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     

Rain depolarization and attenuation measurements at 11.7, 19.04, and 28.56 GHz : A description of the experiment and some preliminary 11.7 GHz results

The authors are investigating rain depolarization and attenuation at 11.7 GHz using the CTS space craft downlink, and at 19.04 and 28.56 GHz using the COMSTAR satellite. The CTS transmitter is righthand circularly polarized while the COMSTAR polarization is linear. Operations at 11.7 GHz began in late May, 1976, and data collection at the higher frequencies began in May, 1977. In each case the earth receiver measures the amplitudes and relative phases of the copolarized and cross-polarized signal components. These will be correlated with data from a network of terrestrial weather instruments and a weather radar. This paper describes the receiving equipment used and presents a summary of the 11.7 GHz data collected thus far. Several rain events with fades in excess of 30 dB at 11.7 GHz are included; these were accompanied by sharp reductions in the cross polarization isolation. The statistical characteristics of 11.7 GHz attenuation and depolarization are also described.     

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     

Bistatic rain-scatter experiment at 34.8 GHz

A bistatic rain-scatter experiment was conducted at 34.8 GHz. The effect of severe rain-induced attenuation on received power from rain scattering was examined with the aid of a 5.33 GHz rain radar installed at the receiving site. It was found that an isolated small rainfall region with high rainfall rate caused a strong received power from rain scattering at 34.8 GHz, though the occurrence of such an event was not frequent. Angular dependence of rain scattering at 34.8 GHz was also examined.     

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。     

Rain-rate duration statistics over a five-year period: a tool forassessing slant path fade durations

A rain gauge network of 10 tipping bucket rain gauges on the Mid-Atlantic coast of the United States has been in continuous operation since June 1, 1986. Rain-rate distributions and estimated slant path fade distributions at 20 and 30 GHz covering the first five-year period have been derived from the gauge network measurements and published data of Goldhirsh, Krichevsky and Gebo (see ibid., vol.40, no.11, p.1408, 1992). In this article, we present rain-rate time duration statistics. The conversion of rain-rate duration statistics derived from in situ measurements to slant path fade duration statistics is complicated because of the vertical and lateral inhomogeneity of the rain. A benchmark set of fade duration statistics at 20 and 30 GHz for a vertical path is derived from the rain-rate duration statistics employing Crane's (1980) global model. These results may be used by investigators for comparison with and/or conversion to slant path fade duration statistics. Such statistics are important for better assessing optimal coding procedures over defined bandwidths     

The effects of rain clutter on millimeter radar performance

The effects of rain clutter on millimeter radar performance are investigated at 35, 94 and 140GHz frequencies, including rain attenuation, radar reflectivity, maximum radar range and equivalent target cross section.     

Path diversity in propagation of millimeter waves through rain

Cumulative distributions of attenuation due to rain are computed for the frequencies 11,18.5, and 30 GHz. The results are derived from a four-year sample of rainfall data obtained on a highly resolving areal rain gauge network in Bedfordshire, England. Lack of correlation of attenuation on both orthogonal and parallel paths is discussed and the effect of path diversity is demonstrated for the frequency 30 GHz. Examples of computer-generated maps of intense rain showers at the earth's surface are given.     

Bistatic scatter from rain

An experimental investigation of bistatic scatter from rain was conducted using a 143 km scatter path at frequencies of 4.5 and 7.7 GHz. The ratio of transmitted to received power (transmission loss) was measured for scattering angles ranging from6degto130deg. Simultaneous weather radar observations were made at a frequency of 1.3 GHz. Transmission loss estimates for the bistatic scatter path were computed using the weather radar data, the bistatic radar equation, and a model for the scattering cross section per unit volume of rain based upon Rayleigh scattering by an ensemble of water spheres. The measured and estimated transmission loss values were compared to test the use of the scattering model for the estimation of interference. The averaged ratio of measured-to-calculated transmission loss for the 4.5 GHz data is 1.2pm 0.4dB. The averaged ratio for the 7.7 GHz data is -1.6pm 0.5dB. Both these values are within the combined calibration uncertainties of each measurement system. The results show that the use of the simplified Rayleigh scattering cross section model for an ensemble of water spheres adequately describes bistatic scatter for a wide range of scattering angles and frequencies below 7.7 GHz for the hydrometeor types (rain, snow, and mixed rain and snow) encountered in New England.     

Predictive methods for rain attenuation using radar and in-situ measurements tested against the 28-GHz Comstar beacon signal

A program to measure the rain attenuation of the Comstar beacon signal at 28.56 GHz has been in continuous operation since March of 1977 at Wallops Island, VA. During the summer of 1977 simultaneous radar and disdrometer measurements at the site were also made and used for predicting path attenuation. The best-fit values ofaandbof the relationk = aZ^{b}were deduced for each rain period from the raindrop size measurements, wherekis the attenuation coefficient [dB/km] andZis the reflectivity factor [mm⁶/m³]. The measuredk-Zrelations and the simultaneous radar reflectivity measurements along the beacon path were injected into a computer program for estimating the path attenuation. Predicted attenuations, when compared with the directly measured ones, show generally good correlation on a case-by-case basis and very good agreement statistically after an empirical calibration adjustment is applied to the radar data. A method was also tested for predicting fade statistics at another frequency (e.g., 19 GHz) using simultaneous rain rate and fade distributions (28 GHz) in conjunction with disdrometer data. The predicted distributions showed good agreement with radar-predicted levels. The results demonstrate the utility of using radar in conjunction with disdrometer and rain gauge measurements for predicting fade events, long-term fade distributions, and establishing predictive criteria associated with earth-satellite telecommunications.     

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.     

Model of intense convective rain cells for estimating attenuation on terrestrial millimetric radio links

The movement of intense rain over a line of rapid-response rain gauges is shown to be related to the wind speed at the 700 mbar level. This relationship is used to deduce model rainfall intensity/distance profiles from intensity/time profiles recorded at a point. The model is applicable to the calculation of attenuation at frequencies above about 30 GHz.     

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.     

Derivation of parameters of Y-Z power-law relation from raindrop size distribution measurements and its application in the calculation of rain attenuation from radar reflectivity factor measurements

The work reported herein empirically derived the coefficients of the power-law relation between the specific attenuation Y and each of two other quantities: the radar reflectivity factor Z and the rainfall rate R. The derivation was accomplished using raindrop size distributions (DSDs) measured in Montreal and Toronto using the precipitation occurrence sensor system (POSS) and a Joss-Waldvogel disdrometer (JWD). The specific attenuation was calculated for both spherical and oblate spheroidal raindrops. Prior to doing so, the effects of inaccuracies in small drop detection by the two systems on the resulting Y-Z and Y-R relations were examined. In computing the relations, the influence of grouping the DSDs according to the corresponding values of Z was assessed. The results from the two sites were then combined in a regression analysis to determine the coefficients of the power-law expressions. The final expressions were used to calculate the probability distribution of rain attenuation over several path lengths at 30 GHz. Conclusions that can be drawn from this work include that (i) the uncertainties in the measurements of small drops do not appear to be critical to the derived relations for some frequencies considered in this work, (ii) in computing rain attenuation estimates from radar reflectivity factors, different criteria for grouping the reflectivity factor measurements can be adopted without changing the statistics of the corresponding attenuation estimates, (iii) differences, which are the smallest for frequencies around 20-30 GHz, are observed in the coefficients derived from the measured DSDs compared with those recommended by the ITU-R, and (iv) the attenuation statistics at 30 GHz computed by applying the Y-Z power-law expression are resistant to spatial averaging.     

Prediction model of time diversity using Japan rain radar data

The effects of rain attenuation on communication systems will become more pronounced in future satellite communication systems, especially with the planned use of the 21‐GHz band or higher‐frequency bands. Diversity techniques provide a solution to mitigate rain attenuation effects. This study proposes a time diversity technique, one such technique that is likely to demonstrate high effectiveness. To model the system, rainfall rate statistics are necessary, and reliability is improved as the amount of statistical data increases. This paper derives the cumulative distribution of the rainfall rate across Japan over 4 years using rain radar data from the automated meteorological data acquisition system and ground‐based rain radar network and evaluates the rainfall rate at 23 observation points across Japan. We carry out a performance evaluation for all locations within Japan to confirm the efficiency of the time diversity method. Finally, we propose prediction model of the time diversity gain for Japan and other significant parameter which is time correlation of rainfall rate that was found from the time diversity results for further investigation. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis of algorithms for the retrieval of rain-rate profiles froma spaceborne dual-wavelength radar

The ability to retrieve rain-rate profiles from a dual-wavelength spaceborne radar system operating at 13.6 and 35 GHz is analyzed. The fundamental problem of extracting either the attenuation and/or the reflectivity from the backscatter echo, which contains both contributions, is addressed. Three algorithms, the backscatter, the attenuation coefficient, and the dual-wavelength methods, are examined. These algorithms are tested using four rain-rate profiles derived from radar measurements. In particular, measured (true) values are compared with calculated (retrieved) rain rates applying the algorithms with superimposed uncertainties assuming a suggested spaceborne dual-wavelength radar system. Error values of rain rates are determined where these values reflect failure of the assumptions utilized in the derivation of the algorithms, rain backscatter noise, and instrument noise     

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     

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.     

The application of S-band polarimetric radar measurements toKa-band attenuation prediction

In September 1993, the National Aeronautics and Space Administration's Advanced Communications Technology Satellite (ACTS) was deployed into a geostationary orbit near 100° W longitude. The ACTS satellite employs two Ka-band beacons, one at 20.185 GHz and another at 27.505 GHz. Impairments due to rain attenuation and tropospheric scintillations will significantly affect new technologies for this spectrum. Heavy rain at Ka-band can easily produce 30 dB of attenuation along the propagation path. Propagation experiments being conducted in seven different climatic zones involve multiyear attenuation measurements along the satellite-Earth slant path. Measurements in the B2 climatic zone are made with an ACTS propagation terminal located in northeast Colorado. In order to gain move understanding about the physical processes that are responsible for Ka-band attenuation, the Colorado State University CHILL S-band polarimetric radar is used to take radar measurements along the slant path. The Colorado Front Range experiences a variety of weather conditions throughout the year ranging from upslope rain conditions to winter storms. Four such events measured along the slant path are illustrated in this paper. They include two convective cases and two “bright-band” cases. The S-band polarimetric radar data is used to initialize radar-based attenuation-prediction models, which are applied to the four precipitation events described. The comparisons of predicted attenuation to measured attenuation are quite good. It was also found during the course of the experiment that water droplets standing on the antenna surface can cause appreciable attenuation at Ka-band frequencies. That finding needs to be recognized in future model development and statistical analysis     

Predictions of radiowave attenuations due to a melting layer ofprecipitation

A melting layer model related to the physical constants and meteorological parameters is employed in this investigation. The specific phase shift, together with the specific attenuation, is computed at 1-100 GHz by using the Mie theory. The additional zenith attenuation, which is the difference between zenith attenuation due to the melting layer and attenuation due to the same thickness of the resulting rain, is comprehensively studied. The ratio of the difference to rain zenith attenuation may be over 1 at 1-5 GHz although the difference is much less than 1 dB. The difference can be over 1 dB at frequencies above 20 GHz. A minimum of the ratio is below 0.05 at frequencies about 40-60 GHz but the ratio can become a value of about 0.1 at 100 GHz. The additional attenuation should be taken into account in satellite-Earth communications and radar remote sensing. The power law parameters of the average specific attenuation of the melting layer and rain specific attenuation are tabulated for three raindrop size distributions at rain rates of below 25 mm/h. The power law method could be utilized in the additional attenuation calculation. It is a good approximation of the Mie theory results at 1-50 GHz and a useful estimate at 50-100 GHz