Mind the Gap - Part 3: Doppler Velocity Measurements From Space

Abstract

Convective motions and hydrometeor microphysical properties are highly sought-after parameters for evaluating atmospheric numerical models. With most of Earth’s surface covered by water, space-borne Doppler radars are ideal for acquiring such measurements at a global scale. While these systems have proven to be useful tools for retrieving cloud microphysical and dynamical properties from the ground, their adequacy and specific requirements for spaceborne operation still need to be evaluated. Comprehensive forward simulations enable us to assess the advantages and drawbacks of six different Doppler radar architectures currently planned or under consideration by space agencies for the study of cloud dynamics. Radar performance is examined against the state-of-the-art numerical model simulations of well-characterized shallow and deep, continental, and oceanic convective cases. Mean Doppler velocity (MDV) measurements collected at multiple frequencies (13, 35, and 94 GHz) provide complementary information in deep convective cloud systems. The high penetration capability of the 13 GHz radar enables to obtain a complete, albeit horizontally under-sampled, view of deep convective storms. The smaller instantaneous field of view (IFOV) of the 35 GHz radar captures more precise information about the location and size of convective updrafts above 5–8 km height of most systems which were determined in the portion of storms where the mass flux peak is typically located. Finally, the lower mean Doppler velocity uncertainty of displaced phase center antenna (DPCA) radars makes them an ideal system for studying microphysics in shallow convection and frontal systems, as well as ice and mixed-phase clouds. It is demonstrated that a 94 GHz DCPA system can achieve retrieval errors as low as 0.05–0.15 mm for raindrop volume-weighted mean diameter and 25% for rime fraction (for a −10 dBZ echo).

Processing reflectivity and Doppler velocity from EarthCARE’s cloud profiling radar: the C-FMR, C-CD and C-APC products

“…The EarthCARE CPR uses a larger antenna (2.5 m compare to 1.6 m diameter for CloudSat) and operates at lower altitude (400 km versus 710 km for CloudSat). As a result, the EarthCARE CPR (hereafter EC-CPR) exhibits higher sensitivity (-36 dBZ versus -29 dBZ for CloudSat) and it is the first atmospheric radar in space with Doppler velocity measurement capability (Kollias et al, 2018(Kollias et al, , 2022. A comprehensive list of L2a (single instrument) and L2B (synergistic) data products has been designed and implemented to achieve the EarthCARE mission scientific objectivites.…”

“…The FM algorithm is based on Clothiaux et al (1995); Marchand et al (2008). a sophisticated spaceborne radar simulator that accurately accounts for all the technical specifications (i.e., antenna pattern, range weighting function, along track integration) of the EC-CPR (Kollias et al ( , 2022) . In addition to the ECCC GEM hydrometeor locations, panel (a) also includes the Earth's surface return and gaseous and hydrometeor attenuation at 94-GHz.…”

“…Center Antenna (DPCA), (Kollias et al, 2022) concept that can minimize the impact of the high platform motion. Considering that the antenna (and thus the antenna beamwidth) are fixed, the only remaining parameter that controls the performance depends considerably on the selected PRF (Kobayashi et al, 2002) which varies between 6100 and 7400 Hz in an orbit.…”

“…Considering that the antenna (and thus the antenna beamwidth) are fixed, the only remaining parameter that controls the performance depends considerably on the selected PRF (Kobayashi et al, 2002) which varies between 6100 and 7400 Hz in an orbit. The is significant if we consider that the EC-CPR Nyquist velocity V N is between 5 and 6 ms −1 (Tanelli et al, 2002;Illingworth et al, 2015;Kollias et al, 2022). The result of this broadening is a significant increase in the EC-CPR Doppler velocity measurement uncertainty especially at low Signal-to-Noise (SNR) conditions.…”

In orbit cross-calibration of millimeter conically scanning spaceborne radars

“…In this work a similar rationale is followed, with the conically scanning radars being calibrated by other space-borne radars that are routinely calibrated with the standard ocean surface return procedure. For the Ka-band the GPM-DPR, expected to fly till the end of the decade (Skofronick-Jackson et al, 2016;, represents a solid choice for the reference calibrator thanks to internal and external calibration procedures that reach an accuracy better than 1 dB (Masaki et al, 2022) whereas the EarthCARE CPR and, later in this decade, the radars envisaged to be part of the NASA Atmosphere Observing System (AOS) constellation (Kollias et al, 2022) should provide a viable option of well calibrated W-band radars (within 1 dB as well) obtained via the ocean surface calibration method. The key science question underpinning this work is: what cross-calibration accuracy can be achieved when intercalibrating the conically scanning and the reference radars in a given time period?…”