K. Aydin

Modeling of Microwave Scattering From Cloud Ice Crystal Aggregates and Melting Aggregates: A New Approach

Ice crystal aggregates and their melting process are modeled with a new approach for determining their microwave scattering characteristics and are compared with those obtained using effective dielectric constant representations. The aggregates are constructed from columnar crystals of random lengths (with the width being a function of the length), which are composed of a string of touching ice spheres with diameters equal to the columns width. The aggregates are melted using a model that incorporates the primary aspects of experimentally observed features of the melting process. The generalized multiparticle Mie method is used for computing the scattering cross sections of the dry and melting aggregates. The T-matrix method is used for computations involving a bulk representation of each aggregate with an effective dielectric constant model and an oblate spheroidal shape. The 3- and 35.6-GHz backscattering cross sections show significant differences between the two methods for both dry and melting aggregates. For sizes larger than 3 mm, these differences range from several decibels at 3 GHz to well over 7 dB at 35.6 GHz. Significant differences are also observed in the extinction cross sections during the melting process. It is concluded that the effective dielectric constant models of dry and melting ice crystal aggregates do not represent the interactions between the constituent crystals (and water droplets during melting) of the aggregates very well. Hence, bulk models must be used with caution particularly at millimeter wavelengths.

A computational study of polarimetric radar observables in hail

The effects of melting and shedding of hailstones on various polarimetric and dual-wavelength radar observabies are examined. A melting model is used for oblate spheroidal hailstones composed of an ice core and a water coating. Single scattering effects are illustrated using monodisperse size distributions having one particle per cubic meter. Resnlts for exponential size distributions, including a Gaussian canting angle distribution, are also given. It is shown that melting can significantly affect the radar observahles and that, by combining several observables, estimating the parameters of an exponential size distribution may be possible.

Rain-induced attenuation effects on C-band dual-polarization meteorological radars

An attenuation correction procedure is proposed and evaluated by simulations using raindrop size distribution obtained from ground-based disdrometer measurements. The results show that under certain conditions it is possible to retrieve C-band reflectivity factor (Z/sub H/) and differential reflectivity (Z/sub DR/) radar observables affected by attenuation along rain-filled propagation paths. Rainfall rates estimated from Z/sub H/ and Z/sub DR/ with and without attenuation correction are compared to determine the effects of attenuation and the capability of the correction procedure to account for it. >

Polarimetric Measurements in a Severe Hailstorm

This study explores the utility of polarimetric measurements for discriminating between hydrometeor types with the emphasis on (a) hail detection and discrimination of its size, (b) measurement of heavy precipitation, (c) identification and quantification of mixed-phase hydrometeors, and (d) discrimination of ice forms. In particular, we examine the specific differential phase, the backscatter differential phase, the correlation coefficient between vertically and horizontally polarized waves, and the differential reflectivity, collected from a storm at close range. Three range–height cross sections are analyzed together with complementary data from a prototype WSR-88D radar. The case is interesting because it demonstrates the complementary nature of these polarimetric measurands. Self-consistency among them allows qualitative and some quantitative discrimination between hydrometeors.

Millimeter wave scattering from spatial and planar bullet rosettes

The electromagnetic scattering characteristics of several bullet-rosette ice crystal forms are computationally evaluated at 35-, 94-, and 220-GHz frequencies and compared with those of stellar crystals, hexagonal plates, and columns. One of the bullet rosettes is a planar crystal with four branches, the other two are spatial rosettes with six and eight branches. Two orientation models are used, one represents highly oriented crystals for which side and vertical incidence directions are considered, and the other represents: randomly oriented crystals (the incidence direction does not affect this case). It is observed that the linear depolarization ratio (LDR), as well as the copolarized correlation coefficient (/spl rho//sub h/spl nu//), can be used to differentiate columns from planar (including plates and stellar crystals) and spatial crystals based on their values at vertical incidence or their trends as a function of the elevation angle. For the random orientation case, LDR and /spl rho//sub h/spl nu// can differentiate columns from spatial crystals (except for sizes larger than 1.2 mm at 220 GHz) but not from planar crystals. Furthermore, the elevation angle dependence of LDR and Z/sub DR/ (differential reflectivity) has the potential for differentiating columnar, planar, and spatial crystals for sizes from a few tenths of a millimeter to 2 mm at 220 GHz, and from about 1 to 2 mm at 94 GHz. At 35 GHz, spatial crystals smaller than 2-mm resemble spherical particles in terms of their Z/sub DR/ and LDR signatures. The results for high-density (0.9 g cm/sup -3/) and low-density (representing hollow crystals) crystal models show significant differences in the values of LDR, Z/sub DR/, /spl rho//sub h/spl nu//, and the backscattering cross sections.

Scattering from ice crystals at 94 and 220 GHz millimeter wave frequencies

Polarimetric scattering from cloud ice crystals modeled as hexagonal columns, hexagonal plates and stellar crystals are calculated at 94 and 220 GHz frequencies using the finite difference time domain (FDTD) method. Two orientation models are considered, In the first model the long axes of columns and broad surfaces of plates and stellar crystals are parallel to the horizontal plane and randomly oriented on this plane. The second model assumes uniform random orientation in three dimensions (3-D). The ratio of the backscattering cross sections at 220 and 94 GHz exhibits a steady decrease with increasing size up to about 1000 /spl mu/m (2000 /spl mu/m for columns) for horizontally aligned ice crystals at side incidence as well as for 3-D random orientation. This dual frequency ratio (DFR) may be useful in gauging the size of ice crystals, DFR may also be useful in discriminating between ice crystal types since it shows major differences between columns and planar crystals. The linear depolarization ratio (LDR), i.e. the ratio of the cross-polar to co-polar backscattering cross sections, at vertical incidence for the horizontal alignment model is much higher for columns (over 20 dB) compared to plates and stellar crystals. As a result LDR also has the potential for discriminating columns from planar crystals, Furthermore, the normalized Mueller matrices for horizontally aligned columns at vertical incidence are shown to be diagonal and a function of only LDR. This same result applies to the 3-D random orientation model for all three crystal types. >

Millimeter wave radar scattering from model ice crystal distributions

Dual-polarization radar observables of cloud ice crystals are simulated at 94 and 220 GHz frequencies. Three crystal shapes are considered: hexagonal columns, hexagonal plates, and stellar crystals. A gamma model size distribution and a three dimensional (3D) canting distribution are used. The radar observables are displayed as a function of radar elevation angle. The linear depolarization ratios LDR/sub h/ and LDR/sub v/, the circular depolarization ratio CDR, and the magnitude of the copolarized cross correlation coefficient /spl rho//sub hv/ are found to be useful for differentiating columns from plates and stellar crystals. These radar observables have different trends as a function of elevation angle for the model columns and planar crystals (plates and stellar crystals). Furthermore, at vertical incidence they exhibit significantly different values for the two crystal types.

X-Band Polarimetric and Ka-Band Doppler Spectral Radar Observations of a Graupel-Producing Arctic Mixed-Phase Cloud

AbstractCharacteristics of graupel in an Arctic deep mixed-phase cloud on 7 December 2013 were identified with observations from an X-band scanning polarimetric radar and a Ka-band zenith-pointing radar in conjunction with scattering calculations. The cloud system produced generating cells and strongly sheared precipitation fall streaks. The X-band radar hemispheric RHI observables revealed spatial sorting of polarimetric signatures: decreasing (with increasing range) differential propagation phase shift φDP, negative specific differential phase KDP collocated with negative differential reflectivity ZDR in the upper half of the fall streak, and increasing or near-constant φDP with positive ZDR at the bottom edge of the fall streak. The negative KDP and ZDR, indicating prolate particles with vertically oriented maximum dimensions, were consistent with small, slow-falling conical graupel coexisting with low concentrations of more isometric graupel. The observed negative KDP values were best matched by scatt...

Millimeter wave scattering and propagation in rain: a computational study at 94 and 140 GHz for oblate spheroidal and spherical raindrops

Electromagnetic scattering from raindrops at 94- and 140-GHz frequencies is investigated. The differences in the scattering properties of spherical and oblate spheroidal model raindrops are illustrated for side and vertical incidence cases. Various backscattering and propagation parameters are evaluated using the Marshall-Palmer, Joss thunderstorm, and Joss drizzle drop size distributions. The Doppler spectrum at vertical incidence is substantially affected by the model raindrop shape. The radar reflectivity and the specific attenuation are not significantly affected. >

Relationships between rainfall rate and 35-GHz attenuation and differential attenuation: modeling the effects of raindrop size distribution, canting, and oscillation

Power law relationships of the form R=aA/sup b/ are derived, where R is the rainfall rate, and A is the 35-GHz specific attenuation A/sub h/ or specific differential attenuation /spl Delta/A=A/sub h/-A/sub v/, where the subscripts h and v indicate horizontal and vertical polarizations. The effects of raindrop size distribution, canting, and oscillation on these relationships are evaluated quantitatively. The drop size distributions (DSDs) are obtained from ground-based disdrometer measurements from three different geographical locations around the world. The R-A/sub h/ relationship is negligibly affected by raindrop canting and oscillation. It is affected to some extent by DSD variations, with less than 15% fractional standard error (FSE) in the estimated rainfall rate R/sub Ah/. On the other hand, the R-/spl Delta/A relationship is most sensitive to raindrop oscillation, up to about 35% difference in R/sub /spl Delta/A/ compared to the no-oscillation case, and the effect of canting is about 9% for a standard deviation of 10/spl deg/ of the polar canting angle compared with no canting. The FSE due to variations in the DSD for R 20 mm h/sup -1/, the FSE in R/sub /spl Delta/A/ is comparable (and even lower for R>30 mm h/sup -1/) to that of R/sub Ah/. The exceptions to this are rainfall rates with DSDs dominated by smaller raindrops (diameters less than 2.4 mm). It is also emphasized that because oscillation and canting affect /spl Delta/A but not A/sub h/, they could be used in combination for determining the presence of drop oscillation and canting and for estimating an effective raindrop shape model (axial ratio versus size).