Spaceborne Weather Radar Research Articles

Three-Dimensionally Printed, Shaped, Engineered Material Inhomogeneous Lens Antennas for Next-Generation Spaceborne Weather Radar Systems

In this letter, three-dimensionally (3-D) printed shaped inhomogeneous dielectric lens antennas are synthesized using a novel approach based on geometrical optics (GO) and the particle swarm optimization (PSO) method. The GO method is used to trace rays to the aperture following the amplitude, phase, and polarization along curved space trajectories through inhomogeneous media to evaluate potential designs. These designs are specified by power series polynomials describing the surface topology and permittivity functions filling the volume of the shaped lens. The optimum design is obtained by integrating the GO analysis engine with the PSO into an optimization loop. The complex designed materials that result can be printed using new 3-D printers. New techniques are developed to print structures using fused deposition modeling (FDM), which contain complex overhanging features. Measured and simulated results are provided for all 3-D printed synthesized inhomogeneous lenses.

Novel Scanning Strategy for Future Spaceborne Doppler Weather Radar With Application to Tropical Cyclones

Severe tropical cyclones (TCs) are one of the most devastating natural disasters along the coastal regions from tropical to temperate zones. Understanding the three-dimensional (3-D) wind fields in TCs helps in assimilating their dynamics and predicting their evolution. Unfortunately, up to now, there is no spaceborne weather radar with Doppler capability to measure the 3-D wind components in the global scale. This paper presents a novel scanning strategy for future spaceborne Doppler weather radar mission to retrieve 3-D wind fields of TCs, which has three downward-pointing (with three different fixed tilt angles) and conically scanning beams. With spaceborne Doppler detection, the radar system enables to measure the vertical motion of hydrometeors, which is important in the estimation of latent heat fluxes and in the study of energy transportation in the structure of TCs on a global scale. A novel scanning strategy is presented and optimized to construct three noncollinear radar observations. Three-dimensional wind fields are retrieved using the least-squares method. A model simulation of TCs is used to validate the proposed scanning strategy. The radar beams of the proposal are able to cover TC's full area of interest. The results demonstrate a suitable accuracy of the proposed scanning strategy on retrieving the TC's 3-D wind fields with a resolution of 10 km × 10 km × 0.25 km grid cell, showing a promising use in the future spaceborne Doppler radar.

Evaluation of Surface Clutter for Future Geostationary Spaceborne Weather Radar

Surface clutter interference will be one of the important problems for the future of geostationary spaceborne weather radar (GSWR). The aim of this work is to provide some numerical analyses on surface clutter interference and part of the performance evaluation for the future implementation of GSWR. The received powers of rain echoes, land and sea surfaces from a radar scattering volume are calculated numerically based on the derived radar equations, assuming a uniform rain layer and appropriate land and sea surface scattering models. An antenna pattern function based on a Bessel curve and Taylor weighting is considered to approximate the realistic spherical antenna of a GSWR. The power ratio of the rain echo signal to clutter (SCR) is then used to evaluate the extension of surface clutter interference. The study demonstrates that the entire region of surface clutter interference in GSWR will be wider than those in tropical rainfall measuring mission precipitation radar (TRMM PR). Most strong surface clutter comes from the antenna mainlobe, and the decrease of clutter contamination through reducing the level of the antenna sidelobe and range sidelobe are not obvious. In addition, the clutter interference is easily affected by rain attenuation in the Ka-band. When rain intensity is greater than 10 mm/h, most of rain echoes at off-nadir scanning angles will not be interfered by surface clutter.

Small-Scale Variability of the Raindrop Size Distribution and Its Effect on Areal Rainfall Retrieval

AbstractThe drop size distribution (DSD) describes the microstructure of liquid precipitation. The high variability of the DSD reflects the variety of microphysical processes controlling raindrop properties and affects the retrieval of rainfall. An analysis of the effects of DSD subgrid variability on areal estimation of precipitation is presented. Data used were recorded with a network of disdrometers in Ardèche, France. DSD variability was studied over two typical scales: 5 km × 5 km, similar to the ground footprint size of the Global Precipitation Measurement (GPM) spaceborne weather radar, and 2.8 km × 2.8 km, an operational pixel size of the Consortium for Small-Scale Modeling (COSMO) numerical weather model. Stochastic simulation was used to generate high-resolution grids of DSD estimates over the regions of interest, constrained by experimental DSDs measured by disdrometers. From these grids, areal DSD estimates were derived. The error introduced by assuming a point measurement to be representative of the areal DSD was quantitatively characterized and was shown to increase with the size of the considered area and with drop size and to decrease with the integration time. The controlled framework allowed for the accuracy of retrieval algorithms to be investigated. Rainfall variables derived by idealized simulations of GPM- and COSMO-style algorithms were compared to subgrid distributions of the same variables. While rain rate and radar reflectivity were well represented, the estimated drop concentration and mass-weighted mean drop diameter were often less representative of subgrid values.

Modified Hitschfeld–Bordan Equations for Attenuation-Corrected Radar Rain Reflectivity: Application to Nonuniform Beamfilling at Off-Nadir Incidence

Abstract As shown by Takahashi et al., multiple path attenuation estimates over the field of view of an airborne or spaceborne weather radar are feasible for off-nadir incidence angles. This follows from the fact that the surface reference technique, which provides path attenuation estimates, can be applied to each radar range gate that intersects the surface. This study builds on this result by showing that three of the modified Hitschfeld–Bordan estimates for the attenuation-corrected radar reflectivity factor can be generalized to the case where multiple path attenuation estimates are available, thereby providing a correction to the effects of nonuniform beamfilling. A simple simulation is presented showing some strengths and weaknesses of the approach.

Winter precipitation fields in the Southeastern Mediterranean area as seen by the Ku-band spaceborne weather radar and two C-band ground-based radars

The spaceborne weather radar onboard the Tropical Rainfall Measuring Mission (TRMM) satellite can be used to adjust Ground-based Radar (GR) echoes, as a function of the range from the GR site. The adjustment is based on the average linear radar reflectivity in circular rings around the GR site, for both the GR and attenuation-corrected NearSurfZ TRMM Precipitation Radar (TPR) images. In previous studies, it was found that in winter, for the lowest elevation of the Cyprus C-band radar, the GR/TPR equivalent rain rate ratio was decreasing, on average, of approximately 8dB per decade. In this paper, the same analysis has been applied to another C-band radar in the southeastern Mediterranean area. For the lowest elevation of the “Shacham” radar in Israel, the GR/TPR equivalent rain rate ratio is found to decrease of approximately 6dB per decade. The average departure at the “reference”, intermediate range is related to the calibration of the GR. The negative slope of the range dependence is considered to be mainly caused by an overshooting problem (increasing sampling volume of the GR with range combined with non-homogeneous beam filling and, on average, a decreasing vertical profile of radar reflectivity). To check this hypothesis, we have compared the same NearSurfZ TPR images versus GR data acquired using the second elevation. We expected these data to be affected more by overshooting, especially at distant ranges: the negative slope of the range dependence was in fact found to be more evident than in the case of the lowest GR elevation for both the Cypriot and Israeli radar.

On the Equivalence of Dual-Wavelength and Dual-Polarization Equations for Estimation of the Raindrop Size Distribution

Abstract For air- and spaceborne weather radars, which typically operate at frequencies of 10 GHz and above, attenuation correction is usually an essential part of any rain estimation procedure. For ground-based radars, where the maximum range within the precipitation is usually much greater than that from air- or spaceborne radars, attenuation correction becomes increasingly important at frequencies above about 5 GHz. Although dual-polarization radar algorithms rely on the correlation between raindrop shape and size, while dual-wavelength weather radar algorithms rely primarily on non-Rayleigh scattering at the shorter wavelength, the equations for estimating parameters of the drop size distribution (DSD) are nearly identical in the presence of attenuation. Many of the attenuation correction methods that have been proposed can be classified as one of two types: those that employ a kZ (specific attenuation–radar reflectivity factor) relation, and those that use an integral equation formalism where the attenuation is obtained from the DSD parameters at prior gates, either stepping outward from the radar or inward toward the radar from some final range gate. The similarity is shown between the dual-polarization and dual-wavelength equations when either the kZ or the integral equation formulation is used. Differences between the two attenuation correction procedures are illustrated for simulated measurements from an X-band dual-polarization radar.

Melting layer model evaluation using fall velocity spectra at Ku-band

A melting layer model used for the rainfall retrievals from spaceborne weather radars is examined using Doppler radar measurements at 13.8 GHz, taken in the zenith-pointing mode. From the pulse-to-pulse variation of the phase measurements, the fall velocity spectra were obtained for each of the range gates from 1 to 5 km, including the melting layer. The spectra show a narrow distribution in the snow region with a mean velocity of 1.5 ms−1, getting wider as the melting progresses. The results indicate that the BB thickness is intensity dependent, which is consistent with previous studies in mid-latitude regions but not reflected in the predictions from the model considered in the paper. Measurements made at lower frequency, i.e. at L-band, also indicate the deficiency of the model in terms of the bright-band thickness. However, because the model has been shown to be consistent with measurements in the tropics, it is suggested that any modification and/or enhancement to the model be applied to mid–latitudes only. Implications for propagation prediction methods are also discussed.

Comparison of TRMM Precipitation Radar and Airborne Radar Data

Abstract The first spaceborne weather radar is the precipitation radar (PR) on the Tropical Rainfall Measuring Mission (TRMM), which was launched in 1997. As part of the TRMM calibration and validation effort, an airborne rain-mapping radar (ARMAR) was used to make underflights of TRMM during the B portion of the Texas and Florida Underflights (TEFLUN-B) and the third Convection and Moisture Experiment (CAMEX-3) in 1998 and the Kwajalein Experiment (KWAJEX) in 1999. The TRMM PR and ARMAR both operate at 14 GHz, and both instruments use a downward-looking, cross-track scanning geometry, which allows direct comparison of data. Nearly simultaneous PR and ARMAR data were acquired in seven separate cases. These data are compared to examine the effects of larger resolution volume and lower sensitivity in the PR data relative to ARMAR. The PR and ARMAR data show similar structures, although the PR data tend to have lower maximum reflectivities and path attenuations because of nonuniform beam-filling effects. Non...

Accuracy Evaluation of Doppler Velocity on a Spaceborne Weather Radar through a Random Signal Simulation

Abstract Digitized signals for a spaceborne weather radar are generated to inspect a Doppler signal processor, in which the digital signals are converted to analog through an arbitrary wave generator. A conventional Rice harmonic analysis is applied to include fluctuations of Fourier coefficients explicitly in a time series rather than in an ensemble. The accuracy of Doppler velocity is studied through this simulation for a mean estimator of contiguous pulse-pair operation in a space mission, characterized by a low signal-to-noise ratio (SNR) and a short coherence time. A linear perturbation formula is shown to deviate from the simulation as the SNR decreases and the pulse-pair interval increases. Furthermore, a theoretical limit in measurement accuracy is derived, beyond which the correlation signal is to be practically regarded as white noise, losing the physical meaning of measurement.

Doppler Velocity from Sea Surface on the Spaceborne and Airborne Weather Radars

The Doppler operation applied to the sea surface is studied for the spaceborne and airborne weather radars. In the space mission, a small amount of beam misalignment can cause large contamination of a platform velocity into a measured Doppler velocity. In this paper, theoretical bases for retrieving water droplet velocity via the Doppler signal from a sea surface are considered for pulse-pair operation in the nadir direction. On a fast-moving platform, Doppler fading is so crucial that the finite beamwidth of an antenna is taken into account in the formalism based on a scalar Kirchhoff theory. Correlation signals are derived for both the coherent and incoherent scatterings that have substantial differences in Doppler velocity. Furthermore, the incoherent Doppler velocity is shown to include a path-difference term, which is characteristic of the incoherent surface scattering, set apart from the incoherent body scattering. Applying the retrieval methodology to the Doppler signal from a ground surface is also discussed.

Differential‐frequency Doppler weather radar: Theory and experiment

To move toward spaceborne weather radars that can be deployed routinely as part of an instrument set consisting of passive and active sensors requires the development of smaller, lighter‐weight radars. At the same time, the addition of a second frequency and an upgrade to Doppler capability are essential to retrieve information on the drop size distribution (DSD), vertical air motion, and storm dynamics. One approach to the problem is to use a single broadband transmitter‐receiver and antenna where two narrowband frequencies are spaced apart by 7–10%. Use of Ka‐band frequencies (26.5–40 GHz) provides adequate spatial resolution with a relatively small antenna. Moreover, the differential reflectivity and mean Doppler signals in this band are directly related to the median mass diameter of the snow and raindrop size distributions. We present in the paper theoretical calculations of the differential reflectivity and Doppler for several frequency pairs including those proposed for the Global Precipitation Mission (GPM) at 13.6 and 35 GHz. Measurements from a zenith‐directed radar operated at 9.1 and 10 GHz are used to investigate the qualitative characteristics of the differential signals. Disdrometer data taken at the surface, just below the radar, show that the differential signals are related to characteristics of the raindrop size distribution. The stability of the DSD estimation procedure is tested using a simulation. The results indicate that reasonably stable estimates of the particle size distribution are feasible with a [31.5 GHz, 35 GHz] combination as long as a large number of independent samples are obtained.

Statistical Methods of Estimating Average Rainfall over Large Space–Timescales Using Data from the TRMM Precipitation Radar

Abstract Data from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar represent the first global rain-rate dataset acquired by a spaceborne weather radar. Because the radar operates at an attenuating wavelength, one of the principal issues concerns the accuracy of the attenuation correction algorithms. One way to test these algorithms is by means of a statistical method in which the probability distribution of rain rates at the high end is inferred by measurements at the low to intermediate range and by the assumption that the rain rates are lognormally distributed. Investigation of this method and the area–time integral methods using a global dataset provides an indication of how well methods of this kind can be expected to perform over different space–timescales and climatological regions using the sparsely sampled TRMM radar data. Identification of statistical relationships among the rain parameters and an understanding of the rain-rate distribution as a function of time and space may h...

Measurement simulations from spaceborne weather radar

Monitoring the meteorological phenomena evolution in those areas of the Earth not covered by ground-based meteorological radar, is now possible with spaceborne weather radars. The Spaceborne Weather Radar Simulator is a software program developed to simulate the reflected power from ground, sea and meteorological targets. It also provides an estimate of the mirror image return and 0f the Doppler shift of the received signal caused by the satellite motion. The analysis is carried out first by computing the received signal due to a single transmitted pulse and then by considering the transmission of a train of pulses at a given repetition frequency. The code has a versatile structure able to accept inputs concerning different instrument and target specifications. In this paper the results obtained by simulating the observation of a stratiform cloud profile from a 13.78GHz radar, at the orbit height of 400Km and beamwidth equal t0 0.57°, are presented. For the purpose 0f these calculations we have considered two pulse lenghts: 10μs (range resolution 1500m) and 2μs (range resolution 300m). The related sensitivities here estimated are respectively 8dBZ (single-10μs pulse) and 22dBZ (single-2μs pulse).

Simulations of mirror image returns of air/space-borne radars in rain and their applications in estimating path attenuation

The mirror image (MI) rain echo, received through the double reflection of the radar pulse from the surface, may provide useful information in estimating the rainfall rate from airborne and spaceborne weather radars. However, because of the complicated scattering mechanisms involving the surface and rain and the relatively small amount of measured data, studies of the MI effect have been few. In this paper, a more rigorous model of the MI return power has been constructed that yields the co-and cross-polarized components of the MI and bistatic returns as a functions of the radar parameters (antenna beamwidth and radar altitude) and the scattering properties of the rain and surface. As a test of the model, the mirror image return, as estimated from theory, is compared with the measured MI range profile, and reasonably good agreement is obtained. For meteorological applications, algorithms for estimation of the rain path attenuation are developed based on the difference of the direct and MI returns at the same distance from the surface. The accuracies of the algorithms are analyzed and compared with those of the surface reference technique (SRT) through the simulations for airborne and spaceborne [the case of the Tropical Rain Measuring Mission (TRRM)] geometries.

Preliminary Results from Multiparameter Airborne Rain Radar Measurement in the Western Pacific

Preliminary results are presented from multiparameter airborne radar measurements of tropical storms. The experiment was conducted in the western Pacific in September 1990 with the NASA DC-8 aircraft that was equipped with a dual-wavelength radar at X and Ka bands and several microwave radiometers. The modification to dual-polarization at X-band radar enabled measurements of the linear depolarization ratio (LDR). Vertical profiles of dual-polarization and dual-frequency observables for an example of stratiform rain and three examples of convective rain cells are examined. It is shown that at nadir incidence the LDR measurement often can be used to distinguish the phase states of the hydrometeors and to identify the melting layer. In addition to the information concerning particle shape and orientation from LDR, the ratio of the radar reflectivity factors in two frequency bands (X and Ka bands) provides insight into particle size. The capabilities of dual-wavelength and dual-polarization radar in the identification of particle size and phase will be important considerations in the design of future spaceborne weather radars.

A Study of Rain Estimation Methods from Space Using Dual-Wavelength Radar Measurements at Near-Nadir Incidence over Ocean

A question arising from the recent interest in spaceborne weather radar is what methods can be used to estimate precipitation parameters from space. In this paper, dual-wavelength airborne radar data obtained from flights conducted during 1988 and 1989 are used to compare rain rates derived from backscattering and attenuation methods. We begin with a survey of path-averaged rain rates estimated from six methods over four flights. The fairly large number of high rain-rate cases encountered during these experiments allows for the first tests of the surface-reference method applied to the low-frequency (10-GHz) data. To help interpret the results the surface reference methods are studied by means of scatterplots of the surface cross sections at the two frequencies under rain and no-rain conditions. Approximate criteria are given on combining attenuation and backscattering methods to increase the effective dynamic range of the radar. The dual-wavelength capability of the radar is also used to examine the vertical structure of the precipitation: critical to the success of most methods is the ability to distinguish rain from mixed-phase precipitation. Another factor affecting the accuracy of the methods is the drop-size distribution. In the final section of the paper a procedure to estimate the profiled drop-size distribution is applied to the measured radar data.

Range profiling of the rain rate by an airborne weather radar

One of the quantities of interest in a spaceborne weather radar is the rain rate or liquid water content as a function of height. In the presence of attenuation, however, the reconstruction of the profile is possible with usual methods only if the rain rate is uniform or if the relationships between the radar measurables and the rain characteristics are well known. Even with a dual-wavelength radar, estimates of attenuation between adjacent range gates tend to be noisy. In this paper we investigate a class of methods that are based on a measure of path attenuation which is used to constrain the Hitschfeld-Bordan solution. Methods of this type have been investigated for lidar, radar, and combined radar-radiometer applications; their function can be interpreted as allocating the attenuation in proportion to the strength of the measured reflectivity. Four estimates of rain rate are described and tested using data from a dual-wavelength airborne radar at 10 GHz and 35 GHz. When attenuation is significant, the estimates are generally more accurate than those without attenuation correction. This suggests that such methods can be used to extend the effective dynamic range of the radar to higher rain rates. Comparisons between the estimates also provide some diagnostic capability for the detection of radar calibration errors. Instabilities in the estimates can occur, however, in cases of low signal to noise ratios and in situations where partially melted hydrometeors contribute to the attenuation.

System design and examination of spaceborne microwave rain-scatterometer

The remote sensing of precipitation, especially rain by the spaceborne weather radar is an important subject which has not been realized. The system design and examination of the spaceborne weather radar which is called as the spaceborne microwave rain-scatterometer are performed by considering user's requirements to observe rain, the restrictions derived from the interface with satellite and the state of the art and the possibility of spaceborne microwave technology. Characteristic parameters for spaceborne microwave rain-scatterometer are determined based on the user's requirements for the system. Functions and performances of each subsystem (antenna, transmitters, receivers and data transmission processor) with the detailed block diagram of the total system of spaceborne microwave rain-scatterometer are shown. All of the critical items of hardware in the development of spaceborne microwave rain-scatterometer are also discussed.