2-3 november 2000eos-ids team meeting1 november 2000 the simpson debacle failures in detecting...

2-3 November 2000 EOS-IDS Team Meeting 1November 2000 The Simpson Debacle

Failures in Detecting Volcanic Ash from aSatellite-Based Technique

James J. Simpson,* Gary Hufford, David Pieri, and Jared Berg*

Remote Sensing of Environment 72:191217 (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 T4-T5 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 !


A major difficulty with Simpsons 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.


Simpson refers to these as magic shapes


b=ratio of extinction coefficients at 11 µm and 12 µm

Ice/water cloudAsh cloud Opaque cloud

Magic Shapes ?


T4-T5 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


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 T4-T5 vs T4 distribution and allows a quantitative estimate of the fraction of ash in a pixel to be determined.


Precipitable water


T4-T5 Simulations


Upper bound:

DTwv=exp[20T* - 18]

Lower bound:

DTwv=exp[6T*- b]

T*=T4/Tmax

b is determined from the data


Original distribution

Water-vapour corrected distribution


DT = F DTs [ Z - Z b]

Z = 1 - DT4

F DTs

DT4 = T4 - Tc

DTs = Ts - Tc

Volcanic Ash Absorption Model

= b ratio of extinction coefficients F = ash fraction


Ash Fraction MapsA quantitative product for aviation use (?)


Simpson eye-ball detection

T4-T5 detection

Simpson concludes T4-T5 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


Are all plumes volcanic ?

Simpsons 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 ?


Negative T4-T5 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


Negative T4-T5 temperature differences often occur at night over the clear land surface. In some of Simpsons examples these effects can be seen. This is a well-known effect (see Platt and Prata, 1993). The image shows negative T4-T5 (coloured green) over a region of central Australia, well away from any erupting volcanoes. Context is important.

Negative T4-T5 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., 45:127-136.

2-3 november 2000eos-ids team meeting1 november 2000 the simpson debacle failures in detecting...

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