Ground-based multifrequency microwave radiometry for rainfallremote sensing

Inversion algorithms for ground-based microwave radiometric retrieval of surface rain-rate, integrated cloud parameters, and slant-path attenuation are proposed and tested. The estimation methods are trained by numerical simulations of a radiative transfer model applied to microphysically-consistent precipitating cloud structures, representative of stratiform and convective rainy clouds. The discrete-ordinate method is used to solve the radiative transfer equation for plane-parallel seven-layer structures, including liquid, melted, and ice spherical hydrometeors. Besides ordinary multiple regression, a variance-constrained regression algorithm is developed and applied to synthetic data in order to evaluate its robustness to noise and its potentiality. Selection of optimal frequency sets and polynomial retrieval algorithms for rainfall parameters is carried out and discussed. Ground-based radiometric measurements at 13.0, 23.8, and 31.7 GHz are used for experimentally testing the retrieval algorithms. Comparison with rain-gauge data and rain path-attenuation measurements, derived from the three ITALSAT satellite beacons at 18.7, 39.6, and 49.5 GHz acquired at Pomezia (Rome, Italy), are performed for two selected cases of moderate and intense rainfall during 1998     

Microwave Signal Attenuation Over Terrestrial Link at 26?GHz in Malaysia

The effect of rain on the microwave systems is more critical especially for countries located in tropical and equatorial region that experience high rainfall rate throughout the year. In order to predict a reliable and an accurate rain prediction model, it is required to determine the one-minute integration time of rainfall rate together with direct measurement of rain attenuation. In order to counter the current trend of employing higher frequencies especially in tropical and equatorial regions, there is an urgent need to carry out studies related to the effect of rain in order to get a better rain attenuation prediction model. Therefore, the purpose of this study is to investigate the effect of rain on terrestrial microwave system operating at 26?GHz in Malaysia. The rain intensity with one minute integration time is measured at Universiti Teknologi Malaysia-Skudai (UTM-Skudai) and 99 rain gauges located throughout the Peninsular Malaysia. This study explains the detailed experimental set up and analyses of both rain rate and rain attenuation measurements. The analysis on large-scale study area includes the comparisons between the measured rainfall data and the Drainage and Irrigation Department (DID) rainfall data and also with the Malaysia Meteorological Services (MMS) rainfall data. This study has successfully proposed a new rain rate and rain attenuation prediction model and the obtained results show satisfactory performance and good agreement.     

Statistical rainstorm models: Their theoretical and physical foundations

Precipitation has become a serious source of attenuation as higher frequencies are being employed for microwave communications. System performance is strongly influenced by the quantity and character of precipitation that occurs over the links of the system. Rain appears to be the precipitation form that accounts for most of the serious attenuation occurrences. It is also for rain that the Scattering theory is most complete. Rain attenuation can be accurately predicted if the drop-size distribution along the propagation path is known. The drop spectrum determines as well the rainfall rate, radar reflectivity, and microwave emission of rain. Consequently, it is possible to make estimates of rain attenuation through indirect measurements by raingauge, radar, and radiometer. Recent experiments have confirmed that these estimates are sufficiently accurate for practical purposes. From propagation experiments and studies of the fine-scale structure of rain, data are becoming available on the horizontal extent of heavy rain areas and the way this structure influences system performance. These data have been used to formulate statistical raincell models that permit prediction of the performance of single-path and path-diversity systems. The current status of raincell models is reviewed and suggestions for future research are offered.     

Rainfall propagation impairments for medium elevation angle satellite‐to‐earth 12 GHz in the tropics

Rain attenuation is the dominant propagation impairment for satellite communication systems operating at frequencies above about 10 GHz. The rainfall path attenuation at 12.255 GHz measured at Universiti Sains Malaysia (USM) for 4 years (2 January to 5 December) is presented. This paper presents an empirical analysis of rain rate and rain attenuation cumulative distribution functions obtained using 1‐min integrated rainfall data and comparison of the measured data with data obtained from well‐established rain model attenuation predictions. Copyright © 2008 John Wiley & Sons, Ltd.     

Rain backscatter measurements at millimeter wavelengths

The US Army Ballistic Research Laboratory (BRL) conducted an experiment in 1973 to measure the properties of radar backscatter from rain at millimeter wavelengths. Rain backscatter and attenuation were measured with pulse radars operating simultaneously at 9.375, 35, 70, and 95 GHz over a wide range of rain intensities while continuous measurements of raindrop size and rainfall rate were made. This report describes the measurement technique, details of the instrumentation, the data analysis procedure, and the rain backscatter data obtained from A-scope photographs and video tapes     

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。     

5G毫米波短距离链路雨衰减特性分析

雨衰减严重影响第5代(the 5th Generation,5G)移动通信系统性能.在路径长度小于1 km的短距离链路情况下,现有雨衰减预测模型调整因子大于1,导致预测雨衰减随路径变短而增大,无法支撑毫米波短距离链路系统设计.通过分析国际电信联盟无线电通信部(Radiocommunication Sector of International Telecommunication Union,ITU-R)短距离链路雨衰减试验数据,发现湿天线衰减与路径雨衰减相当,得到毫米波短距离链路必须考虑湿天线衰减的结论.建立了湿天线衰减与降雨率的关系模型,分频段对模型参数进行了拟合,拟合结果与实测数据吻合得较好.提出了考虑湿天线衰减的短距离雨衰减建模新思路,可解决短距离雨衰减预测问题.研究结果有助于提高5G毫米波系统余量设计的可靠性.     

一种地面视距链路雨衰减预测模式

基于指数雨胞雨衰减物理模型得到的降雨率调整因子概念,通过对ITUR雨衰减数据库实验数据的分析,提出了一种降雨率调整因子公式,并利用ITUR雨衰减数据库数据回归得到了降雨率调整因子公式的参数,建立了一种利用全概率降雨分布的地面视距链路雨衰减预测模式通过与其它模式的比较表明：该模式预测精度优于其它利用全概率降雨的雨衰减预测模式,预测精度与利用概率转换的ITUR模式相当,且预测过程更加简便。     

Rain induced cross polarization at millimeter waves in a Xi'an environment

The rainfall intensity accumulative distribution, raindrop size and canting angle distributions in Xi'an(108.9 E, 34.3 N) have been obtained from data measured over a long period of time. Rain induced crose polarization in this environment has been studied. The crose polarization discrimination, XPD, was compu ted over a frequency range of 19.3 to 300 GHz for non spherical drops. The variations of XPD with frequency, rainfall rate and copolar attenuation, CPA, were investigated. The computational resulte are in good agreement with experimental data. A mathematical relationship was established between the XPD and CPA, raindrop canting angle, which result is in good agreement with experimental data. The cumulative distribution of rain induced cross polarization for eleven years and in month have been ob tained with the cumulative distribution of the rain rate from the data measured from 1975 to 1985. The role of multiple scattering also has been discussed, it is shown that the multiple scattering play an important part in short millimeter wave.     

Rainfall Dynamics Over Two Indian Tropical Stations for Radio Communication

The influence of rainfall on radio wave propagation at frequencies above 10 GHz is crucial over ground to space radio links, especially in tropical regions because of the high intensity rainfall. This often makes direct-to-home; very small aperture terminal systems operating at these frequencies to be subjected to many fade occurrences due to heavy rain. In addition to annual and worst month’s cumulative statistics, dynamic characteristics of seasonal and diurnal variations, together with average event length and separation over the day are needed to give the detailed insights for system design. This paper presents the characteristics of rain distribution based on 5-year rainfall data over two locations in India. Rain intensities for the average year and the average worst months, and monthly and diurnal dynamic characteristics of rain events were obtained. The resulting rain intensities are also compared with the relevant ITU-R recommendations. These results are useful to estimate signal outages in a year over the region and for proper planning of radio-communication in the location.     

Intercomparison of inversion algorithms to retrieve rain rate from SSM/I by using an extended validation set over the Mediterranean area

The capability of some inversion algorithms to estimate surface rain rate at the midlatitude basin scale from the Special Sensor Microwave Imager (SSM/I) data is analyzed. For this purpose, an extended database has been derived from coincident SSM/I images and half-hourly rain rate data obtained from a rain gauge network, placed along the Tiber River basin in Central Italy, during nine years (from 1992 to 2000). The database has been divided in a training set, to calibrate the empirical algorithms, and in a validation one, to compare the results of the considered techniques. The proposed retrieval methods are based on both empirical and physical approaches. Among the empirical methods, a regression, an artificial feedforward neural network, and a Bayesian maximum a posteriori (MAP) inversion have been considered. Three algorithms available in the literature are also included as benchmarks. As physical algorithms, the MAP method and the minimum mean square estimator have been used. Moreover, in order to test the behavior of the algorithms with different kinds of precipitation, a classification of rainy events, based on some statistical parameters derived from rain gauge measurements, has been performed. From this classification, an attempt to identify the type of event from radiometric data has been carried out. The purposes of this paper are to determine whether the use of an extended training set, referred to a limited geographical area, can improve the SSM/I skill in rain detection and estimation and, mainly, to confirm the validity of the physical approach adopted in previous works. It will be shown that, among all the estimators, the neural network presents the best performances and that the physical techniques provide results only slightly worse than those given by empirical methods, but with the well-known advantage of an easy application to different geographical zones and different sensors.     

Rain rate statistics and fade distributions at 20 and 30 GHzderived from a network of rain gauges in the mid-Atlantic coast over afive year period

The authors examine five years of rain rate and modeled slant path attenuation distributions at 20 and 30 GHz derived from a network of ten tipping bucket rain gauges located on the mid-Atlantic coast of the US in the vicinity of Wallops Island, VA. Distributions are derived from the variable integration time data from 1-min averages or rain rate time series. It is demonstrated that for realistic fade margins at 20 GHz and above, the variable integration time results are adequate to estimate slant path attenuations using models which require 1-min averages. An accurate empirical formula is developed to convert the variable integration time rain rates to 1-min averages. Rain rate and fade distributions corresponding to the overall network average, yearly network average, and site distributions averaged over the five-year period are derived. Significant differences in the distributions are noted for the year-to-year (temporal variability) and the worst year site-to-site (spatial variability) cases     

Effect of Atmospheric Parameters on Satellite Link

The rainfall path attenuation measured at Universiti Sains Malaysia (USM) for 4 years (January 02 to January 06) is presented. The data obtained are useful to investigate the impairment due to rainfall attenuation in satellite links operating in tropical and equatorial climates. It shows that the logarithmic function with ground rain rate deviates at very high rain rate. A rainfall rate of 130 mm/h causes the rainfall attenuation threshold (> 20 dB) to be exceeded. Maximum exceedences for rain rate and attenuation were observed during the wet months. The cumulative distributions of attenuation derived from the measured data are presented and compared with those obtained with existing prediction methods.     

Étude de l’affaiblissement de propagation à 13 GHz dû aux précipitations tropicales

Measurements of rainfall rates and attenuation due to rain at 13 GHz have been carried out in Ivory Coast during copt 81 experiment in May and June 81. Rainfall attenuation distributions for horizontal and vertical polarizations and rain rate distributions along the path are presented. Values of differential attenuation and specific attenuation are derived from experimental data as a function of rain rate. Some rain attenuation prediction models are tested against our experimental data at 13 GHz on a 19,2 km path length.     

Twelve years of rain attenuation statistics of Earth–space propagation experiment at Ka band in Toulouse

Since 2009, ONERA has been running Ka band propagation experiments in Toulouse (France, latitude 43.57°N, longitude 1.47°E). A rain gauge was also deployed on site to collect rainfall rate measurements concurrently to beacon data. Since April 2011, the beacon receiver has been recording the 20.2 GHz (vertical polarisation) Astra 3B beacon signal along a slant path of 35.1° of elevation angle. All years of the experiment had excellent data availability, hence giving 12 years of data at Ka band. First, the propagation experiment and the data processing methodology are described. Second, a statistical analysis of rain attenuation and rainfall rate is conducted. Comparisons are then performed with the prediction methods of Recommendations ITU-R P.837-7 (rainfall rate), P.618-13 (rain attenuation) and P.678-3 (variability of propagation phenomena).     

Results of 11.6 GHz radiometric experiment in Cameroon: second year

Three experiments were set up within a joint African radiometric propagation measurement program to provide data at frequencies above 10 GHz for predictive modelling. The authors report the results of the second year of measurements in Douala, Cameroon, along with mean results for the two years. These show significant departures from the ITU-R (formerly CCIR) rain climate and path attenuation models     

The GSMaP Precipitation Retrieval Algorithm for Microwave Sounders—Part I: Over-Ocean Algorithm

We develop an over-ocean rainfall retrieval algorithm for the Advanced Microwave Sounding Unit (AMSU) based on the Global Satellite Mapping of Precipitation (GSMaP) microwave radiometer algorithm. This algorithm combines an emission-based estimate from brightness temperature (Tb) at 23 GHz and a scattering-based estimate from Tb at 89 GHz, depending on a scattering index (SI) computed from Tb at both 89 and 150 GHz. Precipitation inhomogeneities are also taken into account. The GSMaP-retrieved rainfall from the AMSU (GSMaP_AMSU) is compared with the National Oceanic and Atmospheric Administration (NOAA) standard algorithm (NOAA_AMSU)-retrieved data using Tropical Rainfall Measuring Mission (TRMM) data as a reference. Rain rates retrieved by GSMaP_AMSU have better agreement with TRMM estimates over midlatitudes during winter. Better estimates over multitudes over winter are given by the use of Tb at 23 GHz in the GSMaP_AMSU algorithm. It was also shown that GSMaP_AMSU has higher rain detection than NOAA_AMSU.      

Modelling of the rainfall rate cumulative distribution for the design of satellite and terrestrial communication systems

The design of line of sight (L.O.S.) or satellite links requires annual point rainfall rate statistics for several percentages of time in the locations where the radiocommunication links are planned. Rain rate data within an average year, indeed, allow the estimation of the time percentage during which attenuation due to rain is significant, and consequently the future link margin. To meet ITU Radiocommunication Sector's Recommendations, we propose a probability law model allowing the prediction of the rainfall rate cumulative distribution in temperate and tropical climates. This model, which can be easily used, may serve as a very useful and accurate enough tool for radiocommunication engineers and scientists.     

Spatial scales of tropical precipitation inferred from TRMM microwave imager data

The local spatial scales of tropical precipitating systems were studied using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) rain rate imagery from the TRMM satellite. Rain rates were determined from TMI data using the Goddard Profiling (GPROF) Version 5 algorithm. Following the analysis of Ricciardulli and Sardeshmukh (RS), who studied local spatial scales of tropical deep convection using global cloud imagery (GCI) data, active precipitating months were defined alternatively as those having greater than either 0.1 mm/h or 1 mm/h of rain for more than 5% of the time. Spatial autocorrelation values of rain rate were subsequently computed on a 55/spl times/55 km grid for convectively active months from 1998 to 2002. The results were fitted to an exponential correlation model using a nonlinear least squares routine to estimate a spatial correlation length at each grid cell. The mean spatial scale over land was 90.5 km and over oceans was 122.3 km for a threshold of 0.1 mm/h of rain with slightly higher values for a threshold of 1 mm/h of rain. An error analysis was performed which showed that the error in these determinations was of order 2% to 10%. The results of this study should be useful in the design of convective schemes for general circulation models and for precipitation error covariance models for use in numerical weather prediction and associated data assimilation schemes. The results of the TMI study also largely concur with those of RS, although the more direct relationship between the TMI data and rain rate relative to the GCI imagery provide more accurate correlation length estimates. The results also confirm the strong impact of land in producing short spatial scale convective rain.     

Rain cell size statistics as a function of rain rate for attenuation modeling

Rain cell size statistics as a function of rain rate have been deduced by employing a radar data base of rain reflectivity data acquired over a three-year period at Wallops Island, VA. These cell statistics have important applications in slant path rain attenuation modeling and remote sensing of the earth's surface from space at frequencies above 10 GHz.