ISSN:0048-6604    

DOI:10.1029/RS019i001p00001

年代:1984

数据来源: WILEY

摘要:

This paper is a critical survey of the measurement of rainfall by remote sensors. It is shown that single‐parameter radar rainfall measurements are limited because both reflectivity and rain rate are dependent on at least two parameters of the drop size distribution, viz., representative raindrop size and number concentration. Simulations are presented which use experimental raindrop size spectra and show the improvement in accuracy attainable through the use of combinations of two and three remote measurables. The spectrum of remote measurables is then reviewed. These include path integrated techniques of radiometry and of microwave and optical attenuation. A carefully designed short‐path microwave attenuation experiment which employs these techniques is sufficiently persuasive to show that the disappointing results achieved in many others was due largely to a combination of rain sampling problems and vertical air motions between the path and the gages. Other experiments reviewed show that when paths are colinear, attenuation deduced from radar and radiometry is in good agreement with that measured directly. Several dual‐wavelength radar methods are described which were aimed at improved measurements in small range increments but have produced generally disappointing results. However, when the total path attenuation estimated in this way, or by radiometry, is used as a constraint on the retrieval of rain profiles from the radar, the results are more promising. Selected experiments involving combinations of two or more of the three measurables, radar reflectivityZ, attenuation, and/or radiometry, show considerable promise when adequate account is taken of the sampling and air motion problems. The best results in gate‐by‐gate measurements have been achieved with dual polarization or differential reflectivity (ZDR). However, even these are subject to potentially large errors when the rainfall does not behave according to a priori assumptions embodied in the technique. An accompanying paper in this issue shows that the use of a third remote parameter in addition toZandZDRoffers great promise. With a growing appreciation of the needs and the capabilities of the various techniques, the future of highly improved remote rainfall measurements see

ISSN:0048-6604    

DOI:10.1029/RS019i001p00003

年代:1984

数据来源: WILEY

摘要:

From the earliest years of radar meteorology, efforts have been devoted to multiparameter observations: the measurement of one or more signal properties, ordinarily in addition to the signal intensity, to give added information about the precipitation. Studies employing variable polarization or Doppler frequency measurement were among the first such techniques and over the years have proved fruitful in terms of the information provided. During the past decade there has been such an increase in the use of Doppler radar in meteorological research that radar‐measured wind fields are becoming nearly as routine as reflectivity fields. Research has continued in variable polarization techniques, which appear to provide a means of distinguishing remotely between various precipitation types. Investigations of the correlation between the copolarized and cross‐polarized components of the received signal have given new insight into the falling behavior of precipitation particles. A great deal of new information will evidently be made available by combining coherent signal processing techniques with variable polarization measureme

ISSN:0048-6604    

DOI:10.1029/RS019i001p00023

年代:1984

数据来源: WILEY

摘要:

Multiple‐parameter radars have now been developed to the point at which they are being used, usually by radio scientists (but not necessarily so), to provide data for the community not primarily committed to radio science (e.g., those concerned with radio engineering, meteorology, hydrology, or aerospace). At the same time, the full potential of such radars is still being explored. Most progress in these applications has been in providing data from which to model conditions on radio paths, but their use for examining the spatial distribution of various hydrometeor forms is no less promising, and, indeed, the two are closely related. This paper examines various practical applications to which multiple‐parameter radars are being

ISSN:0048-6604    

DOI:10.1029/RS019i001p00037

年代:1984

数据来源: WILEY

摘要:

A feature of the URSI Commission F Open Symposium on Multiple‐Parameter Radar Measurements of Precipitation was a lively workshop session covering each of the three broad topic areas of the meeting. Though reports of successful workshop sessions can never be fully comprehensive, this report covers the main points discussed in the three area

ISSN:0048-6604    

DOI:10.1029/RS019i001p00045

年代:1984

数据来源: WILEY

摘要:

A description is given of the effects of variations in the shape or breadth of the drop size distribution (DSD) on rainfall parameters deduced from a measurement technique which employs the differential reflectivity factorZDRand the reflectivity factor at horizontal polarizationZH. The mathematical form of the DSD used is a gamma distribution. Justification for such a form is given through consideration of varying DSD shape in nature as implied by the results of empirical analyses of other workers. Theoretical expressions are derived for rainfall rateR, liquid water contentW, and median volume diameterD0in terms of ZDR, ZH, and size distribution dependent factors. The latter calculations assume backscattering cross sections for oblate, nonoscillating raindrops falling in still air with equilibrium shapes. These expressions are used to assess quantitatively the effects of changes in DSD breadth on values ofR,W, andD0deduced from ZDRand ZH. They are also used to show the effects of measurement errors in ZDRand ZHonR,W, andD0. The potential improvement in accuracy which is possible when account is taken of DSD shape variations is shown by simulating a (ZDR,ZH) dual‐measurement method using experimental raindrop size spectra. Methods by which DSD shape variations could be detected through the use of a third remote measurable are discusse

ISSN:0048-6604    

DOI:10.1029/RS019i001p00049

年代:1984

数据来源: WILEY

摘要:

The differential reflectivity (ZDR) radar signal contains information on the shape and alignment of ice phase hydrometeors. Under certain circumstances, then, this signal and reflectivity factor can be used to identify the presence of hail particles. Such interpretations require knowledge about the scattering properties of various types and shapes of hydrometeors, including ice, water‐coated ice, and graupel. These hydrometeors are examined for their differential scattering properties obtained from computations using the transition (T) matrix method of Waterman and an extension of this theory to two‐layered bodies. Differential radar cross sections together with circular depolarization ratios are presented, and several instances of possible hail detection by radar using ZDRsignatures are illustra

ISSN:0048-6604    

DOI:10.1029/RS019i001p00058

年代:1984

数据来源: WILEY

摘要:

Raindrop oscillations have been modeled using a potential flow, ellipsoidal variation about an equilibrium distortion along with a potential energy function that includes surface and gravitational energy. Two degenerate modes were investigated, the axisymmetric (vertical) oscillation and the ellipsoidal (horizontal) oscillation, and found to have different time‐averaged axis ratios and oscillation frequencies. Averaged axis and backscatter ratios were calculated for oscillations in a steady state balance between collisional energy and viscous dissipation. The results indicate that the polarization properties of rain are significantly affected by the nonequilibrium distortion of large drops in heavy rainfal

ISSN:0048-6604    

DOI:10.1029/RS019i001p00067

年代:1984

数据来源: WILEY

摘要:

Closed form theoretical solutions have been obtained that relate the eccentricity of raindrops to charge and electric fields. The dependence of drop shape on electric field is highly nonlinear; it appears that commonly observed electric fields in clouds have little effect on drop shape whereas for somewhat stronger fields the growth in distortion is explosive. This is true irrespective of the charge on the drops. In the absence of an electric field the natural oblateness of drops is enhanced by the presence of charge. This increase in eccentricity might be detected as an enhancement of the differential radar reflectivity. With increasing vertical electric field, drops elongate vertically, starting from an oblate shape and passing through spherical and prolate shapes before breaking up. The role of charge in this process is to enhance preexisting oblateness or prolateness.

ISSN:0048-6604    

DOI:10.1029/RS019i001p00075

年代:1984

数据来源: WILEY

摘要:

Differential reflectivity (ZDR) and circular depolarization ratio (CDR) radar signals are directly related to each other and to the median drop size diameter of rainfall when the drop sizes are exponentially distributed. These polarization parameters are examined for both static and oscillating raindrop shapes along with their relationships with rainfall rate and liquid water content estimates derived from ZDRor CDR and reflectivity factor (Z) measurements. Propagation effects atSband wavelengths are also considered and shown to be particularly important in CDR measurements.

ISSN:0048-6604    

DOI:10.1029/RS019i001p00081

年代:1984

数据来源: WILEY