3622 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 48, NO. 10, OCTOBER 2010 Simultaneous Observations and Analysis of Severe Storms Using Polarimetric X-Band SAR and Ground-Based Weather Radar Jason P. Fritz, Senior Member, IEEE, and V. Chandrasekar, Fellow, IEEE Abstract—Recent advances in synthetic aperture radar (SAR) technology have revived meteorological applications with this type of radar. SARs are designed for surface imaging, but now that several X-band multipolarization SAR satellites are in orbit, the attenuation and backscatter caused by precipitation can be better studied. The results presented here demonstrate some of the possibilities by analyzing observations from dual-polarization (HH, VV) TerraSAR-X (TSX) acquisitions over central Florida surrounding severe storms in August 2008. Simultaneous to the SAR acquisitions, WSR-88D ground weather radars in Melbourne and Tampa Bay, FL, collected reflectivity and radial velocity data; the observed strong precipitation cells from convective storms are colocated with severe attenuation in the corresponding SAR images. The observations from SAR measurements are explained quantitatively by converting ground radar reflectivity into space- borne radar attenuation via a theoretical model. In addition, polarization analysis comparing the SAR image to two additional TSX acquisitions 11 days apart and without rain provides an indication of storm-induced propagation effects on X-band SAR. Specifically, the copolar ratio Z dr and the copolar correlation differences exhibit behavior that is better explained by the precip- itation impact versus surface changes. Multiple regions with vary- ing ground cover, including urban, and storm characteristics are analyzed to highlight the complexity of meteorological research using SAR while revealing a potential use of the technology to investigate the storm structure. Index Terms—Attenuation, meteorological radar, polarimetric synthetic aperture radar, propagation losses, spaceborne radar, TerraSAR-X. I. I NTRODUCTION T HE GLOBAL hydrological cycle has a profound influ- ence on life on earth, and apprehension of its effects is a major scientific pursuit by researchers around the world. Understanding the variability and surface impact of rainfall is crucial to understanding our dynamic planet, and spaceborne sensors can provide valuable information to meet this goal, particularly in regions with sparse or nonexistent ground radar coverage. This need led to the highly successful Tropical Rain- fall Measurement Mission (TRMM), a joint project launched Manuscript received August 26, 2009; revised December 24, 2009 and March 11, 2010. Date of publication July 23, 2010; date of current version September 24, 2010. This work was supported by the NASA Earth and Space Science Fellowship Program under Grant NNX07AO55H and the NASA PMM Program. The authors are with the Department of Electrical and Computer Engineer- ing, Colorado State University, Fort Collins, CO 80523-1373 USA (e-mail: jpfritz@ieee.org; chandra@engr.colostate.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2010.2048920 in 1997 between the National Aeronautics and Space Ad- ministration (NASA) and the Japanese Aerospace Exploration Agency (JAXA) [1], [2]. The success of the TRMM has in turn prompted the follow-on Global Precipitation Measure- ment (GPM) mission [3] scheduled to be launched in 2013. In addition to microwave radiometers, a dual-frequency (Ku-/ Ka-band) nadir scanning radar will provide the core satel- lite of GPM with a more detailed precipitation measurement instrument. Observations of meteorology from spaceborne radar, how- ever, began with early synthetic aperture radar (SAR) missions. Atlas and Black [4] analyzed storms over the ocean as observed by the Seasat satellite [5]. Atlas and Moore [6] also developed theoretical expressions to measure precipitation using SAR, whereas Pichugin and Spiridonov [7] presented a geometric model from a real aperture side-looking radar. In 1994, the NASA Space Shuttle Radar Laboratory missions provided the first multipolarization and multifrequency radar observations of precipitation [8]–[10] from space. Observations of storms were made both at traditional SAR look angles (oblique and side looking) and at nadir in preparation for TRMM. However, the X-band SAR that was part of the SIR-C/X-SAR missions had only one polarization: vertical. These studies showed the potential of SAR for precipitation measurement, particularly at X-band, where the signal is attenuated the most. C-band SAR observations of storms over the ocean have also been compared with ground-based weather radar using the European Space Agency’s ERS platform [11], [12]. Since the turn of the century, several X-band SAR (X-SAR) missions have been successfully launched or are planned for the near future, providing an opportunity to investi- gate global precipitation with high-resolution spaceborne radar. In 2007, the Deutches Zentrum für Luft und Raumfahrt (DLR) launched the multipolarization X-SAR TerraSAR-X (TSX) [13] and is planning the follow-on mission TanDEM-X. The first two of four satellites in the COnstellation of small Satellites for the Mediterranean basin Observation (COSMO)-SkyMed multipolarization X-SAR constellation by the Agenzia Spaziale Italiana (ASI) were also propelled into orbit in 2007. As of August 2009, three COSMO-SkyMed systems are orbiting the planet. Within the next several years, more X-SARs will be launched by Korea (KOMPSat-5), Russia (Severjanin on METEOR-M), and others. As a result of these new sensors, and the fact that precipitation has more of an impact on X-band wavelengths, renewed interest in SAR precipitation measurement has surfaced. Danklmayer et al. [14] reported the first images of storms observed by TSX during the 0196-2892/$26.00 © 2010 IEEE