The WInd VElocity Radar Nephoscope (WIVERN) mission concept, a candidate for ESA’s Earth Explorer 11 program, aims at globally observing vertical profiles of reflectivity and line-of-sight (LoS) winds in cloudy and precipitating regions. WIVERN uses a 94-GHz dual-polarization Doppler radar with conical scanning to address the limited coherence duration between radar transmitted from low-Earth satellites with small antennas. This system transmits closely spaced pairs of horizontally and vertically polarized pulses, which are better correlated than pulses of the same polarization separated by longer intervals. The polarization diversity pulse pair (PDPP) technique is then used to estimate radar observables such as reflectivities, differential reflectivities, Doppler velocities, and differential phase. This article introduces an efficient method for generating H- and V-I and Q time series from the covariance matrix of the autocorrelation function. This method treats the signal as a nonstationary stochastic process, making it suitable for the PDPP pulse sequence from a rapidly rotating antenna and more computationally efficient than inverse fast Fourier transform techniques. It also accounts for interfering cross-polar signals and decorrelation from the scanning antenna. This method is included in the mission’s end-to-end simulator, which processes data from raw I and Q to Level 1 estimates of polarimetric variables. For scientific applications, averaging at least 5 km (40 polarization diversity (PD) pairs) is necessary to reduce noise in polarimetric variables and Doppler velocities. Under optimal conditions, uncertainties at 5-km integration are 0.7 dB for reflectivities, 0.3 dB for$Z_{\text {DR}}$, 0.4 m/s for Doppler velocities, and 1.9° for$\Phi _{\text {DP}}$.

WIVERN, short for WInd VElocity Radar Nephoscope ( www.wivern.polito.it , [1] , [2] ), has been recently proposed in the ESA Earth Explorer 11 call with the specific goal of filling the observational gap of global wind observations inside storms and precipitating systems [3] . WIVERN, currently in Phase A, is one of the two remaining candidate missions with the final selection scheduled for July 2025.

WIVERN, a space mission proposed within the ESA Earth Explorer program, aims to provide new insight in the cloud and dynamical structure of tropical cyclones (TC). The mission hinges upon a 800 km swath conically scanning 3-mm Doppler radar. Through notional simulations of WIVERN observations based on the ClouSat nadir looking 3-mm radar, our study demonstrates that WIVERN could profile most of the TCs, particularly the glaciated part of the cloud above the freezing level and the precipitating stratiform regions. Because of its lower sensitivity, the WIVERN radar would provide 75% observations of clouds and 45% accurate horizontal winds in TCs in comparison to where CloudSat detects clouds. However, thanks to its rapid conical scanning, WIVERN would indeed provide ∼50 times more observations of clouds and 30 times more observations of horizontal winds in comparison to the number of clouds sampled by CloudSat.The proposed observing system has the potential to complement the sparse observations from aircraft-reconnaissance measurements and by ground-based and airborne Doppler radars, thus providing additional constraints into numerical weather prediction models and hopefully further improving forecasts of tropical cyclone intensity and track.

Millimitere (mm) and sub-millimiter (sub-mm) radiometer observation of the atmosphere from space is an appealing topic given the variety of information obtainable. The exploitation of window frequencies and various gaseous absorption bands at 50/60, 118, 183 allow for a better representation of tropospheric temperature profiles, water vapor and cloud liquid contents, as well as for hail detection, and to some extent, rainfall and snowfall estimates. These observations have shown tangible impacts on numerical weather prediction and data assimilation, climate benchmarking, hydrometeorology, extreme weather nowcasting, and civil protection. Further benefits for ice cloud retrievals are expected from observations at higher frequency, such as 243 and 664 GHz channels foreseen in the upcoming EUMETSAT Polar System-Secon Generation (EPS-SG) Ice cloud imager (ICI) sensor [1] , [2] . The increase in frequency, and consequently the reduction in wavelength, from mm to sub-mm also gives the technological advantage of reduced size of the overall system, maintaining performances unchanged, thus making it easier to implement constellation of radiometers with the glaring benefit of incrementing the repetition time of the satellite overpasses. A precursor on this topic was proposed by Prof. Marzano in 2009 [3] with the FLOwer constellation of MM-wave RADiometers (FLORAD) mission. The FLORAD concept consisted in tree small satellites (<500 kg each) in a pseudo-stationary orbit (also termed as resonant or floreal orbit) to have a repetition rate of 1 hour over the Mediterranean area with a cross-track scanner sensor named FLOMIS (FLORAD microwave imager-sounder) with channels ranging from 90 to 230 GHz. Two evolutions of FLORAD were proposed later, adding radio occultation [4] or cloud radar [5] . Ten years later, technological progress allowed the deployment of a proof-of-concept radiometer on a cubesat (1.23 kg), named TEMPEST-D [6] , as well as TROPICS [7] , a six-radiometer constellation (5.34 kg each). These missions exploit satellites that are orders of magnitude smaller and cheaper than traditional satellites operated by federal agencies, revolutionizing the next-generations of Earth-observations [8] . In Europe, the ESA/EUMETSAT prototype satellite of the Arctic Weather Satellite (AWS) mission has been recently approved. The AWS Microwave Radiometer (MWR) is a 19 channel cross-track scanning radiometer consisting of a rotating antenna focusing the incoming radiation onto four feedhorns (one for each group of channels) and four receivers, covering the frequency range 50–325 GHz. The AWS will be the forerunner of the potential EPS-Sterna mission, a constellation of small (120 kg) polar-orbiting satellites based on the AWS, each carrying a single microwave radiometer providing frequent coverage of the Earth and full coverage of the polar zones with no gaps. The EPS-Sterna would complement the MetOp series as well as the US NOAA’s Joint Polar Satellite System by providing more frequent observations mainly for temperature and humidity sounding but also for improving precipitation monitoring at high latitudes.

The WIVERN (Wind VElocity Radar Nephoscope) mission, one of the four ESA Earth Explorer 11 candidate missions, aims at globally observing, for the first time, simultaneously vertical profiles of reflectivities and line of sight winds in cloudy and precipitating regions. WIVERN adopts a dual-polarization Doppler radar in order to overcome the short decorrelation time between successive radar pulses transmitted from low Earth-orbiting satellites with finite beamwidth antennas. WIVERN transmits a single polarization state at a time (H or V), receives in both polarization states, and uses the Polarization Diversity Pulse Pair (PDPP) technique to estimate the Doppler velocity. The weaker cross-polar signals can sometimes interfere with the co-polar ones, causing ghost signals in the measurements that hinder the system’s overall performance. Additionally, with the envisaged radar trigger mode, parameters such as Linear Depolarization Ratio (LDR) and Differential Reflectivity (ZDR) cannot be directly measured because of the nearly simultaneous transmission of H and V pulses. To overcome these challenges, this article presents a novel technique based on the Optimal Estimation (OE) algorithm for retrieving LDR, ZDR, and co-polar reflectivity for radars operated in PDPP mode. The performance of the proposed method is evaluated using a realistic climatology of profiles simulated from CloudSat data. Results demonstrate that co-polar reflectivity can be accurately retrieved in regions with a good signal-to-noise ratio and in the absence of simultaneous cross-talk interference in both channels (which occurs very rarely). The LDR retrieval on the other hand is typically driven by the a-priori with a substantial impact of measurements only for the surface returns. The impact of cross-talk is also assessed on the reduction of precise Doppler measurements. Findings confirm that a selection of the separation between the two polarization diversity pulses (THV) of 20 μs achieves a good balance between the large errors originated by the strong dependence on the Doppler phase noise at smallTHVs and those caused by the drop in correlation and unambiguous Nyquist velocity at largeTHV.

Supercooled liquid clouds are very frequent in high-latitude regions. In addition to their substantial effect on visible and infrared radiation, they affect the signal of millimeter radars by producing nonnegligible attenuation. Such attenuation must be properly corrected if the information of millimeter radars is used in quantitative retrievals for inferring ice microphysical properties. This study proposes a multisensor scheme for refining the vertical distribution of supercooled liquid water content (SLWC) compared to state-of-the-art methods that equipartition the liquid water path measured by microwave radiometer to all pixels identified as cloudy by the radars and warmer than −40 °C. Our methodology is applicable in high-latitude, mixed-phase environments based on the synergy between radar and lidar binary cloud phase masking, microwave radiometer, and radio sounding observations. The technique is demonstrated via data collected by the U.S. Department of Energy (DoE) Atmospheric Radiation Measurement (ARM) Program climate research facility at the North Slope of Alaska (NSA) and compared with the state-of-the-art methods. Path integrated attenuation (PIA) at W- and G-band frequencies (>95 GHz) is then assessed. Results indicate that the different in-cloud distributions of the liquid condensate lead to round-trip PIA discrepancies of cloudy volumes that range in [2, 5] dB at W- and G-band frequencies. These differences far exceed those encountered when changing some of the algorithm’s arbitrary assumptions and weighting functions.

Attenuation from clouds and precipitation hinders the use of Ka-band in SARs, radar altimeters and in satellite link communications. The NASA-JAXA Global Precipitation Measurement (GPM) mission, with its core satellite payload including a dual-frequency (13.6 and 35.5 GHz) radar and a multifrequency passive microwave radiometer, offers an unprecedented opportunity for better quantifying such attenuation effects. Based on four years of GPM products, this article presents a global climatology of Ka-band attenuation caused by clouds and precipitation and analyses the impact of the precipitation diurnal cycle. As expected, regions of high attenuation mirror precipitation patterns. Clouds and precipitation cause two-way attenuation at 35.5 GHz in excess of 3 dB about 1.5% of the time in the regions below 65°, peaking at as much as 10% in the tropical rain belt and the South Pacific Convergence Zone and at circa 5% along the storm tracks of the North Atlantic and Pacific Oceans. Confirming previous findings, the diurnal cycle is particularly strong over the land and during the summer period; while over the ocean, the diurnal cycle is generally weaker some coherent features emerge in the tropical oceans and in the northern hemisphere. Results are useful for estimating data loss from (sun-synchronous) satellite adopting active instruments/links at a frequency close to 35 GHz.

This work describes a conically scanning space-borne Dopp-lerised 940Hz radar Earth Science mission concept, WIVERN, Wind VElocity Radar Nephoscope. WIVERN will provide global measurements of in-cloud winds using the Doppler shifted radar returns from hydrometeors. The conically scanning radar is expected to provide wind data with daily revisits poleward of 50° at 50 km horizontal resolution and approximately 1km vertical resolution. The measured winds, when assimilated into weather forecasts and provided they are representative of the larger scale mean flow, should lead to further improvements in the accuracy and effectiveness of forecasts of severe weather and better focusing of activities to limit damage and loss of life. Surface clutter should impede wind measurement below 1 km above the ocean surface and below 2 km over land. The potential impact of the WIVERN winds on reducing forecast errors is estimated by comparison with the known positive impact of winds as measured by cloud motion and aircrafts. The main thrust of WIVERN is observing in-cloud winds, but WIVERN will also provide global estimates of ice water content, cloud cover and vertical distribution of cloud and ice water contents, thus continuing the data series started by CloudSat. The conical scan will provide increased coverage. As with CloudSat, estimates of rainfall and snowfall rates should also be possible. These additional products may also have a positive impact when assimilated into weather forecasts.

By comparing the observations of heavy storms performed by the Dual-frequency Precipitation Radar (DPR) and the corresponding hydrometeor classification based on ground-based polarimetric measurements of the Next Generation Weather Radar (NEXRAD) we showed that the DPR measurements are heavily affected by non-uniform beam filling (NUBF). The presence of heavily rimed particles within the instrument field of view generates significant signal enhancements at the Ka band caused by multiple scattering (MS). MS and NUBF introduce large ambiguities in the estimate of effective reflectivity below the freezing level (FL), especially at Ka band, which strongly reduces DPR capabilities for detecting hail at the ground.

This study compares the instantaneous surface rain rate estimates over Great Britain and Ireland (GBI) from the spaceborne dual-frequency precipitation radar (DPR) and the GPM microwave imager (GMI) on board the GPM Core Observatory (GPM-CO) to estimates from the ground-based United Kingdom Meteorological Office's ground-radar network. In particular, the version-5, level-2 DPR and DPR-GMI (CMB) combined products (5 km resolution) and the Radarnet 4 radar composite product (1 km resolution) are used for the three year study (May 2014 - April 2017). Products are collocated both temporally and spatially, and subject to quality control, prior to the comparison where the Radarnet product is considered to be the “ground truth”. The GPM products are found to underestimate the surface rain rates detected by the Radarnet product from a sample of 575512 collocated 5 km data. The CMB product (bias -2% and correlation 0.49) performs better in comparison to the DPR product (bias -17% and correlation 0.42). Large standard deviations of around 132% suggest that the results are highly variable.

This paper presents the results of a recent flight campaign conducted over the Great Lakes region and reports the first observations of the W-band normalized backscattered cross section ($\sigma _{0}$) for V and H polarization and the linear depolarization ratios (LDRs) from different types of surfaces at moderate incidence angles ($\sigma _{0}$behaves as previously reported at small incidence angles, it features a marked decrease with increasing incidence angles between 20° and 50°. There is a strong dependence of normalized backscattered cross sections both on the wind speed and on the wind direction, with larger values found in the presence of higher wind speeds and when the radar antenna is looking upwind. This is in line with theoretical models (though models tend to overpredict the range of variability at a given incidence angle) and with observations at lower frequencies. The LDRs are steadily increasing from values certainly lower than −30 dB, at vertical incidence, to the values of about −10 dB, at the incidence angles of about 60°–70°, with a good matching between observations and theoretical predictions. On the other hand, land surface backscattering properties are not characterized by a strong angular dependence:$\sigma _{0}$and LDR values typically range between −20 and 0 dB and between −15 and −5 dB, respectively. This paper is relevant for spaceborne concepts of W-band radars, which envisage moderate incidence angles to achieve a broad swath needed for global coverage.

Antenna-pointing techniques that rely on Earth's surface Doppler velocity measurements are expected to play a crucial role in enabling antenna mispointing corrections in spaceborne radar systems. Here, the impact of the EarthCARE cloud profiling radar (CPR) receiver saturation on the quality of the surface Doppler velocity measurements is discussed. The CPR linear receiver is expected to saturate always from surface echoes. Our results based on an I/Q simulation framework show that for the EarthCARE radar configuration: 1) biases introduced by saturation will be negligible; 2) the standard deviation of the velocity estimates will increase by 30-50% when moving from unsaturated to completely saturated surface return for a corresponding pulse repetition frequency in the range between 6.1 and 7.5 kHz. As a consequence longer integration times will be necessary to achieve the same accuracy in presence of complete saturation.

The EarthCARE (EC) radar will be the first atmospheric cloud-profiling radar in space with Doppler capabilities. The mitigation of mispointing uncertainties related to thermal distortions and vibrations of the antenna structure, and/or attitude determination errors represents one of the several challenges to overcome for the reaching of high-quality Doppler velocity products. In addition to the possibility of adopting mispointing correction techniques based on the surface Doppler velocity in clear sky regions, we propose to use the presence of abundant natural targets (ice clouds) with climatologically very well characterized Doppler velocities as calibration points. This is demonstrated by showing that the ice clouds present on average in each EC orbit (emulated with CloudSat data) can be used to accurately retrieve the effect of an orbital harmonic, which represents the most relevant component in the modeling of the antenna mispointing error expected for the EC satellite. The residual errors associated to the orbital harmonic mispointing are negligible (<; 0.03 m/s) compared with the other errors typically involved in Doppler velocities observed from fast-moving platforms. On the other hand, high-frequency components of the torque spectrum cannot be mitigated and, according to the current thermal and mechanical models for the EC antenna, are estimated to contribute to an RMS error of 0.2 (0.3) m/s in the nominal (worst case) scenario.

Multiple scattering (MS) strongly affects CloudSat's W-band cloud-profiling radar (CPR) reflectivity when the satellite is overpassing moderate and heavy precipitation systems. With the upcoming deployment of W-band Doppler radars in space-Earth Clouds, Aerosols, and Radiation Explorer's CPR in primis-and the goal of retrieving vertical motions within convective systems, there is an urgent need to assess the impact of MS onto the Doppler signatures. A Monte Carlo code capable of simulating the reflectivity enhancement due to higher orders of scattering has been extended to include the Doppler effects. This paper presents the main guidelines for the inclusion of the Doppler analysis into the Monte Carlo scheme. To our knowledge, this simulator is the first one capable of simulating realistic Doppler signals in the presence of MS. The case studies are first presented in uniform beam-filling conditions for the profiles extracted from a cloud-resolving model simulation of deep convection (i.e., 1-D profiles are used to characterize a stratified atmosphere). The simulations demonstrate that, at ranges where MS contributions affect the overall radar signal, two main features appear as the following: 1) The Doppler spectrum tends to broaden with increasing MS enhancement, adding up to the single-scattering (SS) Doppler fading due to the satellite motion; and 2) the mean Doppler of the backscattered signal departs from the mean Doppler determined by the combined effect of the vertical-wind and hydrometeor-terminal velocities at all range bins below the altitude where the MS contribution significantly overcomes the SS. The simulator can be run in nonuniform-beam-filling conditions as well (i.e., a 3-D field is used to characterize the atmosphere at scales smaller than the radar resolution). With its cutting-edge capabilities, it provides a unique tool for the evaluation of the performances of the upcoming high-frequency spaceborne Doppler radars.

During the Convective and Orographically Induced Precipitation Study, advanced microwave radiometer for rain identification has continuously acquired measurements at the Atmospheric Radiation Measurement Mobile Facility in the Black Forest from the beginning of August until December 2007. The radiometer has six channels measuring in horizontal and vertical polarizations at 10.65, 21.0, and 36.5 GHz. Rainy events have been selected out of the entire database according to collocated gauges and, subsequently, analyzed. Measured brightness temperatures and (vertical-horizontal) polarization differences are interpreted by comparing with radiative transfer simulations, which account for the presence of nonspherical particles in preferential orientation. Measurements confirm the importance of the polarization signal for separating the effect introduced by non-Rayleigh scatterers and, therefore, the rain from the cloud component. More quantitative interpretation of the signal requires a better understanding of the role played by melting particles and an identification of the 3-D structure of the precipitating system under observation. Both aspects will be tackled in the near future by exploiting the synergy with a coinstalled micro rain radar.

A 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.