Notes on the Simpson et al manuscript

Rene: This manuscript could add some interesting things to the ash cloud detection problems by highlighting a plume bifurcation case, and trying to critically examine what really happened.  There is lack of clarity at separating science from operations, and some real misunderstandings that can be eliminated from the paper.

Here are some comments that would help improve the paper. 
 p 1, 1st paragraph:  “Only total avoidance  of the ash ensures flight safety...”   This is a goal that is unreachable, since even if we had a flawless satellite detector that worked every second, there will always be ash that that is below detection limits.  So if we state this goal we can never reach it. Operationally we should try for avoidance of everything detectable or predictable.

p1, 2nd paragraph:  “Class separation is especially difficult as delta T approaches zero”  This is a useless statement which doesn’t inform the reader.  You could say:  “The identification of volcanic clouds using negative brightness temperature differences (BTD) is harder when the burden of ash is low (small eruptions) and is particularly difficult when the eruption is small (Rose and Mayberry, 2000, Geophys Res Lett, 27: 3097-3100) and there are complicating environmental conditions such as : 1.  high, cold clouds underlying the volcanic cloud (Schneider, D. J., W. I. Rose and L. Kelley, 1995, U. S. Geol. Survey Bull 2139: 27-36). , 2. ice in the volcanic cloud (Rose, W. I., D. J. Delene, D. J. Schneider, G. J. S. Bluth, A. J. Krueger, I. Sprod, C. McKee, H. L. Davies and G. G. J. Ernst, 1995, Nature, 375: 477-479); or 3. large amounts of water vapor in the lower (0-3 km) troposphere (measured as precipitable water) which when in excess of about 2 cm can make weak ash clouds indistinguishable unless a correction is made (Yu, Rose and Prata, JGR in press).”  These latter  words are all well established by careful research results--none of which are referred to in this paper--how can we get anywhere with this stuff unless we try to learn from each other?  

p1, 3rd paragraph:  ...make accurate tracking of an ash plume difficult.”  Actually this is a very misleading conclusion.  The tracking of the Cleveland ash plume is actually quite advantageous.  It is aided by the look angle of the GOES at high latitude which strengthens its signal, and by the low amounts of precipitable water in the lower trop.  This conclusion is made prematurely and probably incorrectly, as will be seen in the comments below.

p 3, Polar Atmosphere section: “Four of these stations were also very dry....”  This is not an informative statement unless it is quantified.  It is vital to quantify this moisture if you really wish to know whether there was any detectable ash in the southeast moving parts of the cloud.

p 4, 2nd paragraph under overview:  “The arctic air was dry.  The extratropical storm brought moisture...”  You must say how dry and how much moisture.  We now know how to correct for these effects so we can see if there really was detectable ash in the southern part of the volcanic cloud areas (Yu et al, in press, JGR).  It will also require knowledge about cloud heights underneath and possibly ice conditions, but it would be worthwhile to pursue this.

p 5, 2nd paragraph under SSE component:  This is a critical section which can be greatly clarified, by the following:  1. Figure 1 can be much, much better--a. use a proper cutoff value of BTD which would eliminate many of the cold cloud and misalignment erroneous signals and b. show the GOES view angle in panel a so we can see clearly that view angle is part of the favoravle detection and c. use a dual cutoff color reporting so the low BTD (< -4 K) values that represent values that contain high enough ash burdens that they cannot be overcome by any lower tropospheric moisture moisture (Yu et al, in press).   Also this statement that “The split window detected the SSE portion of the plume from 1600 to 0000...” cannot be verified in the figure because the cutoff selection is so poorly visible.  In the MODIS image I have examined at this time range (2310 UT), attached, has a proper cutoff which demonstrates the potential clarity possible with careful work, and shows the SE trending portion of the cloud, but barely.  It is quite possible to estimate the amounts of ash in this portion (Wen & Rose, 1994), even given the uncertainty with  respect to high clouds and atmospheric moisture (this can be corrected using the Yu et al corrections scheme), because we have known for years how to retrieve ash mass from such images (the one I attach has a total of 19000 MT of ash, and the atmospheric correction applied only reduces the mass to 18000).  It also would be possible to subset the SSE portion of the cloud to get its mass.  I have not done that, but it must be very small--probably less than 1000 MT, very similar in scale to the 1999 Montserrat volcanic clouds.  Since it is straightforward to do, and since this quantifies the scale of the ash in both limbs--why not do it?  It should be part of a scholarly examination.  As it stands, the evidence for any ash in the SSE trending part of this case is either weak or non existent, since a bad cutoff was used in the figure, and the only other evidence is the aircraft encounter.  the inference that “much of the SSE plume was entrained in the comma cloud...”  is highly speculative.

p 6 at top of page: “this deformation and entrianment process added to the total component of ash being transported..”   Again this is too highly speculative to include in a rigorous paper.
“The extratropical storm complicated interpretation of the split window”  There is no real evidence for this at all--in fact this whole paragraph is uninformative with respect to how much ash might be in this part of the cloud, and the conclusion that” available moisture complicates”  this is likely to be wrong, as the most likely difficulty would come from high cloud cover.

beginning on p6: The section on numerical simulations is the strongest part of the paper.  This shows that ash could be in the SSE part of the cloud, and is quantitative. It also shows that most of the ash went to the northeast, which agrees with the GOES data.  If the rest of the paper was quantititative, as it easily could be, then a useful evaluation of the problem would be possible.

p 11, rapid notification: “Notification of the event did not occur until about 3.5 hours after the initial eruption” This is urprising in view of the fact that the in the next sentence the authors say that a strong negative BTD signal was seen by GOES at 1500 UT, only minutes after the eruption started.  Why?   This should be the focus of the paper.  Why didn’t the VAAC pick up and investigatee this right away.  It implies that training might be needed in interpretation of the GOES data, as does the very sloppy processing procedure shown in figure 1.  

p 11, plume height assignment: This discussion is critical to the paper, but most unilluminating.  The paper should report that there are a variety of methods possible to assign the height--which were used here?  What were used and why was there uncertainty in this?

p 12  reliance on the split window: The main conclusion here is that the ash was entrained into the SSE cyclonic storm--but this wasn’t supported by good evidence from the satellite.  The trajectory data suggests that this was only a minor amount.  What seems most likely to me is that the ash amounts in the SSE trending system were very small, so they really were not detected at all above the GOES noise level, and the only exaserbating environmental condition was high cloud, not moisture.   Even a small amount of ash and sulfur is detected by an airplane in the cockpit, however.  Thus the example shows (again) that a real encounter and the pilot’s noses are more sensitive than the satellite.

Some ancillary information: MODIS has several images of the same cloud.  It detects the cloud with many IR bands and band combos.  SO2 is easily seen.  The comprehensive MODIS perspective helps greatly to resolve several uncertainties and also gives quantitative water vapor data.  It is odd that data from this detector is shown (figure A.2), but only a band of possible interest is shown.  Why not use all the data which is known to work?  Especially since you could have a quantitative result with it, instead of a qualitative one.