The WIVERN (WInd VElocity Radar Nephoscope) mission, now in Phase 0 of the ESA Earth Explorer program, promises to complement Doppler wind lidar by globally observing, for the first time, the vertical profiles of winds in cloudy areas. This work describes an initial assessment of the performances of the WIVERN conically scanning 94 GHz Doppler radar, the only payload of the mission. The analysis is based on an end-to-end simulator characterized by the following novel features tailored to the WIVERN radar: the conically scanning geometry, the inclusion of cross-polarization effects and the simulation of a radiometric mode, the applicability to global cloud model outputs via an orbital model, the incorporation of a mispointing model accounting for thermoelastic distortions, microvibrations, star-tracker uncertainties, etc., and the inclusion of the surface clutter. Some of the simulator capabilities are showcased for a case study involving a full rotational scan of the instrument.
Preliminary findings show that mispointing errors associated with the antenna's azimuthal mispointing are expected to be lower than 0.3 m s
The simulator will be used for studying tradeoffs for the different WIVERN configurations under consideration during Phase 0 (e.g., different antenna sizes, pulse lengths, and antenna patterns). Thanks to its modular structure, the simulator can be easily adapted to different orbits, different scanning geometries, and different frequencies.
Accurate forecasts save lives and support emergency management and the mitigation of impacts, thus preventing losses from severe weather while creating substantial revenue
The WIVERN (WInd VElocity Radar Nephoscope) concept has been recently proposed within the ESA Earth Explorer 11 call in order to strengthen the wind, cloud, and precipitation observation capability of the Global Observing System. The mission has been selected for Phase 0 studies. It hinges upon a single instrument, i.e., a dual-polarization Doppler W-band scanning cloud radar with a 3 m circular aperture non-deployable main reflector. The WIVERN antenna conically scans around nadir at an off-nadir angle of 38
Artistic impression of the WIVERN concept. A 94 GHz Doppler radar with 3 m antenna scanning at 12 rpm tracing out a cycloidal track with an incidence angle of 41.6
The aim of the mission is to complement Doppler lidar winds acquired in clear-sky conditions and from the tops of optically thick clouds unprecedented wind observations inside tropical cyclones and mid-latitude windstorms that will routinely reveal the dynamic structure of such destructive systems; observations of convective motions that will validate the representation of convection in models; global profiles of cloud properties and precipitation over an 800 km swath that will better quantify the hydrological cycle and the atmospheric and surface energy budget; and the first direct observation of tropospheric winds that will underpin the predictions of transport and dispersion of trace gases and pollutants in atmospheric chemistry and air quality models. Extending the lead time of useful prediction skills of hazardous weather (e.g., wind storm, cyclones, and floods) by direct assimilation of wide swath winds from clouds and profiles of radar reflectivity of clouds and precipitation into numerical weather prediction (NWP) models. Improving numerical models by providing new metrics and observational verification to assess different NWP parameterization schemes within such models. NWP and climate models use similar schemes, so better NWP models will also augment confidence in climate models. Establishing a benchmark for the climate record of cloud profiles, global solid/light precipitation, and, for the first time, in-cloud winds, crucial for a better quantification of the Earth's hydrological cycle, and energy budgets, with a significant reduction in the sampling errors of current and planned cloud radar missions.
These advances in the observational capabilities are expected to address the following three science objectives
World Meteorological Organization (WMO) requirements for data assimilation into global NWP
WMO (World Meteorological Organization) requirements for horizontal winds for numerical weather prediction (NWP) and the expected performance of WIVERN.
In order to achieve these targets, WIVERN will adopt the following:
polarization diversity (i.e., the use of successive pulses with independent a large antenna (3 m) in order to achieve a narrow beam, thus giving a fine vertical resolution and fewer issues related to non-uniform beam-filling (NUBF) biases
Previous studies
End-to-end (E2E) simulators are paramount tools for evaluating instrument performances in preparatory mission studies. They provide a high-fidelity performance prediction of the overall system. The focus of this study is in the mission performance assessment and error budget computation with a detailed partitioning of the different error contributors. Several radar simulators have been developed in the recent years to simulate space-borne atmospheric radars (e.g.,
Our simulator capitalizes on recent refinements of radar simulators developed within different ESA projects. In particular, it benefits from the inclusion of polarization diversity pulse pair processing and wide swath scanning
The simulator developed in this work can cope with data produced by state-of-the-art, high-resolution cloud-resolving models as the basis for creating scenes that are used as input to the various instrument simulation modules. These outputs can be linked with sun-synchronous orbits produced by an orbital model derived from the two-body problem theory, with the addition of
Flow chart illustrating the overall structure of the Wivern E2E simulator. The integrated hydrometeor content and the 4.0 km height winds are shown at the top as examples of input fields from the reference global model whereas outputs of the simulator (reflectivities and line-of-sight (LOS) winds) for a WIVERN cross section that will be examined later (Figs.
Current and future capabilities of the WIVERN E2E simulator.
A schematic for the overall structure of the simulator is depicted in Fig.
The description of the different modules of the simulator is detailed in the following subsections. The radar specifics used throughout this paper are the ones recently proposed to the ESA Earth Explorer 11 and are listed in Table
Specifics of the radar for the simulation. The configuration adopted here is the one proposed for WIVERN in a recent ESA Earth Explorer 11 call. The E2E simulator can study various tradeoffs to optimize mission, system, and instrument parameters.
The global storm-resolving models
Here, output from a GSRM that participated in the DYAMOND project, the System for Atmospheric Modeling (SAM;
The orbit selected for WIVERN is sun-synchronous, with a mean inclination of 97.4
Example of a simulation of five WIVERN orbits with the ground tracks (red lines) and the 800 km WIVERN scanning swath (red-shaded region) plotted over the hydrometeor one-way path integrated W-band attenuation (the color bar scale is in dB). A single model snapshot is used for the simulation.
The radar is sounding the atmosphere down to the ground with a range resolution of 500 m. Figures
Illustration of the satellite-scanning geometry. The boresight direction (solid green arrow) is identified by the elevation angle
Schematic illustrating the 2D projection onto the antenna elevation cut of the WIVERN observing geometry. The specifics of the radar are detailed in Table
Scattering properties (extinction and backscattering coefficients, single scattering albedo, asymmetry parameters, and Doppler velocities) at each model grid point are computed by adding up the contributions from the different hydrometeors (cloud water, cloud ice, rain, and snow). Gas attenuation is computed according to the
The normalized surface backscattering cross sections (
The point target response (PTR) is assumed to be a simple top hat with a pulse length,
Since the WIVERN antenna is circular, a simple Gaussian antenna pattern is assumed with a one-way gain equal to the following:
The Doppler velocity in radar systems is derived by measuring phase shifts between successive pulses (pairs). Since phases are measured with a
WIVERN will transmit pairs of 3.3
The fundamental radar quantities are the range dependent
The power received by the radar from the atmosphere,
The power received by the radar from the surface at a range
The total reflectivity signal is obtained by adding up the atmospheric and the surface contributions, e.g., for the
To simulate a Doppler radar with polarization diversity profiles, cross-polar returns are also needed. These are obtained by performing the same integrals but using the cross-polar reflectivities via LDR and the cross-polar surface NRCS,
Similarly, the signal received in the
Since the Doppler spectral widths,
The radar Doppler velocities also have a component associated with the hydrometeor and one with the surface. The former is given by the following:
Similar to Eq. (
Doppler velocities estimated via pulse pair processing also have intrinsic noise associated with the phase and thermal noise and to the cross-polarization interference. Uncertainties depend on the signal-to-noise ratio (SNR), the radar Doppler spectral width, and the number of averaged samples
where we have introduced definitions for the signal-to-noise (SNR) and signal-to-ghost (SGR) ratios. A Gaussian random noise with a standard deviation corresponding to Eq. (
For accurate winds, the pointing of the radar beam formed by the antenna must be known very accurately. For instance, a 140
The amplitude of the two-sided spectrum of the signal is calculated from the two-sided PSD by taking the square root and adding to each sample a random phase in the [0, 2
WIVERN is also envisaged to have a radiometric mode. During the 250
The simulator rationale is demonstrated for a case study simulating an overpass over Labrador, Canada, on 5 September 2017, with a cold front moving eastward from inland. The satellite is moving northward and is scanning counterclockwise. The satellite ground track over North America is shown in Fig.
Conceptual model of the azimuth absolute knowledge error PSD. Contributions from different mechanisms are expected in different regions of the spectrum. For instance, sharp peaks are expected in correspondence to the scan harmonics.
Antenna weighted hydrometeor content in grams per cubic meter (g m
Atmospheric
Reflectivities and mean Doppler velocities for the atmospheric and surface targets computed, according to the methodology described in Sect.
The surface Doppler velocities, sampled at very fine range resolution (Fig.
Linear depolarization ratio (LDR;
The LDR values shown in Fig.
Reflectivity and Doppler velocity results corresponding to a full revolution of the WIVERN antenna, as shown in Fig.
The two panels of Fig.
The presence of ghosts arising from surface cross-talk is obvious around an altitude of
Another WIVERN product is the
Simulated brightness temperatures for
The E2E simulator represents a useful tool for studying the performances of the WIVERN mission. Apart from the errors related to the Doppler estimators in the pulse pair processing (Eq.
The wind shear errors which tend to occur when reflectivity and velocity gradients are present at the same time within the backscattering volume, as can happen at the boundaries of clouds, can be computed from the difference between
Errors induced by wind shear
Estimates of the NUBF errors can be obtained by comparing the expression of
Histogram of NUBF-induced error as a function of the azimuthal scanning angle. The color indicates the
This study introduces a state-of-the-art E2E simulator tailored to simulating space-borne conically scanning Doppler radars adopting polarization diversity with the inclusion of a radiometric mode. The WIVERN configuration, as proposed to the ESA Earth Explorer 11 call (see specifics in Table
Preliminary findings show that mispointing errors associated with the antenna azimuthal mispointing are expected to be lower than 0.3 m s
The characterization of the errors and the isolation of each single error source makes the E2E simulator a useful tool to verify mission performances and compliance with requirements, which will be part of the Phase 0 studies that started in December 2021 and due to end in October 2023. Different problematic areas will be investigated with the introduction of new features (see Table By changing the antenna gain (Eq. More sophisticated surface modeling could be introduced by including the dependence on the surface winds over the ocean and different surface types over land. Cloud scenes at finer horizontal resolution ( A multiple scattering module, based on the two-stream approximation Additional polarimetric variables like specific differential phase ( Further studies on mispointing effects will be performed once power spectral densities of azimuth and elevation knowledge error are better specified by industrial studies. In particular, the E2E simulator will be able to assess how frequently and with which accuracy the surface return could be used to check the elevation pointing. The E2E simulator will also serve as the basis to develop mitigation algorithms for NUBF, wind shear, mispointing, and vertical wind corrections that will be needed in order to produce horizontal line of sight winds, which will be the product directly assimilated by numerical weather prediction models.
Thanks to its modular structure, the simulator can be easily adapted to different orbits, a gamut of scanning geometries (e.g., cross-track), and various frequencies (by simply changing the look-up tables). Therefore, the simulator could be applied to simulate other space-borne Doppler atmospheric radars as well.
The simulation inputs are available on request. The E2E simulator code is not yet available it is because part of ongoing ESA studies.
AB wrote most of the text and has built most of the modules of the simulator. PM implemented the code in MATLAB and produced most of the figures. EC and LP provided inputs for the orbital model, and to the mispointing model plus, they contributed to the scientific discussion. FS provided supervision to PM and participated to the discussion on radar mispointing issues. PK and AI contributed to the discussion, the editing, and the formulation of the WIVERN idea and definition.
At least one of the (co-)authors is a member of the editorial board of
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Paolo Martire's work has been funded by Compagnia di San Paolo, Turin, Italy. This research used the Mafalda cluster at Politecnico di Torino.
This research has been supported by the European Space Agency (Doppler Wind Radar Science Performance Study; ESA contract no. 4000130864/20/NL/CT).
This paper was edited by William Ward and reviewed by two anonymous referees.