2-3 November 2000EOS-IDS Team Meeting1 November 2000 The Simpson Debacle Failures in Detecting Volcanic Ash from a Satellite-Based Technique James J. Simpson,* Gary Hufford, † David Pieri, ‡ and Jared Berg* Remote Sensing of Environment 72:191 – 217 (2000) The Simpson Debacle

2-3 November 2000EOS-IDS Team Meeting2 November 2000 The Simpson Debacle •Highlights problem with water vapour (Already well-known from work of Rose and Prata)  Uses ‘ eye-ball ’ method as ‘ truth ’ to test T 4 -T 5 method  Misunderstands radiative transfer - suggests detection is based on ‘ magic numbers ’ and ‘ magic shapes ’ •Misconstrues operational use and ignores other satellite- based methods (e.g. TOMS, movie-loops) •Ignores other effects on radiances and disregards context •Comment by Prata, Bluth, Rose, Schneider and Tupper •Reply by Simpson et al is 42 pages long !

2-3 November 2000EOS-IDS Team Meeting3 November 2000 The Simpson Debacle A major difficulty with Simpson ’ s paper is that it does not propose or use an objective independent method against which the T4-T5 method can be tested. Instead, he introduces another method and assumes it is 100 % correct.

2-3 November 2000EOS-IDS Team Meeting4 November 2000 The Simpson Debacle Simpson refers to these as ‘ magic shapes ’

2-3 November 2000EOS-IDS Team Meeting5 November 2000 The Simpson Debacle  =ratio of extinction coefficients at 11 µm and 12 µm Ice/water cloudAsh cloudOpaque cloud Magic Shapes ?

2-3 November 2000EOS-IDS Team Meeting6 November 2000 The Simpson Debacle • T 4 -T 5 detection assumes no water vapour is present in cloud • This is generally not true and the problem has been known from studies since 1989 • Recent research by Rose and Prata, Yu et al and others has been addressing this problem Water Vapour Effects

2-3 November 2000EOS-IDS Team Meeting7 November 2000 The Simpson Debacle Water vapour effects have been modelled using MODTRAN-3 and an empirical correction scheme devised to remove water vapour effects. The correction effectively rotates the T 4 -T 5 vs T 4 distribution and allows a quantitative estimate of the fraction of ash in a pixel to be determined.

2-3 November 2000EOS-IDS Team Meeting8 November 2000 The Simpson Debacle Precipitable water

2-3 November 2000EOS-IDS Team Meeting9 November 2000 The Simpson Debacle T 4 -T 5 Simulations

2-3 November 2000EOS-IDS Team Meeting10 November 2000 The Simpson Debacle Upper bound:  T wv =exp[20T* - 18] Lower bound:  T wv =exp[6T*- b] T*=T 4 /T max b is determined from the data

2-3 November 2000EOS-IDS Team Meeting11 November 2000 The Simpson Debacle Original distribution Water-vapour corrected distribution

2-3 November 2000EOS-IDS Team Meeting12 November 2000 The Simpson Debacle  T = F  T s [ Z - Z  ] Z = 1 -  T 4 F TsF Ts  T 4 = T 4 - T c  T s = T s - T c Volcanic Ash Absorption Model  ratio of extinction coefficients F = ash fraction

2-3 November 2000EOS-IDS Team Meeting13 November 2000 The Simpson Debacle

2-3 November 2000EOS-IDS Team Meeting14 November 2000 The Simpson Debacle Ash Fraction Maps A quantitative product for aviation use (?)

2-3 November 2000EOS-IDS Team Meeting15 November 2000 The Simpson Debacle Simpson ‘ eye- ball ’ detection T 4 -T 5 detection Simpson concludes T 4 -T 5 is wrong because it does not agree with the ‘ eye-ball ’ method. But, what is the truth ? Perhaps both are in error ? Negative values over clear land at night Pixel mis-alignment effects

2-3 November 2000EOS-IDS Team Meeting16 November 2000 The Simpson Debacle Are all plumes volcanic ? Simpson ’ s methodology assumes that a plume is volcanic regardless of its context and origin. But there are many meteorological instances where a plume is not volcanic, even though it may be near or coincide with a known volcano. Ulawun volcano Drifting plume ?

2-3 November 2000EOS-IDS Team Meeting17 November 2000 The Simpson Debacle Negative T 4 -T 5 differences can (and do) occur because the instantaneous fields-of-view (IFOVs) of channels 4 (11 µm) and 5 (12 µm) are not concentric. For a 2% mis- alignment, T4-T5 is negative near the edge of a cloud at a temperature different to that of the surface

2-3 November 2000EOS-IDS Team Meeting18 November 2000 The Simpson Debacle  Negative T 4 -T 5 temperature differences often occur at night over the clear land surface. In some of Simpson ’ s examples these effects can be seen. This is a well- known effect (see Platt and Prata, 1993). The image shows negative T 4 -T 5 (coloured green) over a region of central Australia, well away from any erupting volcanoes. Context is important. Negative T 4 -T 5 over clear land at night Platt, C. M. R., and A. J. Prata, 1993, Nocturnal effects in the retrieval of land surface temperatures from satellite measurements, Rem. Sensing Environ., 45:

2-3 November 2000EOS-IDS Team Meeting19 November 2000 The Simpson Debacle