Use of NDVI Satellite Data to Identify and Document Destructive Hail Swaths

Hailstorms cause millions of dollars in damage each year. Yet these storms remain poorly documented within Storm Data with many events comprising only a handful of hail and wind reports from point sources. As a result, the full impact and areal extent of hail damage may be underestimated. Recent research has shown promise in using Normalized Difference Vegetation Index (NDVI) from MODIS and LANDSAT to identify hail swaths. This can be done by comparing two images – one before and one after the hail storm – and taking the difference in NDVI. In these cases, it was found that the hail swath could be identified by large changes in NDVI. It was documented that the change in NDVI varied along the hail track perhaps indicating periods during the hail storm where it was much more destructive. As a result, a research study was implemented with the goal of conducting and relating damage surveys of destructive hailstorms, and corresponding NDVI differences, to damage to crops and other vegetation. Additional goals were to determine if changes in NDVI differences along the storm path could provide validation of the Warning Decision Support System – Integrated Information (WDSS-II) Maximum Expected Hail Size (MESH) products and be related to changes in storm evolution observed within radar data. This study takes the method used by Gallo et al. 2012 and applies it to an event in northeast Nebraska on 18 August 2011. A supercell moved from Yankton, South Dakota southeastward across Dixon County Nebraska and into west central Iowa. Hail up to 10.1 cm (4 in) in diameter was accompanied by winds over 30 m s-1 producing widespread agricultural damage as well as damage to homes. On 25 August 2011, a damage survey was done over Dixon County, Nebraska and photographs were taken to document the damage to agriculture. This was followed by a phone survey within one township of Dixon County in which estimated hail size and wind speeds were recorded. Areas where there were both winds exceeding 20 m s-1 and hail greater than 4.5 cm had the greatest documented damage and the largest NDVI changes. In addition, the variation in NDVI differences along the storm path generally corresponded to observed variations in damage to crops. The NDVI difference were also compared to different radar signatures and algorithms, such as composite reflectivity above predefined isothermal surfaces, to determine if the vertical extent of high reflectivity values above these surfaces could serve as a predictor to aid forecasters in accurately warning the public of a destructive hailstorm.