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The polarimetric technique consists of simultaneous transmission and reception of Horizontally and Vertically polarized waves. Thus, transmitted polarization will be elliptical with equal horizontal and vertical components and arbitrary phase between the two. This scheme has been tested at NSSL on the NOAA research and development WSR-88D radar. Here reports on backscatter differential phase measurements with this radar. The radar also has a mode of transmitting horizontal polarization and receiving both the strong H and the weak V component (for short we refer to this mode as Linear Depolarization Ratio mode, LDR). Backscatter differential phase is considered a secondary polarimetric variable of lesser value for precipitation measurements than (the forward) specific differential phase or differential reflectivity. This is because backscatter differential phase is seldom significant. At 10 cm wavelength the only precipitation that can cause appreciable backscatter differential phase is hail. But it turns out that backscatter differential phase from other scatters provides two benefits. (1) it is suitable for discrimination between nonmeteorological scatters and precipitation and (2) it offers a way to calibrate (measure) the system differential phase. Of relevant and special importance is that, for some scatters, the measured total differential phase in the SHV scheme can differ from the value obtained in the sequential H, V scheme. The radar system contributes two distinct phases to the total differential phase. One is the differential phase in the transmission channel, the other is the differential phase in the receiver channel

In this paper we present an overview of the method of surface differential reflectivity (SDR) for the study of semiconductor surfaces. We will describe the principles of the technique and the experimental apparatus together with some theoretical considerations concerning the connection of SDR with the microscopic properties of the surface. In particular, the analysis of SDR is done in the frame of a macroscopic three layers model in which the media involved (vacuum, surface and substrate) are assumed to have definite anisotropic dielectric functions. A few experimental results, indicative of the potential of the technique, will be shown.

estimated from power measurements. Comparison of the direct estimate of propagation phase (F m ) measurement, which is unaffected by absolute calibration of the radar system, with the estimated propagation phase (F e ) from power measurements is the basis for the calibration method. Polarization measurements such as reflectivity (Z ), differential reflectivity (ZDR), and differential propagation phase (FDP) are sensitive to drop size distribution (DSD) and mean drop shape. It is important to devise a calibration technique relatively unperturbed by changes in DSD and drop shape. Statistical fluctuation in F e is derived to estimate the accuracy of the calibration procedure, raindrop shape, and attenuation. The proposed method is applied for calibrating reflectivity measurements of the National Center for Atmospheric Research (NCAR) S-band polarization radar (S-Pol) in midlatitude, subtropical, and tropical rain events. INDEX TERMS: 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; 6904 Radio Science: Atmospheric propagation; 6969 Radio Science: Remote sensing; KEYWORDS: polarization radar calibration, reflectivity, differential reflectivity, differential phase, raindrop size distribution

We investigate pump induced modulation in the temporal evolution of the excitonic coherent polarization generated by a probe pulse in GaAs multiple quantum wells. For this, we perform fs time resolved pump-probe reflectivity measurements using frequency upconversion at both positive and negative pump-probe delays. A contribution arising from interference of excitonic linear and nonlinear polarization, which is usually not considered in describing pump-probe experiments, is required to understand the results., Comment: 7 pages, 6 figures, submitted to Phys. Rev. B

Presents observations of the co-polar correlation coefficient |/spl rhosub hv/(0)| made with a ground based and airborne weather radars along the vertical. Also differential phase and differential reflectivity at a 10/spl deg/ off nadir are shown. Simple models of prolate and oblate spheroids are used to qualitatively explain the observations. >

Some interesting aspects of millimeter-wave (35, 94, 220 GHz) scattering properties of various types of ice crystals are presented. These ice crystals include three types of bullet rosettes which are compared with hexagonal columns and hexagonal plates. The elevation angle dependence of differential reflectivity and linear depolarization ratio for certain bullet rosettes show distinct signatures that are different than columns and plates.

The purpose of this paper is to present details of the meteorological radar algorithm for classification of hydrometeors and a methodology for sensitivity analysis to the various polarimetric variables used for classification. The classification algorithm described belongs to the "fuzzy logic" family. In its essence a classifier assigns (maps) an observed point X in the multiparameter space to a class j (which refers to the bulk hydrometeor type such as rain, hail, graupel, snow, etc.). In this paper the point X has for coordinates the following six variables: (1) the reflectivity factor for horizontally polarized waves Z/sub h/, (2) the differential reflectivity Z/sub DR/, (3) the specific differential phase K/sub DP/, (4) the cross correlation coefficient between the horizontally and vertically polarized (copolar) waves /spl rho//sub hv/(0), (5) the linear depolarization ratio L/sub DR/, and (6) the environmental temperature T. The crux of a hydrometeor classification scheme is to partition this six-dimensional space of observed variables into subsets such that each can be associated with a specific hydrometeor type (j). An obvious solution is to decouple the variables and consider each individually and then make a final judgement on the basis of weighted averages.

Errors arise when using conventional radar reflectivity, Z, to estimate rainfall rate, R, and these can be particularly severe during severe convective storms; the very events when accurate estimates are needed so that action can be taken to mitigate the effects of flooding. Concentration is on three problems associated with heavy rainfall: hail, attenuation and absolute calibration of the radar, and consider how polarisation radar parameters, differential reflectivity, ZDR, and specific differential phase shift KDP, might lead to their alleviation. It is essential to consider the fundamental limits to the accuracy with which these parameters can be estimated. If ZDR can be measured to an accuracy of 0.2 dB, then it provides a measure of mean raindrop shape which is sufficiently precise to improve rainrate estimates. This can be achieved at S-band (10 cm), but seems very difficult for operational C-band (5 cm) radars; differential attenuation by the heavy rain introduces a negative bias into ZDR which increases with range. However, the magnitude of this bias at C-band can then be used to correct for the total attenuation of Z. Differential phase, KDP has the advantage that it is a phase measurement and so is unaffected by attenuation. It only responds to the rainfall and is unaffected by the hail, but KDP is a noisy parameter and is only useful for heavy rainfall above 30 – 60 mm hr-1. Fortuitously, KDP and ZDR are not independent and one use of KDP and ZDR may well be to exploit this redundancy to identify rain areas as opposed to hail, and in rainfall to use the redundancy to provide an automatic calibration of the absolute reflectivity, Z, to 0.5 dB (12%). Finally, the noisy character of both ZDR and KDP together with the low level of the co-polar correlation coefficient provide the first reliable means of detecting and removing anomalous propagation which is a major operational problem for all weather radars. Keywords: polarisation radar, rainfall calibration, attenuation, hail, anomalous propagation

We have investigated the room temperature deposition of ultrathin bismuth layers on GaAs(110) surfaces by using Surface Differential Reflectance (SDR) spectroscopy. In SDR spectra measured at the “as grown” surfaces, optical transitions related to the new electronic structure of the overlayer appear at 0.8 eV and 1.35 eV, below the gap of bulk gallium arsenide. Because of their energy position and dependence upon deposited Bi thickness, we interpret them as related to Bi-overlayer and Bi-substrate bonds, respectively. Subsequent annealings of the overlayer at temperatures in the range 200–350 °C have not produced any improvement in the quality of the interface, but a new (weak) transition at about 1.1 eV possibly related to Bi crystallites present at the surface. Annealing atT > 350 °C produces desorption of the adsorbed bismuth.

For a long time, radars have been used extensively for both meteorological and hydrological applications, including remote-sensing of rainfall, warning of severe weather and flash flood prediction. Today, most weather radars installed in European countries operate at C-band, mainly to reduce costs. However, at this frequency, the advantage is often offset by the effect of attenuation caused by the intervening rainfall. An improved method to correct attenuation using reflectivity (Z/sub H/) and differential reflectivity (Z/sub DR/) is developed and described. The technique relies on the fact that, for an exponential raindrop size distribution, the measured Z/sub DR/ results in an overestimated median drop diameter (D/sub 0/), whereas the measured Z/sub H/ yields a under-estimated N/sub 0/. The estimated (N/sub 0/D/sub 0/) pair can then be used to obtained attenuation parameters which are close to those that would have been derived from true Z/sub H//Z/sub DR/ values.

We have measured the differential optical reflectivity ( λ = 0.78 mm) of YBa 2 Cu 3 O 7 − x single crystals in the temperature range of 20–300 K using a thermal wave technique. A sharp signal maximum with a temperature width ∼ 10 K was observed below T c , and was attributed to the electronic contribution of the optical reflectivity. It is concluded that the two components of the differential reflectivity signal are related to accumulation of the superconducting phase and temperature variation of the electron-scattering time.

International audience; Band structure calculations of complete InAs monolayer in AlGaAs/GaAs quantum wells are performed within the framework of the extended-basis sp3d5s* tight-binding model. We show that the optical properties can be tuned from the quantum well energy below the GaAs band-gap depending on the well thickness and the position of the probe. The results are supported by differential reflectivity measurements and represent a concept for optoelectronic devices with an operation wavelength widely tuneable around 850 nm employing GaAs process technology.

Angle-resolved photoelectron spectroscopy and surface differential reflectivity have been used to study the electronic structure of Si(100):Sb-1\ifmmode\times\else\texttimes\fi{}1 and Si(100):Sb-2\ifmmode\times\else\texttimes\fi{}1 surfaces. For both surfaces, one occupied surface-state band has been mapped along the [010] and [011] directions. Both surfaces show a semiconducting behavior with a gap of 1.6 eV for the 1\ifmmode\times\else\texttimes\fi{}1-Sb and 1.4 eV for the 2\ifmmode\times\else\texttimes\fi{}1-Sb surface. A minimum-energy position at the Fermi level is derived for the empty surface-state band. The results are also compared with those obtained for the clean Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface.

A brief account of the most important experimental and theoretical methods employed in surface physics is given. The use of optical spectroscopy in the study of clean semiconductor surfaces is described in detail.

We present surface differential reflectivity results on the polarization dependence of optical transitions and on the oxidation of InP(110) surfaces in the energy range between 2.0 and 4.0 eV. The surface dielectric function has been computed for light electric vector along [110] and [001] directions. Three well defined optical peaks have been detected at the following energies: 2.6 eV (light electric vector along [110] direction), 3.1 eV (unpolarized), and 3.5 eV (light electric vector along [001] direction). Absorption kinetics is analyzed by following the reflectivity variation between 1×102 and 2×106 L of molecular oxygen at three selected spectral energies. Several well defined steps of the oxidation process are clearly resolved and discussed in terms of disappearance of intrinsic surface states, creation of acceptor and donor defect states, growth of In2O3.

Small time (1 to 15 min) and space (0.5 to 1.5 km) measurements of the mean rainfall rate have been made within 10 km from a dual polarization radar, during several stratiform and convective precipitation events. At first, the time and space characteristic scales of the gage and of the radar data are evaluated in order to find the minimum time interval Δt and spatial resolution Δx for which the mean rainfall rate values have a large enough statistical significance. Then, the quantitative comparisons are made between the mean rainfall rate deduced from the gage data and that deduced from the radar measurements, either without or with using the differential reflectivity Z DR . During the same experiments, the hailfall parameters (total hailstone number N t , total hailfall kinetic energy E t ) have been estimated from the data of a hailpad network situated within 30 km from the radar. The values of these hailfall parameters have been approximated by functions of several radar parameters, deduced from the time evolution of the radar reflectivities (Z H and Z DR ) above each hailpad.

We report the transient differential reflectivity ΔR/R in Y 1−x−y Pr x Ca y Ba 2 Cu 3 O 7 thin films. The transient differential reflectivity measurement reveals that the effect of Pr substitution for Y ions on the related Fermi level position is similar to that of the oxygen deficiency in YBa 2 Cu 3 O 7-δ system, however, it is different from the effect of divalent Ca substitution for Pr.

The performance of an S-band dual-polarisation meteorological radar is investigated using a physically-based radar simulator, in particular, the impact of the drop size distribution on the variance of the estimate of differential reflectivity (Z/sub DR/) is addressed and discussed.

Two radars of different characteristics were used on a clear night in early summer to investigate the nature of the clear air echoes. It is deduced that most of the echoes on this evening were due to insects rather than atmospheric turbulence or birds 1) by the magnitude of differential reflectivity; 2) by the lack of characteristic bird wing beat signature; 3) by the inability to recognize birds using a collimated spotlight; 4) by the strength of the signal return; and 5) by the migration schedules of birds in central Illinois. The CHILL radar measured differential reflectivity, and showed that, at low levels, the insects were flying with a common heading. At higher levels, the differential reflectivity showed no preferential heading of the insects, but was still greater than for atmospheric turbulence. It is proposed that differential reflectivity provides a means of differentiating between echoes due to atmospheric turbulence and insects.

Whereas different hydrometeor types cannot be readily distinguished using radar reflectivity alone, this paper shows how the differential reflectivity available from dual-polarization radars can be used as an indicator of hydrometeor type (when supported by reflectivity factor also), and that the spatial variability of differential reflectivity is a useful addition. Various hydrometeor types, ground targets, and burnt chaff from straw fires may be identified, whereas conventional radar reflectivity measurement alone would require considerable pattern recognition to make the same distinctions.

This paper summarizes some of the highlights of the symposium, synthesizes them in the context of prior observations and understanding, and attempts to resolve apparent conflicts. The differential reflectivity ZDR technique has shown excellent results in the quantitative measurement of rainfall. Systematic differences were found to be due to deviations from the originally assumed exponential drop size distribution. The consensus was that the circular depolarization ratio (CDR) method is more versatile and powerful but that ZDR is simpler and cheaper. Precautions in the use of ZDR have been noted. The apparent conflict between CDR and ZDR measurements near the melting layer seems to have been resolved. A spectrum of scientific and practical applications has been identified. Most importantly, the ZDR method is recommended for further development and use wherever accurate rainfall measurements are required.

A comparison between radar-derived rainfall rates and those obtained from a ground-based disdrometer located at the radar site is presented. The radar measurements were made in the vicinity of Chicago, IL, on August 2, 1978, with the CHILL radar of the Illinois State Water Survey. Three radar-derived profiles of rainfall rate versus distance from the radar were obtained for a single storm in the direction of the storm path toward the radar. These profiles indicated the storm motion and the variability of rainfall as the storm progressed toward the radar. The last two profiles support the hypothesis that the storm was in steady state as it passed over the radar site. This provided an opportunity to compare the radar-derived rates with an apparent rate versus distance profile derived from a disdrometer time record of rainfall rate obtained at the radar site. Excellent agreement between the radar and disdrometer measurements of rainfall resulted.

Radar measurements of average rainfall rate over two separate 550 km2 areas am compared with raingage measurements in the same areas over time intervals of ∼1 h. The measurements were performed to test the differential reflectivity (ZDR) technique of Soliga and Bringi (1976) which provides a means of estimating rainfall rate (R)by combining measurements of radar reflectivity factors at horizontal (ZH) and vertical (ZV) polarizations; ZDR(dB) is defined as the ratio of these reflectivities, i.e., ZDR = 10 log(ZH/ZV). Results from an experiment performed near Chicago, Illinois, on 9 August 1978, using the University of Chicagco-Illinois State Water Survey (CHILL) radar and the Illinois State Water Survey's Chicago Hydrometeorological Area Project (CHAP) raingage network are presented. ZDR estimates of rainfall rate compared very favorably with the raingages measurements and were significantly better than estimates obtained from two Z-R relationships, one of which was obtained by raingage calibratio...

One of the Doppler radars operated by the National Severe Storms Laboratory can measure the difference between propagation phase constants K/sub Dp/ at horizontal and vertical polarization. This study examines the use of this parameter K/sub Dp/ in addition to the reflectivity factor Z/sub H/ and the differential reflectivity Z/sub DR/, to obtain information about rain and hail. It is shown from theory and experiments that a third parameter of the drop size distribution, obtained from K/sub DP/, can be used to support Z/sub DR/ measurement and/or to point out mixed-phase hydrometeors. Quantitative information on hail size distribution can be obtained for small hailstones when their major axes are nearly vertically aligned, giving rise to negative Z/sub DR/ and K/sub DP/ values. >

Empirical relationships for estimating rainfall rate, liquid water content and median volume diameter from radar measurements of reflectivity factor and differential reflectivity are derived from a disdrometer record of a highly variable, heavy rainfall event in central Illinois. Comparisons with relationships representing exponential and gamma model drop-size distributions are made. Simulations, employing these and Z-R relationships for rainfall estimation, are performed. Statistical measures are tabulated for comparing results. These show an excellent agreement between the disdrometer- and radar-derived rainfall parameters when the latter are obtained from the empirical relationships.

New accurate surface differential reflectivity data of the GaAs and GaP cleavage faces are presented. Surface state transitions have been detected near 2.9 eV in GaAs(110), while in GaP(110) they appear at 2.8 and 3.5 eV. In both cases a small bulk-like Franz-Keldish effect is superimposed on the surface contribution to differential reflectivity. A method for subtracting this term out is described. The relevance of the present results in the context of the debate on surface excitons is also discussed.

The optical properties of rough Ag films quenched on substrates cooled to 150 K are studied by differential reflectivity during annealing to room temperature. An abnormal optical absorption at about 2.5 eV is assigned to surface roughness.

The statistical characteristics of three estimators of the differential reflectivity (ZDR) radar signal are examined theoretically and compared with pulse-to-pulse measurements of reflectivity at horizontal and vertical polarizations. The square-law and log-ratio estimators provide satisfactory means for measuring ZDR while the log-ratio estimator does not. In addition, the theory is consistent with radar data, clearly showing that ZDR could be measured accurately from a sequence of alternately polarized waves.

A novel approach to making differential reflectivity measurements in multiple polarization planes has been proposed. This paper describes a versatile technique using a mechanical polarization switch and the systematic errors in differential reflectivity measurements which result. Initial results of simultaneous X band slant path measurements of attenuation and depolarization at 11.6 GHz and differential reflectivity measurements at 2.8 GHz using a slow polarization switch are presented.

Calculations have been made of the radar backscattering, differential reflectivity, and circular depolarization ratio of large oblate spheroids composed of spongy ice. Results are compatible with laboratory measurements by earlier investigators. As expected, scattering of 10-cm radiation depends to an important extent on the size, water content, and axial ratios of the spheroids. Observations of differential reflectivities close to zero in hailstorms can be explained, as was done by V. N. Bringi and his associates, as resulting from the irregular shapes and tumbling of hailstones. But such observations could also be explained by size-distributed oblate spheroids with equivalent diameters greater than about 3.5 cm, falling with vertical symmetry axes.

Backscatter amplitude ratios are defined for horizontal and vertical polarizations. These new parameters of linear polarization provide not only a coherent interpretation of the magnitude of the cross-correlation function between horizontally and vertically copolarized backscattered signals (ρL) but also give new meaning and applications for differential reflectivity (ζ). In particular, it is demonstrated that ρL and ζ can be used to determine the mean and variance of the amplitude ratios. In rain these quantities can be transformed into estimates of the mean and variance of drop axis ratios. This information can potentially improve polarization estimates of rainfall, provided ρL is measured accurately. Amplitude ratios also appear to be useful for investigating melting processes. Measurements indicate the presence of prolate-like scatterers even where ζ is greater than unity in some bright bands. It is hypothesized that this scattering from apparent prolates is, at least in part, a result of the...

The differential reflectivity (ZDR) measures the mean shape of hydrometeors and provides an estimate of the mean size of raindrops Observations of ZDR for rain may be combined with the conventional radar reflectivity factor (Z) and fitted to any two-parameter raindrop size distribution and this information used to derive more accurate rainfall rates. In such work the precise shape of raindrops is a critical parameter. Recently available data suggest that large raindrops are more oblate than previously believed. These new shapes support the idea that ZDR values above 3.5 dB can be attributed to rain. Average values of ZDR as a function of Z obtained in heavy rain by the Chilbolton radar agree very closely with those predicted using the new shapes. Statistics are also presented of the natural variability of raindrop spectra in heavy rain. Analytic expressions are proposed for computing rainfall rate from Z and ZDR.

The potential use of differential reflectivity measurements at orthogonal polarizations to determine rain-fall rate is examined. The method involves measurements of ZH and ZV, the radar reflectivity factors due to horizontally and vertically polarized incident waves respectively. The differential reflectivity, ZDR = 10 log (ZH/ZV), which should be precisely determinate, occurs as a result of the distortion of raindrops as they fall at terminal velocity. The approximate theory of Gans for electromagnetic scattering by spheroids is applied to the distorted raindrops. Assuming a general exponential form for the raindrop size distribution, equations are derived relating the distribution parameters to the measurements. The determination of rainfall rate follows directly. Finally, the sensitivity of the distribution parameters to radar inaccuracies is examined, and several methods of implementing the measurements are suggested. It is concluded that good estimates of rainfall rate using a single non-att...

This paper presents an analysis of the accuracy of rain rate estimates from data observed with a radar that has alternating horizontal and vertical polarization. Theoretical accuracies of rain rates from the reflectivity, the differential reflectivity and the differential propagation phase shift are considered via-a-vis the drop size distribution (DSD) variability, using a computer simulation procedure. First measurements of the differential propagation phase shift have been provided by the National Severe Storms Laboratory's dual-polarized radar, in addition to the reflectivity and the differential reflectivity. An examination of the radar data has revealed factors that could affect the rain rate estimates to a greater extent than the often contended DSD variability in the case of differential reflectivity method. Errors caused by sidelobe contamination significantly affect the differential phase shift data, so that a large spatial scale averaging is required to obtain reasonably accurate rain r...

We compute the optical properties of semiconductor surfaces starting from the self-consistent electronic structures determined using local pseudopotentials with a plane-wave basis set. Calculations for a 9 layer (110) slab of GaP are carried out with both clean and H covered surfaces. Differential reflectivity spectra and their anisotropies are interpreted in terms of transitions involving surface states.

An overview of the method of surface differential reflectivity (SDR) for the study of semiconductor surfaces is presented. This includes the principles of the technique, the experimental apparatus, and some theoretical considerations concerning the connection of SDR data with the microscopic properties of the surface. Experimental results are also presented as an example of the application of this spectroscopy to the study of semiconductor surfaces.

The optical properties of the GaAs(110) surface are studied by means of self-consistent local-density slab calculations. It is found that a large contribution to the calculated anisotropy of the clean surface reflectance is related to transitions which do not involve surface states. Comparison is made with differential reflectivity and reflectance anisotropy experiments.

We present a determination of surface dielectric functions (SDF) of Si(111)2×1, Ge(111)2×1, GaAs(110) and GaP(110) surfaces obtained using polarized surface differential reflectivity (SDR) technique.

Results of the first measurements of differential reflectivity (ZDR) in rain are reported. ZDR = 10 log (ZH/ZV) dB involves copolar measurements of ZH and ZV, the radar reflectivity factors at horizontal and vertical polarizations, respectively. The data were obtained with the University of Chicago-Illinois State Water Survey (CHILL) radar facility in Oklahoma during Spring 1977 and confirmed theoretical expectations that ZDR should be positive, ranging between around 0-4 dB. By combining ZDR measurements with ZH, it is shown how estimates of N0 and D0, the parameters of an exponential raindrop size distribution, can be obtained. These estimates were subsequently used to compute rainfall rates as a function of range along a radar ray. These results illustrate that ZDR can have important applications in the quantitative, remote measurement of drop-size distributions and rainfall.

For improving hail suppression strategies much more detailed knowledge on the individual hail formation process within the thermodynamical as well as the dynamical framework of hail producing cloud systems is needed. One possibility to obtain such knowledge on microphysical and dynamical processes simultaneously is with polarimetric Doppler radar measurements. The advanced polarimetric Doppler radar Oberpfaffenhofen, in operation since summer 1987, is described. Its unique capabilities, such as the real time estimation and display of the three Doppler moments reflectivity, Doppler velocity and spectral width as well as polarimetric parameters, such as depolarization ratios and the differential reflectivity are presented. Furthermore, from polarimetric measurements a hail signal is implemented which can be displayed in real time, too. The microphysical as well as dynamical structure of a squall line has been observed and a conceptual model of the hail formation process within this system is presented, thus illustrating the detailed insights into cloud processes possible with this new radar system.

Use of the lognormal form of raindrop size distributions in simulations of differential reflectivity (ZDR) measurements is investigated. Using two remotely measured variables and an empirical relation, the three parameters of the lognormal distribution can be deduced and the spectrum integrated to obtain rain rate. This is demonstrated by a simulation of the ZDR method using ground-based drop size distributions. Drop axis ratio and sampling time effects are also investigated and results compared to those obtained using a gamma distribution. It is shown that the lognormal representation is easily adaptable for use in the ZDR method. Using our dataset, we show that the lognormal size distribution provides lower average absolute deviations of theoretically determined rain rates from actual ones (10.7%) than those obtained using either the exponential (41.0%) or gamma distributions (11.8%).

Multiple parameter radar measurements (dual frequency hail signal DFR-HS or differential reflectivity ZDR) can be strongly affected by the mismatch of the antenna illumination functions at the two wavelengths or polarizations. The resulting instrumental error can be estimated, with some approximations, from the independently measured reflectivity at one wavelength or polarization by using the actual radar illumination functions. This objective estimation of the antenna-induced error is made in some real cases and its possible conseqences on the data analysis are discussed.

Radar observations of the differential reflectivity (ZDR) of developing convective clouds are presented. This parameter provides a measure of a mean hydrometeor shape, and when analysed in conjunction with the conventional reflectivity (Z), it gives an indication of whether the precipitation is ice or liquid water; for rain it enables an estimate of raindrop size and concentration to be made. the results suggest that some isolated echoes which subsequently develop into large cumulonimbus clouds initially have very small concentrations of large (diameter greater than 4 mm) supercooled raindrops, a very different size distribution from that normally observed in mature convective clouds. an alternative explanation in terms of an even lower population of much larger ice particles in wet growth is technically possible but does not seem physically realistic.

The results of an investigation by differential reflectivity of the reflectivity change due to the deposition of small amounts of Ag (10 to 200 A) on a cold (140 K) polycrystalline smooth Ag substrate are presented. The modifications of the optical properties due to the roughening of the surface are analysed, the annealing of the deposited Ag overlayers is investigated and their relationship with Surface-Enhanced Raman Scattering (SERS) is discussed Etude des variations de la reflectivite dues au depot de faibles quantites d'Ag (10 a 200 A) sur un support lisse d'Ag polycristallin refroidi a 140 K. Analyse des modifications des proprietes optiques causees par la rugosite a la surface et du recuit des couches d'argent; discussion de leur relation avec la diffusion Raman «geante»

Processing of simultaneous measurements of differential polarization parameters (differential reflectivity, ZDR, and differential phase shift, ϕDP) and Doppler spectral moments is discussed. It is shown how ZDR and ϕDP modulate the Doppler signal and what effects they have on the autocovariance at lag 1 if a sequence of alternately polarized fields (linear horizontal, H, and vertical, V) is transmitted. A scheme to overcome these effects is proposed and demonstrated on recorded time series data from a radar with polarization diversity.

The results of reflectivity and differential reflectivity measurements at non-normal incidence for p-polarized light of thin films of silver on different substrates are presented. It was found that the peak associated with the silver plasma frequency broadens and shifts towards higher energies for thinner layers. A model of a thin layer with bulk silver optical constants cannot be used to explain this behaviour. Agreement with experiment is obtained if we take into account not only the shorter relaxation time of the free electrons but also the non-local effects associated with the longitudinal wave (polarization wave) near the plasma frequency (in our configuration these effects are particularly important).

A grazing incidence monochromator has been constructed using the principle of differential reflectivity for order sorting. Some difficulties associated with earlier designs of this type have been solved and various refinements incorporated. The instrument includes a phase grating of the laminar type, as developed for use in the X-ray region. Details of the design and construction of the monochromator together with measurements of performance are reported, and a comparison with the calculated properties of the grating and optical system is made.