Failures in detecting volcanic ash from a satellite-based technique - Response

Prata et al. [Remote Sens. Environ. (2001)] state that our analysis [Remote Sens. Environ. 72 (2000) 191] "suffers from a fundamental flaw in its meth odology and numerous errors in fact and interpretation." We assert that Pra ta et al. [Remote Sens. Environ. (2001)] are incorrect. Our original analys is, augmented herein, shows that from an aviation safety perspective, their T-4-T-5 volcanic ash detection algorithm does not meet the requirements of the aviation industry. For arbitrary satellite scenes, their algorithm: (1 ) underdetects airborne volcanic ash; (2) yields numerous false alarms; and (3) does not satisfy the 5 min warning imperative mandated by the aviation industry. Independent evidence and unique in situ validation data from the NASA DC-8 encounter with volcanic products from the recent Hekla eruption further support our original analysis and conclusions. Factors affecting th e usefulness of their algorithm within the context of aviation safety inclu de, but are not limited to, ambient atmospheric water vapor, ground and juv enile water in the magma as well as its chemical composition, cloud cover, atmospheric ice crystals, and the general applicability of the theoretical assumptions underlying their T-4-T-5 volcanic ash detection algorithm. The new analyses presented herein, as well as those of Simpson et al. [Remote S ens. Environ. 72 (2000) 191], show that new approaches are needed to addres s the complex problem of accurate and rapid detection of airborne volcanic ash. 2001 Elsevier Science Inc. All rights reserved.