气球载雷达云雨杂波建模与仿真

气球载雷达升空工作,面临强大的地海杂波和云雨杂波,尤其当目标处于云雨杂波区时,雷达的探测性能将受到严重影响。该文主要研究分析气球载雷达的云雨杂波特性。首先建立了气球载雷达云雨杂波的几何模型,然后在分析影响云雨杂波特性的各种因素的基础上,导出了气球载雷达在指定距离门内云雨杂波模型的表达式,最后通过仿真给出了气球载雷达云雨杂波的距离方位维杂噪比和距离-多普勒谱特性,其结果可以为气球载雷达杂波抑制和信号处理方法的研究提供参考。     

Rain Measurement Results Derved from a Two-Polarzation Frequency-Diversity S-band Radar at Wallops Island, Virginia

A dual-polarization radar located at the NASA Wallops Flight Facility, Wallops Island, Virginia is described. This radar operates with a slow polarization switch having a cycle time of 0.7 s and also incorporates a frequency diversity technique to achieve independent sampling over short intervals of time. Rain-rate measurements derived from the dual-polarization radar and low-and high-resolution rain gauges located at a remote site are compared. Average percent differences in rainfall of less than 5 and 16 percent were demonstrated when comparing the dual-polarization radar measurement with the low-and high-resolution rain gauges, respectively. Excellent correlation of the rain rates was in evidence during one rain day. All rain measurement cases examined were limited to only light rain rates (less than 7 mm/h).     

An ERS-1 synthetic aperture radar image of a tropical squall linecompared with weather radar data

A radar image acquired by the C-band synthetic aperture radar (SAR) aboard the European Remote Sensing satellite ERS-2 over the coastal waters south of Singapore showing radar signatures of a strong tropical squall line (“Sumatra Squall”) is compared with coincident and collocated weather radar data. Squall line features such as the gust front, areas of updraft convergence, and rain areas are identified. Possible attenuation effects from the rain drops in the atmosphere under very heavy rain (rain rate >100 mm/h) is suggested. In addition, the possibility of extracting the associated geophysical parameters, i.e., rain rate and wind speed from SAR imagery is investigated. The rain rate is estimated from the attenuation signature in the SAR image. Comparison between the estimated rain rate and weather radar rain rate shows consistency. Wind speed associated with the squall line is estimated based on the CMOD4 wind scatterometer model. The estimated wind speed pattern appears to be in agreement with the observed squall line structure. Possible errors in the wind estimation due to effects of rain are suggested     

Conceptual design of rain radar for the tropical rainfall measuring mission

Specifications for a spaceborne rain radar for tropical rainfall measurement are described. A spaceborne rain radar has problems peculiar to rain observation from space. The radar must have a fast scanning mechanism to cover a large swath. Very weak rain echoes compared to the sea or land surface signal must be detected. These capabilities must be attained under the severe power consumption and mass limitations of the satellite bus. The fast scanning requirement forces application of an electrically scanning mechanism. This requirement also causes a severe limitation of the available number of independent samples. The requirement for weak rain echoes excludes application of the pulse compression technique, which is a very conventional technique for other active microwave sensors on board satellites. Under these constraints, a rain radar with an electrically scanning planar antenna at 13-8 GHz is proposed.     

Detection of rain/no rain condition on the ground based on radarobservations

Detection of rain/no-rain condition on the ground is important for application of radar rainfall algorithms. A radial basis function neural network-based scheme for rain/no-rain determination on the ground using vertical profiles of radar data is described. Evaluation based on WSR-88D radar over central Florida indicates that the rain/no-rain condition can be inferred fairly accurately     

用TOGA／COARE试验中机载雷达资料进行衰减效应订正的个案研究

为了蛎限度地利是生载测雨雷达，必须实现一些主要条件：高的空间分辨率、宽阔的覆盖面积，大的动态降雨测量范围以及精确的点测量。     

Dimension statistics of rain cell cores and associated rain rateisopleths derived from radar measurements in the mid-Atlantic coast ofthe United States

Employing a multi-year radar database of precipitation in a mid-Atlantic coast region, the authors have characterized rain cell size statistics. The measurements were made with a high resolution, high power radar in which equicircle diameter contours of resolutions of 1 km and greater were identified out to ranges of 100 km from the radar. The rain cell was constructed by its core intensity isopleth and a family of enveloping contours having defined rain rate intervals in the regions between them. The isopleth statistics were extracted from a database of cores with rain level number 7 (18-24 mm/h) and greater     

Monitoring of rain water storage in forests with satellite radar

The sensitivity of radar backscatter to the amount of intercepted rain in temperate deciduous forests is analyzed to determine the feasibility of retrieval of this parameter from satellite radar data. A backscatter model is validated with X-band radar measurements of a single tree exposed to rain. A good agreement between simulation and measurements is observed and this demonstrates the ability of radar to measure the amount of intercepted rain. The backscatter model is next applied to simulate different satellite radar configurations. To account for forest variability, the backscatter difference between a wet and dry forest canopy is calculated for four deciduous tree species, above a wet and a dry soil. On average, the simulated backscatter of a wet forest canopy is 1 dB higher than the backscatter of a dry forest canopy at co-polarized L-band and 2 dB at co-polarized C and X-band. The simulated sensitivity is in agreement with observations. It is argued that current satellites can retrieve the amount of intercepted rain at best with a reliability of 50%, due to the variability in soil moisture, species composition and system noise. The authors expect that the reliability will improve with the launch of the next generation radar satellites. The results of this analysis may also be used to assess the influence of rain, fog or dew upon other radar applications for temperate deciduous forests     

降雨对雷达探测性能的影响

文中拟合得到了由降雨的雷达体反射率反演雨衰减率的公式,适用频率1~100 GHz,并且给出了降雨存在时的雷达方程、最大作用距离以及雷达接收信号信噪比与信杂比的变化。文中的结果对降雨存在时雷达的目标检测具有指导意义。     

一种改进气象雷达测雨性能的新方法

常规气象雷达无法分离雷达反射率和雨衰减率，因此用它测雨具有一些原理上的误差。本文提出一种雷达和卫星系统反演沿地空路径降雨率分布的新方法。推导出反演沿路径雨衰减率的递推方程组。给出了实现该方法的雷达和卫星联合测雨系统，以及应用该系统测量得到沿地空路径降雨率分布的典型结果。     

雨滴谱参数估计优化方案及其微物理资料检验

提出一种新的雨滴谱参数估计优化方案。其最大优点就是：对于任意几束相邻的测雨雷达观测资料,从任意的雨滴谱模型（如Marshall-Palmer经验公式）决定的积分雨参数之间的关系（如k-Z关系）出发,利用雷达视反向率资料并借助雷达海平面或地表面回波信号,可以获取束向上的比较接近实际降雨场的雨滴谱;然后利用校正的雨滴谱参数决定的积分雨参数之间的关系,降雨量反演长法就可以提供较准确的降雨量分布。这个方案可行性被同区域机载2D－P雨滴谱仪实测的微物理资料很好地验证。     

Depolarization of radar signals due to multiple scattering in rain

The linear depolarization ratio (LDR) of radar returns from rain is studied. Pulse intensities of copolarized and cross polarized components are calculated by using the second-order solution of the time-dependent radiative transfer equation for a finite rain layer composed of spherical raindrops. Theoretical results show significant differences in LDR values between X and Ka bands for light to moderate rainfall rates and a rapid increase in the LDR reaching a level of about -8.5 dB at radar ranges near the rear edge of the rain layer. These characteristics of the LDR are in good agreement with observations from an air-borne dual-frequency, dual-polarized radar and suggest that a part of the depolarized radar power is caused by second-order multiple scattering effects     

Melting layer attenuation at 28.6 GHz from simultaneous comstar beacon and polarisation diversity radar data

Attenuation data at 28.6 GHz obtained from measurements of the Comstar beacon show that, for moderate rain, slant path attenuation may significantly exceed that calculated from simultaneous radar reflectivity measurements. Polarisation diversity radar data were used for positive identification of the rain and the melting layer, and for estimating the rain attenuation along the path. These results indicate that the melting layer attenuation is significant.     

Rain Cell Size Statistics Derived from Radar Observations at Wallops Island, Virginia

Results of an investigation are given describing two-di-two-dimensional rain cell size statistics employing radar and disdrometer data bases spanning more than 5 years and all seasons. Radar data were obtained employing the SPANDAR radar located at the NASA Wallops Flight Facility, Wallops Island, Virginia. These data corresponded to approximately 100 low-elevation azimuthal antenna scans encompassing 17 rain days. For each of these rain days, disdrometer data were also obtained. Analysis of the regression relations relating the rain rates to the disdrometer data enabled the determination of least square fit radar reflectivity factors. The individual disdrometer results along with the radar data were employed with a contouring program. More than 22 000 contours were generated, where each isopleth belonged to predefined rain rate intervals. Computing the areas of each of these contours, statistics were generated relating the equi-circle contour diameters and rain rate categories. Two types of contour have been analyzed. One pertained to the above-described isopleth regardless of the rain rate levels interior to it. Another type corresponded to those isopleths in which the rain rates interior to it were equal or greater than the isopleth values. These isopleths were referred to as "total" and "cell" contours, respectively. An abundance of total and cell contours were observed belonging to all rain rate categories. In particular, a dominant number of smallarea contours were observed belonging to the lower rain rate levels. The results showed that the average and median "equi-circle" cell contour diameters were 2.4 and 1.9 km, respectively.     

毫米波测云雷达回波信号衰减补偿仿真研究

针对毫米波测云雷达受云雨衰减的问题,从雷达I、Q信号角度出发,模拟毫米波测云雷达回波I、Q信号并假设一定的探测环境,进行雷达回波信号衰减补偿仿真试验.试验结果表明,雷达电磁波信号穿过雨区和云区时,在远距离处衰减量很大,通过衰减补偿,信号强度明显增强,目标信号突出.     

Rain, radiometry, and radar

A remote sensing system is proposed for quantitative operational measurement of the amount of liquid per unit volume in heavy rain. In particular, a 6-GHz radiometer in conjunction with a 10-cm wavelength radar is discussed. An experimental example involving a discrete heavy rain cell with a derived rainrate of 150 mm/h measured by a 6-GHz radiometer/3-GHz radar combination is given. The effect of clear air brightness temperatures, from oxygen and water vapor, on the 6-GHz radiometric rain measurements is analyzed, and an equation relating the 6-GHz brightness temperature from the rain to the radar backscattered power is derived. Design factors important in an operational observing system are also discussed     

Measurements of cloud contribution to rain millimeter wave attenuation by using a radar and radiometers

The measurement technique of cloud contribution to rain attenuation and the equipment consisted of the coherent pulse Doppler radar at wavelength λ=3.2 cm, the radiometers at λ=0.4; 0.8 and 1.35 cm and apparatus for signal recording and processing is described. The results of such measurements are given. The Doppler spectrum of the rain backscattered radar signal was used for determination of rain drop size distribution height profile then rain attenuation was calculated and cloud attenuation was determined as the difference between the total attenuation measured by using the radiometers and rain attenuation. The results of this work gave possibility to improve the known rain model of P.Misme for prediction of rain attenuation statistics for Earth-satellite links at millimeter wave.     

机载雷达气象方式的降雨补偿方法设计与实现

高空降雨云层作为一种气象目标,也是对雷达波的吸收体,降雨路径上造成的衰减会对气象信息的探测带来严重的影响。文中设计了一种具有降雨路径补偿的机载雷达气象方式,引入了降雨区衰减和降雨率的关系,将近端降雨区的衰减量作为参数解算远端降雨率,并对其原理、算法和实现方法作了详细阐述。通过对实测数据处理验证表明:具有降雨路径补偿的气象方式可以更准确地反映气象环境,保证飞行安全。     

Scattering of radiowaves by a melting layer of precipitation inbackward and forward directions

A melting layer of precipitation is composed of melting snowflakes (snow particles); the assumption of spherical particles along with mass conservation is used. The melting layer is studied by deriving the size distribution of the melting snow particles, the thickness of a melting layer, the density of a dry snow particle, and the average dielectric constant of a melting snow particle. Vertical profiles of radar reflectivity and specific attenuation are computed at 1-100 GHz by using the Mie theory for five raindrop size distributions at rain rates below 12.5 mm/h. The radar bright band is explained with computed radar reflectivities at 3-10 GHz. It is shown that the radar bright band can be absent in the melting layer at frequencies above 20 GHz. This agrees with radar observations at 35 and 94 GHz. The specific attenuation, as well as the average specific attenuation of the melting layer, is divided into absorption part and scattering part. The latter is increasingly significant with the increase of frequency. The total zenith attenuation due to stratiform rain is divided into the rain zenith attenuation and the additional zenith attenuation, which is the difference between zenith attenuation, due to the melting layer, and attenuation, due to the same path length of the resulting rain. The additional zenith attenuation increases with the increase of rain rate even at frequencies above 20 GHz. This should be taken into account in radar remote sensing and satellite-Earth communications     

Rainfall parameters from disdrometer dropsize measurements at a tropical station

The rain dropsize distributions measured by a disdrometer in 21 rain events consisting of 1, 640one-minute dropsize spectra have been analyzed for a tropical station, Ile-Ife (geog. lat. 7.5°N, long 4.5°E) in South-Western Nigeria. Empirical power law relations of the form Y = aR^b have been obtained between the rainfall rate R, the radar reflectivity factor Z, liquid water content M, optical extinction σ and the median volume diameter D₀ for three types of rainfall. A power law relationship was also obtained between the median volume diameter D₀ and the liquid water content M. The variability in a and b with rain parameters, rain events and rain types is discussed. The empirical relations have been compared with existing relations for other parts of the world, for their utilization in radar meteorology and radar engineering.