Multiparameter radar and in situ aircraft observation of graupeland hail

Documenting simultaneous multiparameter radar observations of precipitation in conjunction with in situ hydrometeor sampling is important for the interpretation of multiparameter radar observations. In situ observation using aircraft-mounted probes is one of the best ways to collect such data. In situ observation of hail and graupel in convective storms is complicated due to adverse environment of flight and low concentration of large particles that are difficult to sample. This paper presents one of the first observations of simultaneous multiparameter radar observations and in situ samples of wet hail and graupel in convective storms. The observations are unique because of the excellent coordination between aircraft samples and radar scanning, as well as relatively large sample volumes of aircraft data. Multiparameter radar observations (namely reflectivity, differential reflectivity, linear depolarization ratio, copolar correlation coefficient, and specific differential phase) are documented in graupel and wet hail. The observations indicate that the linear depolarization ratio and copolar correlation measurements, in conjunction with reflectivity levels, can be used to distinguish between graupel and hail. A simple procedure is developed to estimate the average bulk density of graupel and wet hail, comparing radar and in situ observations     

Polarimetric observations and theory of millimeter-wave backscatterfrom snow cover

Polarimetric radar measurements carried out at 95 and 225 GHz are presented for fresh and refrozen snow cover. These data indicate that the Mueller matrix for snow cover consisting of spherical ice particles has a relatively simple form, with 10 of the 16 elements approximately zero. Measurements of new-fallen snow consisting of predominantly nonspherical snow crystals are also presented. The anisotropic structure of such snow cover results in a more complex Mueller matrix, fitting the general form for natural surfaces. An analytic expression for the Mueller matrix of isotropic snow cover is derived by computing the response of a semi-infinite layer of scatterers that are insensitive to the orientation of the incident polarization. This matrix is shown to accurately predict the polarimetric response of the snow cover comprised of spherical ice particles based solely on copolarized and cross-polarized radar cross-section measurements     

Effective permittivity of and scattering from wet snow and icedroplets at weather radar wavelengths

In this parametric study, wet snow and ice droplets are modeled as sparse collections of Rayleigh scatterers (size small compared to wavelength) consisting either of ice or of composite mixtures of air and ice in water. An effective permittivity is calculated using various extended Maxwell-Garnett-type models to account for variations in shape and orientation of the constituents. The backscatter radar cross section is calculated as an incoherent sum of individual particle cross sections, and for various distributions of shape, size, and orientation. The results indicate a dependence of the radar cross section on the polarizations of the incident and reflected fields. This dependence is shown in the differential reflectivity, defined in terms of the ratio of the backscatter cross sections due to two mutually orthogonal linearly polarized incident electric fields     

Polarimetric radar studies of atmospheric ice particles

Single scattering properties of ice crystals are described at microwave frequencies using discrete dipole approximations and Rayleigh scattering techniques. For a given shape, the average bulk densities of ice crystals can be estimated using the ratio of the copolarized radar signal in a linear (horizontal, vertical) polarization basis. Reflectivity depends on the ice content (g×m^-3), and also on both size distribution parameters and average bulk density of the scatterers. Differential propagation phase is primarily a function of shape, ice water content, and is independent of size distribution parameters. Thus, by using a combination of polarimetric radar measurements, average ice content, bulk density, and shape of distributed scatterers call be inferred. These techniques become quite complex in the case of a winter storm where scatterers can exist with varying shape and bulk densities. Polarimetric radar properties of such complex distributed scatterers are modeled. Physical variations in the relation among ice water content, reflectivity, and differential propagation phase are considered with respect to change in the shape of size distribution, bulk density,,and average shape of the scatterers. Also, simultaneous polarimetric radar observations and in situ aircraft measurements are shown to demonstrate practical applicability of the techniques     

非球形冰晶在94/220 GHz毫米波的散射特性模拟计算

针对94/220 GHz双频雷达的数据处理,分析了不同形状冰晶对这两个波段的单散射特性及衰减特性,探讨了单形状冰云及具体冰云模型的回波特性,结果表明:1)当冰晶较大时,冰晶的后向散射及衰减对冰晶形状较敏感,相同最大尺度下,六角形冰晶后向散射及衰减最大、子弹花次之、雪花最小;2)单形状冰晶云的雷达反射率因子对冰晶形状、冰水含量、滴谱的中值尺度较敏感,同样滴谱条件下,220 GHz的衰减系数约是94 GHz的5~25倍;3)具体冰云模型的雷达反射率因子随粒子浓度、冰水含量、中值尺度增加而增加,对粒子谱的形状参数敏感性较低.     

Dual polarization radar observations of anomalous wintertimethunderclouds in Japan

Dual-polarization radar observations of wintertime thunderclouds for the Sea of Japan are presented. The range-height-indicator (RHI) and plan-position-indicator (PPI) scans, respectively, reveal the height and horizontal distributions of ice particles, such as graupel and ice crystals. The overall shape of these ice particles is also confirmed on the ground. The ice crystals are, in general, found at high altitude near the cloud top, whereas the graupel is primarily seen near the center of clouds. The PPI display indicates a bandlike horizontal structure, and the lightning tends to occur around the bandlike gap where the ice crystals are, in advance, accumulated on the windward side of the preceding cloud. Simultaneous field mill observations indicate electric charge separation between these ice particles precipitating from the thunderclouds     

卫星资料在云顶粒子尺度特征分析中的应用

选取NOAA卫星AVHRR通道3(3.55～3.93μm)观测资料,分析云顶粒子的物理性质,并用地面雨量和雷达观测资料对卫星反演分析结果进行验证.分析结果显示通道3反射率小,云顶粒子尺度大时易产生降水.当云层为单一层次时,通道3的反射率小值区与雷达回波区间具有良好的对应关系,证实了在降水过程中卫星分析云顶粒子尺度结果与雷达观测一致.     

Differential propagation constants on slant paths through snow and ice crystals as measured by 16.5 GHz polarization-diversity radar

Radar determinations of differential propagation constants at a wavelength of 1.82 cm on slant paths through heavy snow and ice crystals are reported. Also reported are measurements of differential propagation effects in the melting layer. The slant path results indicate an increase of differential phase shift with height, to a value at 2000 m which may exceed 1.3 deg/ km, being typically several times that at the surface. This effect is attributed to a change in the hydrometeors, as they precipitate, from ice crystals to snow aggregates. Differential propagation effects in the melting layer are small.     

Retrieval of Snow and Rain From Combined X- and W-Band Airborne Radar Measurements

Two independent airborne dual-wavelength techniques, based on nadir measurements of radar reflectivity factors and Doppler velocities, respectively, are investigated with respect to their capability of estimating microphysical properties of hydrometeors. The data used to investigate the methods are taken from the ER-2 Doppler radar (X-band) and cloud radar system (W-band) airborne Doppler radars during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment campaign in 2002. Validity is assessed by the degree to which the methods produce consistent retrievals of the microphysics. For deriving snow parameters, the reflectivity-based technique has a clear advantage over the Doppler-velocity-based approach because of the large dynamic range in the dual- frequency ratio (DFR) with respect to the median diameter D₀ and the fact that the difference in mean Doppler velocity at the two frequencies, i.e., the differential Doppler velocity (DDV), in snow is small relative to the measurement errors and is often not uniquely related to D₀. The DFR and DDV can also be used to independently derive D₀ in rain. At W-band, the DFR-based algorithms are highly sensitive to attenuation from rain, cloud water, and water vapor. Thus, the retrieval algorithms depend on various assumptions regarding these components, whereas the DDV-based approach is unaffected by attenuation. In view of the difficulties and ambiguities associated with the attenuation correction at W-band, the DDV approach in rain is more straightforward and potentially more accurate than the DFR method.     

A dual-polarization rain profiling algorithm

A new attenuation correction algorithm based on profiles of reflectivity, differential reflectivity, and differential propagation phase shift is presented. A solution for specific attenuation retrieval in rain medium is proposed, which solves the integral equations for reflectivity and differential reflectivity with cumulative differential propagation phase shift constraint. The conventional rain profiling algorithms that connect reflectivity and specific attenuation can retrieve specific attenuation values along the radar path assuming a constant intercept parameter of the normalized drop size distribution. However, in convective storms, the drop size distribution parameters can have significant variation along the path. This paper presents a dual-polarization rain profiling algorithm for horizontal looking radars incorporating reflectivity as well as differential reflectivity profiles. The dual-polarization rain profiling algorithm has been evaluated with X-band radar observations simulated from drop size distribution derived from high-resolution S-band measurements collected by the Colorado Statue University CHILL radar. The analysis shows that the retrieved specific attenuation, differential attenuation, reflectivity, and differential reflectivity from the dual-polarization rain profiling algorithm provide significant improvement over the current algorithms.     

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     

On modeling air/spaceborne Radar returns in the melting layer

The bright band is the enhanced radar echo associated with the melting of hydrometeors in stratiform rain. To simulate this radar signature, a scattering model of melting snow is proposed in which the fractional water content is prescribed as a function of the radius of a spherical mixed-phase particle consisting of air, ice, and water. The model is based on the observation that melting starts at the surface of the particle and then gradually develops toward the center. To compute the scattering parameters of a nonuniform melting particle, the particle is modeled as a sphere represented by a collection of 64/sup 3/ cubic cells of identical size where the probability of water at any cell is prescribed as a function of the radius. The internal field of the particle, used for deriving the effective dielectric constant, is computed by the conjugate gradient and fast Fourier transform (CGFFT) numerical methods. To make computations of the scattering parameters more efficient, a multilayer stratified-sphere scattering model is introduced after demonstrating that the scattering parameters of the nonuniformly melting particle can be accurately reproduced by the stratified sphere. In conjunction with a melting layer model that describes the melting fractions and fall velocities of hydrometeors as a function of the distance from the 0/spl deg/C isotherm, the stratified-sphere model is used to simulate the radar bright-band profiles. These simulated profiles are shown to compare well with measurements from the Precipitation Radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite and a dual-wavelength airborne radar. The results suggest that the proposed model of a melting snow particle may be useful in studying the characteristics of the bright-band in particular and mixed-phase hydrometeors in general.     

Supervised Classification and Estimation of Hydrometeors From C-Band Dual-Polarized Radars: A Bayesian Approach

In this paper, a Bayesian statistical approach for supervised classification and estimation of hydrometeors, using a C-band polarimetric radar, is presented and discussed. The Bayesian Radar Algorithm for Hydrometeor Classification at C-band (BRAHCC) is supervised by a backscattering microphysical model, aimed at representing ten different hydrometeor classes in water, ice, and mixed phase. The expected error budget is evaluated by means of contingency tables on the basis of C-band radar noisy and attenuated synthetic data. Its accuracy is better than that obtained from a previously developed fuzzy logic C-band classification algorithm. As a second step of the overall retrieval algorithm, a multivariate regression is adopted to derive water content statistical estimators, exploiting simulated polarimetric radar data for each hydrometeor class. The BRAHCC methodology is then applied to a convective hail event, observed by two C-band dual-polarized radars in a network configuration. The hydrometeor classification along the line of sight, connecting the two C-band radars, is performed using the BRAHCC applied to path-attenuation-corrected data. Qualitative results are consistent with those derived from the fuzzy logic algorithm. Hydrometeor water content temporal evolution is tracked along the radar line of sight. Hail vertical occurrence is derived and compared with an empirical hail detection index applied along the radar connection line during the whole event.     

Influence of microphysical cloud parameterizations on microwavebrightness temperatures

The microphysical parameterization of clouds and rain cells plays a central role in atmospheric forward radiative transfer models used in calculating microwave brightness temperatures. The absorption and scattering properties of a hydrometeor-laden atmosphere are governed by particle phase, size distribution, aggregate density, shape, and dielectric constant. This study investigates the sensitivity of brightness temperatures with respect to the microphysical cloud parameterization. Calculated wideband (6-410 GHz) brightness temperatures were studied for four evolutionary stages of an oceanic convective storm using a rive-phase hydrometeor model in a planar-stratified scattering-based radiative transfer model. Five other microphysical cloud parameterizations were compared to the baseline calculations to evaluate brightness temperature sensitivity to gross changes in the hydrometeor size distributions and the ice-air-water ratios in the frozen or partly frozen phase. The comparison shows that enlarging the raindrop size or adding water to the partly frozen hydrometeor mix warms brightness temperatures by as much as 55 K at 6 GHz. The cooling signature caused by ice scattering intensifies with increasing ice concentrations and at higher frequencies. An additional comparison to measured Convection and Moisture Experiment (CAMEX-3) brightness temperatures shows that in general all but two parameterizations produce calculated T_Bs that fall within the CAMEX-3 observed minima and maxima     

Estimation of raindrop size distribution from spaceborne Radar observations

The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar uses surface reference method to estimate the attenuation encountered in the observation of radar reflectivity. The cumulative attenuation estimated from the surface reference method can be distributed along the radar range using a power law relation between the specific attenuation (k) and reflectivity factor (Z). A physical interpretation of the variability in the k-Z relation can be provided with the normalized drop size distributions. This paper describes an algorithm to estimate the drop size distribution (DSD) parameters from the measured attenuation and reflectivity values obtained from TRMM precipitation radar observations. Coincident data collected with ground polarimetric radar during the TRMM field campaigns is used to cross-validate the estimates of drop size distribution parameters obtained from the TRMM precipitation radar. The results of cross validation show fairly good agreement with the drop size distribution parameters retrieved from TRMM precipitation radar and the ground-radar-based estimates. The algorithm is subsequently used to generate monthly global maps of DSD. The global distribution of DSDs is critically important for development of retrieval algorithms used by the Global Precipitation Mission Radiometers.     

A method for estimating rain rate and drop size distribution frompolarimetric radar measurements

Polarimetric radar measurements are sensitive to the size, shape and orientation of raindrops and provide information about drop size distribution (DSD), canting angle distribution and rain rate. The authors propose and demonstrate a method for retrieving DSD parameters for calculating rain rate and the characteristic particle size. The DSD is assumed to be a gamma distribution and the governing parameters are retrieved from radar measurements: reflectivity (Z_HH), differential reflectivity (ZDR), and a constrained relation between the shape (CL) and slope (Λ) parameters derived from video disdrometer observations. The estimated rain rate is compared with that obtained from more traditional methods and the calculated characteristic size is compared with the measured values. The calculated K_DP based on the retrieved Gamma DSD is also compared with measurements. The proposed method shows improvement over the existing models and techniques because it can retrieve all three parameters of the gamma distribution. For maintaining the continuity of earlier published results, raindrop shape is assumed to be equilibrium     

An airborne 95 GHz dual-polarized radar for cloud studies

A 95 GHz dual-polarization radar system was developed and flown on the University of Wyoming King Air research aircraft, from which it measured reflectivity, depolarization, and Doppler-derived velocity mean and standard deviation of a variety of clouds. This paper describes the radar and a data acquisition system that uses commercially available digitizers, signal processors, and signal generators. The authors also describe the tradeoffs between spatial resolution and ability to estimate reflectivity and velocity. This paper presents the first known airborne measurements of clouds made at 95 GHz; these are thought to be the most highly resolved millimeter-wave cloud images made to date. Depolarization, measured in terms of the linear depolarization ratio (LDR), was especially high in the melting band and in regions containing pristine ice crystals. These measurements demonstrate the advantages that high-spatial-resolution airborne millimeter-wave radars offer for the study of cloud microphysical properties     

Prediction of the effects of rain on satellite communication systems

The major propagation effects for satellite communication systems operating above 4 GHz are caused by rain. With the possible exceptions of depolarization and multiple scattering at frequencies above 20 GHz, these effects may be calculated if the distribution of rain intensity is known in both time and space. The major effects-attenuation and interference-require information about path and volume averaged rain intensities. Current prediction models are not capable of adequately estimating the statistical distributions of path and volume averaged values. Radar observations could provide the required data. The best information currently available for modeling these distributions are statistical cell or storm models derived from radar observations.     

Meteorological passive-active radar observations

The results of passive-active radar investigations of atmospheric objects, including determination of the microphysical characteristics of clouds and precipitation (cloud water content and precipitation intensity) and the characteristics of lightning activity in thunderstorm-hazardous cloudiness, are presented. The possibilities of both active and passive methods have been discussed. It is shown that their combined application makes it possible to acquire data on the cloud water content at different stages of their evolution and detect dangerous weather phenomena related to clouds (thunderstorms, hail, and showers). These data can be employed in supershort-term forecasts of such phenomena. The principles of constructing passive-active meteorological radar systems have been analyzed.     

A New Melting Particle Model and its Application to Scattering of Radiowaves by a Melting Layer of Precipitation

A melting layer of precipitation is composed of melting snow particles, and we modeled them by three-layered spherical particles, in which the innermost layer is air, the middle ice and the outermost water. Based on this model, the radar reflectivity, together with the specific phase shift and the specific attenuation of a melting layer of precipitation, were computed at 1–100 GHZ by using the Mie theory. The radar bright band is explained by this model. We compared our numerical results with that in the literature Zhang (IEEE Transactions on Antennas and Propagation 42(3):347–356, 1994), Zhang (IEEE Transactions on Antennas and Propagation 42(3):492–500, 1994). It demonstrates that the three-layered snow sphere model is appropriate and practicable, so the computed results are more accurate. This study can be used in radar remote sensing and satellite-earth communications.