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 methodology and numerous errors in fact and interpretation." We assert that Prata et al. [Remote Sens. Environ. (2001)] are incorrect. Our original analysis, 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 the usefulness of their algorithm within the context of aviation safety include, but are not limited to, ambient atmospheric water vapor, ground and juvenile 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 Sens. Environ. 72 (2000) 191], show that new approaches are needed to address the complex problem of accurate and rapid detection of airborne volcanic ash. 2001 Elsevier Science Inc. All rights reserved.