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John C. Hubbert, James W. Wilson, Tammy M. Weckwerth, Scott M. Ellis, Mike Dixon, and Eric Loew

Abstract

The National Center for Atmospheric Research (NCAR) operates a state-of-the-art S-band dual-polarization Doppler radar (S-Pol) for the National Science Foundation (NSF). This radar has some similar and some distinguishing characteristics to the National Weather Service (NWS) operational Weather Surveillance Radar-1988 Doppler Polarimetric (WSR-88DP). One key difference is that the WSR-88DP is used for operational purposes where rapid 360° volumetric scanning is required to monitor rapid changes in storm characteristics for nowcasting and issuing severe storm warnings. Since S-Pol is used to support the NSF research community, it usually scans at much slower rates than operational radars. This results in higher resolution and higher data quality suitable for many research studies. An important difference between S-Pol and the WSR-88DP is S-Pol’s ability to use customized scan strategies including scanning on vertical surfaces ([range–height indicators (RHIs)], which are presently not done by WSR-88DPs. RHIs provide high-resolution microphysical structures of convective storms, which are central to many research studies. Another important difference is that the WSR-88DP simultaneously transmits horizontal (H) and vertical (V) polarized pulses. In contrast, S-Pol typically transmits alternating H and V pulses, which results in not only higher data quality for research but also allows for the cross-polar signal to be measured. The cross-polar signal provides estimates of the linear depolarization ratio (LDR) and the co- to cross-correlation coefficient that give additional microphysical information. This paper presents plots and interpretations of high-quality, high-resolution polarimetric data that demonstrate the value of S-Pol’s polarimetric measurements for atmospheric research.

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Scott M. Ellis, Peisang Tsai, Christopher Burghart, Ulrike Romatschke, Michael Dixon, Jothiram Vivekanandan, Jonathan Emmett, and Eric Loew

Abstract

A technique for correcting radar radial velocity Vr in airborne, nadir-pointing radar data using the surface of Earth as a reference is proposed and tested. Operating airborne Doppler radars requires correcting the radial velocity for platform motion. This can be accomplished with accurate beam-pointing and platform motion measurements. However, there are often residual pointing errors due to drift in inertial navigation systems (INS) and/or errors in platform-relative pointing. The technique proposed here takes advantage of the fact that the surface is stationary and the mean of the measured Vr at the surface Vrsurfmeas should be 0 m s−1. Therefore, if a good estimate of the mean Vrsurfmeas is made, it can be subtracted from the measured Vr to correct for errors due to residual pointing errors. The Vrsurfmeas data contain many independent deviations from 0 m s−1 due to various causes, including measurement variance and large deviations due to surface features. These deviations must be filtered out of Vrsurfmeas before the surface reference can be applied to correct the Vr data. A two-step filtering process was developed and tested. The first step removes large deviations in Vrsurfmeas and the second step removes the measurement noise. The technique was examined using data from three field campaigns and was found to improve the quality of Vr in all cases. The Vr bias was removed and the variance was substantially reduced. The approach is generally applicable to nadir-pointing airborne radar data.

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Peter H. Hildebrand, Wen-Chau Lee, Craig A. Walther, Charles Frush, Mitchell Randall, Eric Loew, Richard Neitzel, Richard Parsons, Jacques Testud, François Baudin, and Alain LeCornec

The ELDORA/ASTRAIA (Electra Doppler Radar/Analyese Stereoscopic par Impulsions Aeroporte) airborne Doppler weather radar was recently placed in service by the National Center for Atmospheric Research and the Centre d'étude des Environnements Terrestre et Planetaires in France. After a multiyear development effort, the radar saw its first field tests in the TOGA COARE (Tropical Oceans–Global Atmosphere Coupled Ocean–Atmosphere Response Experiment) field program during January and February 1993. The ELDORA/ASTRAIA radar (herein referred to as ELDORA) is designed to provide high-resolution measurements of the air motion and rainfall characteristics of very large storms, storms that are frequently too large or too remote to be adequately observed by ground-based radars. This paper discusses the measurement requirements and the design goals of the radar and concludes with an evaluation of the performance of the system using data from TOGA COARE.

The performance evaluation includes data from two cases. First, observations of a mesoscale convective system on 9 February 1993 are used to compare the data quality of the ELDORA radar with the National Oceanic and Atmospheric Administration P-3 airborne Doppler radars. The large-scale storm structure and airflow from ELDORA are seen to compare quite well with analyses using data from the P-3 radars. The major differences observed between the ELDORA and P-3 radar analyses were due to the higher resolution of the ELDORA data and due to the different domains observed by the individual radars, a result of the selection of flight track past the storm for each aircraft. In a second example, the high-resolution capabilities of ELDORA are evaluated using observations of a shear-parallel mesoscale convective system (MCS) that occurred on 18 February 1993. This MCS line was characterized by shear-parallel clusters of small convective cells, clusters that were moving quickly with the low-level winds. High-resolution analysis of these data provided a clear picture of the small scale of the storm vertical velocity structure associated with individual convective cells. The peak vertical velocities measured in the high-resolution analysis were also increased above low-resolution analysis values, in many areas by 50%–100%. This case exemplifies the need for high-resolution measurement and analysis of convective transport, even if the goal is to measure and parameterize the large-scale effects of storms. The paper concludes with a discussion of completion of the remaining ELDORA design goals and planned near-term upgrades to the system. These upgrades include an implementation of dual–pulse repetition frequency and development of real-time, in-flight dual-Doppler analysis capability.

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