D.H.O. Bebbington

The Sensitivity of Single Polarization Weather Radar Beam Blockage Correction to Variability in the Vertical Refractivity Gradient

Radars operating in complex orographic areas usually suffer from partial or total beam blockage by surrounding targets at their lowest elevation scans. The need for radar quantitative precipitation estimates in such environments led to the development of beam blockage corrections. This paper aims at evaluating the performance of beam blockage corrections under different electromagnetic propagation conditions with particular interest in anaprop situations. Three years of radiosonde data collected at Barcelona, Spain, a typical Mediterranean coastal site, are used to characterize the behavior of the vertical refractivity gradient near a weather radar. Three different targets surrounding the radar have been chosen and used to evaluate the different beam shielding simulated under different propagation conditions. A simple interception function between the radar beam and the topography is proposed and used for the different targets and propagation conditions considered. Results show that beam blockage correction is generally robust, with departures of 1 dB from the standard propagation conditions correction less than 10% of the time. However, as the presence of extreme anaprop cases would lead to higher differences, the monitoring of the propagation conditions is suggested as a criteria to be considered, among others such as the analysis of the echo structure, as a quality control of the radar quantitative precipitation estimate.

Weather radar anaprop conditions at a mediterranean coastal site

Abstract Many weather radars are affected by some type of anomalous propagation (AP) with a wide variety of frequency and intensity. Some geographical areas have been described to be particularly prone to AP. Even in some sites, AP is not statistically abnormal at all but dominant. Severe AP cases have been reported in coastal areas affected by strong temperature inversions and humidity contrasts such as the Baltic Sea, the Indic Ocean or the Mediterranean. In this paper we present average and extreme propagation conditions in the coastal area of Barcelona (NE Spain) calculated using three years of radiosonde data. Occurrence of different propagation conditions is discussed and related to local prevailing circulations such as the sea breeze with particular interest in the nearby Vallirana weather radar. The effect of some extreme ducting episodes seems to be remarkable in beam blocking correction procedures leading to wrong precipitation estimates when anaprop and rainfall occur at the same time. The use of radiosonde and mesoscale NWP data to derive operationally propagation conditions jointly with other existing techniques based in the analysis of radar data is overviewed.

Measurement and Characterization of Entropy and Degree of Polarization of Weather Radar Targets

To date, few polarimetric weather radars have exhibited the capability to measure full scattering matrices. In contrast, in the synthetic aperture radar (SAR) community, considerable experience has been gained in dealing with complete scattering matrices and their statistical behavior. This paper aims to place weather radar parameters in a wider context in order to exploit more general concepts like target decomposition theorems and polarization basis transformations. Entropy, which is a fully polarimetric variable derived from the Cloude-Pottier decomposition, and the degree of polarization, which is derived from Wolfs coherence matrix, are the subject of this paper. The theoretical analysis carried out in the first part is checked against fully polarimetric data from POLDIRAD, which is the german aerospace center research weather radar. The entropy and the degree of polarization are compared with the copolar correlation coefficient in order to understand whether they can add value to radar meteorological investigations. Because the degree of polarization is available to conventional dual-polarization coherent systems, it is important to assess its potential for operational use.

Extracting Rainfall Rates from X-band CDR Radar Data by Using Differential Propagation Phase Shift

Abstract In this paper, the method of using differential propagation phase shift ΔΦ to measure rain rate R from X-band circular depolarization ratio (CDR) radar data is presented. The principle of this method is based on the fact that there is a relationship between ΔΦ and R. The relationship between ΔΦ and R is investigated under various parameters such as water temperature, raindrop shape, and drop-size distribution form. Differential backscattering phase shift ΔδHV is related to ΔΦ, and this makes it possible to extract ΔΦ from CDR radar data and thus to estimate rain rates. The “cleaning” procedures are utilized to minimize the effects of canting angle. Limitations due to noise and quantization in using ΔΦ to measure R are discussed. Rain-rate measurements obtained by raingage and by radar agree well.

Evaluation of atmospheric anomalous propagation conditions: an application for weather radars

Several meteorological conditions are known to cause anomalous propagation (AP) of microwave radiation. The effect of AP in weather radar measurements my be important as spurious echoes from distant ground targets may appear as precipitation leading to wrong rainfall estimations. AP may also affect dramatically the quality of clear air radar observations. In this study, more than one hundred radiosonde ascents are examined to evaluate the occurrence of AP at the coastal site of Barcelona (Spain). Temperature and humidity profiles are used to calculate refractivity gradients and to estimate the existence of ducting layers. Ducts represent the worst case of super refraction and within them microwaves travel trapped like in a waveguide. To detect thin AP features a vertical resolution higher than that given by standard operational radiosonde data is desirable. For this reason, radiosonde data recorded every 10 s have been used. Results are compared against standard operational radiosonde analysis revealing a significantly higher number of AP layers. The output of a mesoscale numerical weather prediction model is also used to derive refractivity gradients. The ability of the model to simulate the propagation conditions is overviewed in order to assess the feasibility of an operational diagnostic AP product.

Fully polarimetric analysis of weather radar signatures

In this work the concept of depolarization response, namely the degree of polarization as a function of transmit polarization state, is investigated. Application examples are shown in the field of radar meteorology, namely for hydrometeor identification with fully polarimetric weather radar signatures. Data are from POLDIRAD, DLR research weather radar.

An evaluation of the potential of polarimetric radiometry for numerical weather prediction using QuikSCAT

It has been proposed that wind vector information derived from passive microwave radiometry may provide an impact on numerical weather forecasts of similar magnitude to that achieved by scatterometers. Polarimetric radiometers have a lower sensitivity to wind direction than scatterometers at low wind speed but comparable sensitivity at high windspeed. In this paper, we describe an experiment which aimed to determine if an observing system only capable of providing wind direction information at wind speeds over 8 ms/sup -1/ can provide comparable impact to one providing wind vectors at wind speeds over 2 ms/sup -1/. The QuikSCAT dataset used in the experiments has a wide swath and is used operationally by several forecast centers. The results confirm that assimilation of wind vectors from QuikSCAT only for wind speeds above 8 ms/sup -1/ gives similar analysis increments and forecast impacts to assimilating wind vectors at all wind speeds above 2 ms/sup -1/. Measurements from the WindSat five frequency polarimetric radiometer are compared with calculations from Met Office global forecast fields, and this also confirms that WindSat measurement and radiative transfer model accuracy appears to be sufficiently good to provide useful information for numerical weather prediction.

Fourth-moment calculation of optical propagation in a turbulent atmosphere with use of the split-step method. I. Plane wave

The split-step method was used to derive the full spatial dependence of the fourth moment of a plane-wave propagating in a two-dimensional turbulent atmosphere with a power-law spectrum for two values of the scattering parameter γk = 0 and 1 [ J. Opt. Soc. Am. A2, 2133 ( 1985)]. The changes in γk were obtained by the use of two different values of the inner scale of turbulence l0 while the operating wavelength and turbulent strength were kept constant. In this way the influence of l0 on the field statistics can be obtained. The results of intensity scintillation index σI2 and covariance function bI are also presented. The agreement of σI2 with the results of the reference cited above especially for γk = 0 is good. We also used the results of bI to study the asymptotic dependence of the characteristic correlation length on ζ, where ζ is the propagation-range scale.

Geometric Polarimetry—Part I: Spinors and Wave States

A new formal approach for the representation of polarization states of coherent and partially coherent electromagnetic plane waves is presented. Its basis is a purely geometric construction for the normalized complex-analytic coherent wave as a generating line in the sphere of wave directions and whose Stokes vector is determined by the intersection with the conjugate generating line. The Poincaré sphere is now located in physical space, simply a coordination of the wave sphere, with its axis aligned with the wave vector. Algebraically, the generators representing coherent states are represented by spinors, and this is made consistent with the spinor-tensor representation of electromagnetic theory by means of an explicit reference spinor that we call the phase flag. As a faithful unified geometric representation, the new model provides improved formal tools for resolving many of the geometric difficulties and ambiguities that arise in the traditional formalism.

On Mathematical and Physical Principles of Transformations of the Coherent Radar Backscatter Matrix

The congruential rule advanced by Graves for polarization basis transformation of the radar backscatter matrix is now often misinterpreted as an example of consimilarity transformation. However, consimilarity transformations imply a physically unrealistic antilinear time-reversal operation. This is just one of the approaches found in the literature to the description of transformations where the role of conjugation has been misunderstood. In this paper, the different approaches are examined, particularly in respect to the role of conjugation. In order to justify and correctly derive the congruential rule for polarization basis transformation and properly place the role of conjugation, the origin of the problem is traced back to the derivation of the antenna height from the transmitted field. In fact, careful consideration of the role played by Greens dyadic operator relating the antenna height to the transmitted field shows that, under general unitary basis transformation, it is not justified to assume a scalar relationship between them. Invariance of the voltage equation shows that antenna states and wave states must in fact lie in dual spaces, a distinction not captured in conventional Jones vector formalism. Introducing spinor formalism and with the use of an alternate spin frame for the transmitted field, a mathematically consistent implementation of the directional wave formalism is obtained. Examples are given comparing the wider generality of the congruential rule in both active and passive transformations with the consimilarity rule.