2-3 November 2000EOS-IDS Team Meeting1 November 2000 The Simpson Debacle Failures in Detecting...
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Transcript of 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
2-3 November 2000 EOS-IDS Team Meeting 2November 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 !
2-3 November 2000 EOS-IDS Team Meeting 3November 2000 The Simpson Debacle
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.
2-3 November 2000 EOS-IDS Team Meeting 4November 2000 The Simpson Debacle
Simpson refers to these as magic shapes
2-3 November 2000 EOS-IDS Team Meeting 5November 2000 The Simpson Debacle
b=ratio of extinction coefficients at 11 µm and 12 µm
Ice/water cloudAsh cloud Opaque cloud
Magic Shapes ?
2-3 November 2000 EOS-IDS Team Meeting 6November 2000 The Simpson Debacle
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
2-3 November 2000 EOS-IDS Team Meeting 7November 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 T4-T5 vs T4 distribution and allows a quantitative estimate of the fraction of ash in a pixel to be determined.
2-3 November 2000 EOS-IDS Team Meeting 8November 2000 The Simpson Debacle
Precipitable water
2-3 November 2000 EOS-IDS Team Meeting 9November 2000 The Simpson Debacle
T4-T5 Simulations
2-3 November 2000 EOS-IDS Team Meeting 10November 2000 The Simpson Debacle
Upper bound:
DTwv=exp[20T* - 18]
Lower bound:
DTwv=exp[6T*- b]
T*=T4/Tmax
b is determined from the data
2-3 November 2000 EOS-IDS Team Meeting 11November 2000 The Simpson Debacle
Original distribution
Water-vapour corrected distribution
2-3 November 2000 EOS-IDS Team Meeting 12November 2000 The Simpson Debacle
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
2-3 November 2000 EOS-IDS Team Meeting 13November 2000 The Simpson Debacle
2-3 November 2000 EOS-IDS Team Meeting 14November 2000 The Simpson Debacle
Ash Fraction MapsA quantitative product for aviation use (?)
2-3 November 2000 EOS-IDS Team Meeting 15November 2000 The Simpson Debacle
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
2-3 November 2000 EOS-IDS Team Meeting 16November 2000 The Simpson Debacle
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 ?
2-3 November 2000 EOS-IDS Team Meeting 17November 2000 The Simpson Debacle
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
2-3 November 2000 EOS-IDS Team Meeting 18November 2000 The Simpson Debacle
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 2000 EOS-IDS Team Meeting 19November 2000 The Simpson Debacle