1,721,074 research outputs found
Explaining the polarization signal from rain dichroic effects
No full textRadiative transfer simulations for a dichroic medium consisting of horizontally oriented spheroid raindrops in a cloud have been performed to better understand the polarization properties of the radiation field in such kind of media. A successive order of scattering methodology is employed to obtain exact and approximate equations of the rain medium in order to evaluate the contribution of the different scattering orders to the polarization signal for a 1D geometry approximation at all cloud levels. Then a Monte Carlo technique is used to evaluate the impact of 3D effects in the polarization of the downwelling/upwelling radiation field at the bottom/top of the dichroic layer. An accurate analysis of the impact of the emission term onto the total polarization signal is carried out. The 3D effects seem to be more relevant in polarization signatures when the emission/absorption processes dominate. © 2006 Elsevier Ltd. All rights reserved
How does multiple scattering affect the spaceborne W-band radar measurements at ranges close to and crossing the sea-surface range?
No full textA radar simulator capable of treating multiple-breakscattering effects has been upgraded to include the interaction with a Kirchoff surface, which realistically reproduces the effect of water surfaces. Multiple-scattering effects explain in a straightforward way some peculiar features of the first images delivered by the 94-GHz cloud-profiling radar onboard the CloudSat, overpassing precipitating systems. The reflectivity profiles without the usual peaks at surface range are found to be distinctive signatures of strong multiple scattering. Moreover, multiple scattering is responsible for producing long signal tails at apparent ranges far below the surface with a strong sensitivity on the microphysical assumptions of the icy segment of the cloud. The estimates of multiple-scattering enhancement at surface and close to the surface range and the saturation levels for simplified precipitating profiles for both CloudSat and EarthCARE configurations are provided. © 2008 IEEE
A rain-rate retrieval algorithm for attenuated radar measurements
Full text linkAdynamic regularization scheme for rain-rate retrievals from attenuated radar measurements is presented. Most regularization techniques, including the optimal estimation method, use the state-space parameters to regularize the problem, which will always lead to a bias in the solution. To avoid this problem the authors introduce an evolutionary regularization technique, which is based on the spatial derivative of the measured reflectivity profile and allows for a bias-free global solution. The regularization strength is determined by the quadratic eigenvalue solution using the regularized total least squares method. With the new method, the authors perform a retrieval of rain-rate profiles from simulated measurements of a nadir-pointing W-band (94 GHz) radar, in a configuration similar to the cloud radar employed on CloudSat. The simulations assume that multiple scattering is negligible and only liquid hydrometeors are taken into account. The authors compare the results of this method with the outcome of an optimal estimation method and demonstrate that their method is superior in terms of reliability, correlation coefficient, and dispersion to the optimal estimation method for layers experiencing high values of attenuation; therefore, the a priori bias typical for optimal estimation solutions is avoided. © 2010 American Meteorological Society
Evaluation of radar multiple-scattering effects from a GPM perspective, Part II: Model results
Full text linkMultiple-scattering effects as sensed by radars in configurations useful in the context of the Global Precipitation Mission (GPM) are evaluated for a range of meteorological profiles extracted from four different cloud-resolving model simulations. The multiple-scattering effects are characterized in terms of both the reflectivity enhancement and the linear depolarization ratio. When considering the copolarized reflectivity in spaceborne configurations, the multiple-scattering enhancement becomes a real issue for Ka-band radars, though it is generally negligible at the Ku band, except in meteorologically important situations such as when high rain rates and a considerable amount of ice are present aloft. At Ka band it can reach tens of decibels when systems of heavy cold rain are considered, that is, profiles that include rain layers with high-density ice particles aloft. On the other hand, particularly at 35 GHz, high values of the linear depolarization ratio are predicted even in airborne configurations because of multiple-scattering effects. This result should allow the observation of these features in field campaigns. © 2006 American Meteorological Society
Evaluation of radar multiple-scattering effects from a GPM perspective. Part I: Model description and validation
Full text linkA numerical model based on the Monte Carlo solution of the vector radiative transfer equation has been adopted to simulate radar signals. The model accounts for general radar configurations such as airborne/ spaceborne/ground based and monostatic/bistatic and includes the polarization and the antenna pattern as particularly relevant features. Except for contributions from the backscattering enhancement, the model is particularly suitable for evaluating multiple-scattering effects. It has been validated against some analytical methods that provide solutions for the first and second order of scattering of the copolar intensity for pencil-beam/Gaussian antennas in the transmitting/ receiving segment. The model has been applied to evaluate the multiple scattering when penetrating inside a uniform hydrometeor layer. In particular, the impact of the phase function, the range-dependent scattering optical thickness, and the effects of the antenna footprint are considered. © 2006 American Meteorological Society
Multiple scattering effects due to hydrometeors on precipitation radar systems
Full text linkSpace-borne radars are invaluable tools for characterizing clouds and precipitation. At higher frequencies (like those used for the TRMM PR or envisaged for GPM radars) attenuation due to hydrometeors increasingly becomes a relevant issue. Simultaneously when dealing with active sensors, multiple scattering effects could be significant due to the simultaneous increase of the optical thickness and the single scattering albedo of the hydrometeors with frequency. In this study, we investigate multiple scattering due to rainfall and graupel on radar returns for nadir observations at 13 and 35 GHz. A numerical approach, based on the forward fully polarized Monte Carlo technique, which incorporates a Gaussian antenna pattern function with varying beam-widths, is adopted in the study. Results reveal that multiple scattering effects are driven by the interplay between the antenna footprint, the medium scattering coefficient and the depth traveled inside the medium. The multiple scattering effects are generally negligible at 13 GHz for typical spaceborne and air-borne systems while they are relevant to spaceborne but almost negligible in air-borne configurations at 35 GHz. Copyright 2005 by the American Geophysical Union
Evaluation of radar multiple scattering effects in Cloudsat configuration
Full text linkMonteCarlo simulations have been performed to evaluate the importance of multiple scattering effects in coand cross-polar radar returns for 94 GHz radars in Cloudsat and airborne configurations. Thousands of vertically structured profiles derived from some different cloud resolving models are used as a test-bed. Mie theory is used to derive the single scattering properties of the atmospheric hydrometeors. Multiple scattering effects in the co-polar channel (reflectivity enhancement) are particularly elusive, especially in airborne configuration. They can be quite consistent in satellite configurations, like CloudSat, especially in regions of high attenuation and in the presence of highly forward scattering layers associated with snow and graupel particles. When the cross polar returns are analysed [but note that CloudSat does not measure any linear depolarization ratio (LDR hereafter)], high LDR values appear both in space and in airborne configurations. The LDR signatures are footprints of multiple scattering effects; although depolarization values as high as -5 dB can be generated including non-spherical particles in single scattering modelling, multiple scattering computations can produce values close to complete depolarization (i.e. LDR=0dB). Our simulated LDR profiles from an air-borne platform well reproduce, in a simple frame, some experimental observations collected during the Wakasa Bay experiment. Since LDR instrumental uncertainties were not positively accounted for during that experiment, more focused campaigns with air-borne polarimetric radar are recommended. Multiple scattering effects can be important for CloudSat applications like rainfall and snow-fall retrievals since single scattering based algorithms will be otherwise burdened by positive biases
Multiple scattering effects in pulsed radar systems: An intercomparison study
Full text linkIn this paper, two different numerical methods capable of computing multiple scattering effects in pulsed-radar systems are compared. Both methods are based on the solution of the time-dependent vectorial form of the radiative transfer equation: one exploits the successive order of scattering approximation, the other a forward Monte Carlo technique. Different benchmark results are presented (including layers of monodisperse spherical water and ice particles), which are of specific interest for W-band spaceborne cloud radars such as CloudSat's or Earth-CARE's cloud profiling radars. Results demonstrate a good agreement between the two methods. The pros and cons of the two models are discussed, with a particular focus on the validity of the second order of scattering approximation. © 2008 American Meteorological Society
PARSIVEL snow observations: A critical assessment
Full text linkThe performance of the laser-optical Particle Size Velocity (PARSIVEL) disdrometer is evaluated to determine the characteristics of falling snow. PARSIVEL's measuring principle is reexamined to detect its limitations and pitfalls when applied to solid precipitation. This study uses snow observations taken during the Canadian Cloudsat/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Validation Project (C3VP) campaign, when two PARSIVEL instruments were collocated with a single two-dimensional disdrometer (2-DVD), which allows more detailed observation of snowflakes. When characterizing the snowflake size, PARSIVEL instruments inherently retrieve only one size parameter, which is approximately equal to the widest horizontal dimension (more accurately with large snowflakes) and that has no microphysical meaning. Unlike for raindrops, the equivolume PARSIVEL diameter-the PARSIVEL output variable-has no physical counterpart for snowflakes. PARSIVEL's fall velocity measurement may not be accurate for a single snowflake particle. This is due to the internally assumed relationship between horizontal and vertical snow particle dimensions. The uncertainty originates from the shape-related factor, which tends to depart more and more from unity with increasing snowflake sizes and can produce large errors. When averaging over a large number of snowflakes, the correction factor is size dependent with a systematic tendency to an underestimation of the fall speed (but never exceeding 20%). Compared to a collocated 2-DVD for long-lasting events, PARSIVEL seems to overestimate the number of small snowflakes and large particles. The disagreement between PARSIVEL and 2-DVD snow measurements can only be partly ascribed to PARSIVEL intrinsic limitations (border effects and sizing problems), but it has to deal with the difficulties and drawbacks of both instruments in fully characterizing snow properties. © 2010 American Meteorological Society
Microwave radiative transfer intercomparison study for 3-D dichroic media
No full textThree different numerical methods capable of solving the radiative transfer of microwave radiation within 3-D dichroic media are compared. A case study, represented by an intense rain shaft populated by perfectly oriented oblate raindrops, is analysed in detail, including a discussion of the behaviour of all four Stokes components. Results demonstrate an acceptable agreement between all Monte Carlo methods. The method based on a discrete ordinates scheme agrees only qualitatively with the Monte Carlo outputs. Because of its lower computational cost the backward Monte Carlo technique based on importance sampling represents the most efficient way to face passive microwave radiative transfer problems related to optically thick 3-D structured clouds including non-spherical preferentially oriented hydrometeors. © 2006 Elsevier Ltd. All rights reserved
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