Detecting tornado tracks using Synthetic Aperture Radar (SAR) imagery

NASA SPoRT has been working to support the NWS’s use of the Damage Assessment Toolkit (DAT) by integrating multiple satellite datasets into the DAT framework to assist in damage surveys.  Imagery from MODIS, VIIRS, and Landsat 8 are available daily within the application while imagery from higher resolution satellites, such as Terra ASTER and other high-resolution commercial imagery are facilitated by our partnership with the USGS’s Hazard Data Distribution System (HDDSexplorer.usgs.gov). One new area being explored is the application of Synthetic Aperture Radar (SAR) imagery to detect tornado damage.

ClearkLake_WI_SAR_Tornado.gif

Zoomed-in section of the SAR change detection RGB generated from Sentinel-1B imagery from May 10 and May 22, 2017.  The damage indicators show preliminary track information as of June 2, 2017 and are not considered final.

On the evening of May 16th, 2017, a supercell tracked across Wisconsin producing a strong tornado. The resulting 83-mile long track tornado produced EF-3 damage.  Shown in the image above is a prototype change detection RGB using data from Sentinel-1(A/B), a European Space Agency (ESA) satellite with a SAR instrument on board. Unlike optical sensors, which observe surface reflectance and temperature, SAR instruments measure backscatter from the surface, allowing the instrument to be used at all times of the day and in any sky conditions. SPoRT has been working with the Alaska Satellite Facility, NASA’s SAR Distributed Active Archive Center (DAAC) to receive these products for evaluation and put them in the DAT to help with the identifying of damage tracks, especially in areas where damage surveys can be more challenging (i.e. forested areas, poor road network).  The RGB takes advantage of the dual polarization from the sensor, assigning the VV and VH corrected polarization from the post-event granule to the red and green channels of the RGB, respectively.  The blue channel is a difference image of the VH polarization (same as what is used in the green channel) from the before and after granules.  The resulting RGB will show any changes between the two granules in a aqua/periwinkle/purple-color.  Although the RGB will show all change between the granules over the ~12-day period (i.e. agricultural growth), tornado tracks tend to be linear, making it a possible to discern/identify the damage track.  Without the hindrance of clouds that constantly plague damage detection in optical imagery, SAR imagery offers another tool to operational forecasters for use during damage surveys.  The team is also working on other change or anomaly detection techniques to facilitate mapping of tornado and severe weather damage.

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