for the LAS due to the spatial integration. No systematic bias between the estimates was found as a function of wind direction, indicating the homogeneity of the site. Sensitivity analysis of the required aerodynamic parameters shows that careful attention must be paid to the displacement height along the measurement path. Spatial variability of surface sensible heat flux is studied via a second LAS measurement path over the city.     

Representations of inverse covariances by differential operators

In the cost function of three- or four-dimensional variational data assimilation, each term is weighted by the inverse of its associated error covariance matrix and the background error covariance matrix is usually much larger than the other covariance matrices. Although the background error covariances are traditionally normalized and parameterized by simple smooth homogeneous correlation functions, the covariance matrices constructed from these correlation functions are often too large to be inverted or even manipulated. It is thus desirable to find direct representations of the inverses of background error correlations. This problem is studied in this paper. In particular, it is shown that the background term can be written into f dx|Dv(x)|^2, that is, a squared L2 norm of a vector differential operator D, called the D-operator, applied to the field of analysis increment v(x). For autoregressive correlation functions, the D-operators are of finite orders. For Gaussian correlation functions, the D-operators are of infinite order. For practical applications, the Gaussian D-operators must be truncated to finite orders. The truncation errors are found to be small even when the Gaussian D-operators are truncated to low orders. With a truncated D-operator, the background term can be easily constructed with neither inversion nor direct calculation of the covariance matrix. D-operators are also derived for non-Gaussian correlations and transformed into non-isotropic forms.     

一次短时暴雨WP-3000边界层风廓线雷达回波与多普勒雷达回波分析

运用风廓线雷达产品等对2010年7月20日出现在南京浦口区短时暴雨过程进行分析。结果表明：根据风廓线雷达产品和多普勒雷达产品在暴雨过程中的表现特征,发现在整个降水过程的不同时间段,风廓线雷达产品的水平风廓线、信号噪声比、垂直气流和折射率结构常数,以及多普勒雷达产品的基本反射率、风廓线等都表现出非常明显的特征,并且两者产品有着较好的对应关系。利用雷达回波图像可以直观地反映降水过程中大气的变化情况,证明了风廓线雷达产品对暴雨等强对流天气预测起着重要作用。     

SIMULTANEOUS DETERMINATION OF AEROSOL SIZE DISTRIBUTION AND REFRACTIVE INDEX AND SURFACE ALBEDO FROM RADIANCE－PART I: THEORY

Analyzing in detail the dependence of sky radiance on aerosol optical property and surface albedo, we present a new method for simultaneous determination of aerosol size distribution, its wavelength-dependent refractive index and surface albedo. The aerosol scattering phase function near 10o, its weighted phase function near 40o introduced in this paper, which can be inferred from sky radiance data, and the radiance near 90o are respectively used in retrieving the real part of refractive index, its imaginary part and surface albedo. Results in numerical experiments are satisfactory under different given conditions.     

青藏高原冷暖与东亚大气环流的统计分析

本文使用高原温度距平指数代表高原温度。分析指出,高原温度的季序列存在着明显的3年、5年周期,7年和11年周期也较明显。高原温度的持续性容易在秋季发生转换,并且比西太平洋副高、超长波振幅的持续性转换得早。分析还指出,青藏高原温度夏季与南亚高压的南北振荡有密切关系,而冬季又与东亚槽和新疆脊的强度有比较密切的关系。并指出,高原热状况对我国旱涝的影响应引起高度的重视。     

Intermittency of turbulence within open canopies

Eddy covariance data have been analyzed to examine intermittency and clustering properties of turbulence within open canopies. Intermittency consists of two aspects: one is related to amplitude variation and the other to clustering. Using the telegraph approximation (TA), the clustering properties have been separated from amplitude effects. Intermittency of canopy turbulence has been explored via clustering exponent, probability density distribution of inter-pulse period of TA, intermittency exponent and structure kurtosis. Intermittency and clustering properties of turbulence within open canopies show similar features to those within dense canopy but some differences are also noted. Unlike within a dense canopy, temperature does not show larger clustering than velocity, which seems to be due to a different thermal structure of the sub-canopy and larger vertical scale of canopy eddy within open canopies. Within the crown region, the inter-pulse probability distribution of TA does not show the ‘double regime’ which was observed within the crown of a dense canopy, indicating less influence of near-field source on canopy turbulence within open canopies. For TA series of the flow variables, intermittency exponent is higher for temperature than for two velocity components within open canopies, which are opposite within a dense canopy. When comparing intermittency for flow variables and their TA series, it is shown that amplitude variation mitigates intermittency for both velocity components and temperature although amplitude variations play a much larger role in velocity intermittency than in temperature counterpart. Kurtosis analysis demonstrates that structure kurtosis is higher at large scales in stable conditions than in unstable conditions, indicating the existence of global intermittency due to stable stratification. The intermittency features of canopy turbulence within open canopies have been discussed in comparison with those within a dense canopy.     

On the parameterisation of evaporation and sensible heat exchange for shallow lakes

Summary This paper demonstrates that typical air–sea interaction models can be transferred to shallow water conditions and lakes including a depth dependent function. As depth decreases, it is assumed that exchange conditions increase. From our results it follows that the model without the shallow water correction underestimates the latent and sensible heat fluxes of a lake by about 20%. This was shown through the comparison of the model with eddy covariance data for a shallow lake during the LITFASS-98 experiment in Germany. Furthermore, data selection according to the fetch and footprint conditions is presented. These models are also useful for gap filling procedures at measuring stations over lakes with limited fetch conditions in selected directions.     

微波波段大气折射率结构常数的仿真研究

大气折射率结构常数描述了大气湍流起伏的强弱,它表征了大气折射率随机不均匀性的剧烈程度。光波和无线电波在大气中传播时会受到大气湍流的影响而产生各种不良效应,如:光斑漂移、闪烁、相位起伏等。因此,对大气折射率结构常数的研究具有重要的意义。本文利用常规探空资料对微波波段大气折射率结构常数进行了仿真研究,结果表明:在低空,特别是大气边界层之内,大气折射率结构常数主要为湿度所贡献;在高空,大气折射率结构常数主要为温度所贡献。在微波波段,影响大气折射率结构常数最大的气象因子并不是温湿压的大小,而是它们梯度的大小,其中湿度梯度的大小对其影响最大,在实际的低空测量大气折射率结构常数时,主要考虑湿度梯度与温度梯度的影响。     

The Northern Path of Asian Dust Transport from the Gobi Desert to North America

The aerosol index (AI) of the Total Ozone Mapping Spectrometer (TOMS) satellite data (1979 2001) was analyzed to reveal the climatological long-distance path of dust transport from Asia to North America. The AI in the west coast of the United States is highly correlated with that in the Gobi desert. Additionally, from the TOMS satellite images, it can be seen that very strong plumes advect from Asia to the west coast of North America in typical dust storm cases. When applying the sourcereceptor relationship to detect the northern dust transport path between the Gobi source region and the west coast of the United States receptor region, it is evident that the dust plume can be transported northward beyond 60°N from its source region and that it takes 5 to 6 days to reach the west coast of the United States. The cross correlation technique shown in this work is a useful tool that can be applied in other regions to give useful insights into relationships between major dust sources and downwind receptor locations by using remotely sensed dust observations.     

VARIATIONAL DATA ASSIMILATION USING WAVELET BACKGROUND ERROR COVARIANCE: INITIALIZATION OF TYPHOON KAEMI (2006)

Background error covariance plays an important role in any variational data assimilation system, because it determines how information from observations is spread in model space and between different model variables. In this paper, the use of orthogonal wavelets in representation of background error covariance over a limited area is studied. Based on the WRF model and its 3D-VAR system, an algorithm using orthogonal wavelets to model background error covariance is developed. Because each wavelet function contains information on both position and scale, using a diagonal correlation matrix in wavelet space gives the possibility to represent some anisotropic and inhomogeneous characteristics of background error covariance. The experiments show that local correlation functions are better modeled than spectral methods. The formulation of wavelet background error covariance is tested with the typhoon Kaemi (2006). The results of experiments indicate that the subsequent forecasts of typhoon Kaemi’s track and intensity are significantly improved by the new method.     

Atmosphere–Surface Fluxes of CO<Subscript>2</Subscript> using Spectral Techniques

Different flux estimation techniques are compared here in order to evaluate air–sea exchange measurement methods used on moving platforms. Techniques using power spectra and cospectra to estimate fluxes are presented and applied to measurements of wind speed and sensible heat, latent heat and CO₂ fluxes. Momentum and scalar fluxes are calculated from the dissipation technique utilizing the inertial subrange of the power spectra and from estimation of the cospectral amplitude, and both flux estimates are compared to covariance derived fluxes. It is shown how even data having a poor signal-to-noise ratio can be used for flux estimations.     

大口径闪烁仪及其在地表能量平衡监测中的应用

介绍了LAS仪器的测量原理及数据处理方法, 并利用中荷合作项目CEWBMS中获得的河南郑州LAS测站2000年的观测资料, 同时结合其它辅助资料, 对观测点附近地区的能量平衡状况进行了分析。分析结果表明, 由LAS测值得到的显热通量值, 以及结合净辐射资料间接得到的潜热通量值, 合理地反映出了当地能量平衡状况的季节变化, 显示出一年之中当地大部分的净辐射能用于潜热通量的释放。其数据结果所表征的当地下垫面干湿程度的变化与同期的降水及土壤相对湿度相比, 表现出了相当好的一致性, 这为拓展LAS在局地地表能量平衡监测中的应用提供了物理依据。