Conclusions

We have assessed the performance of a variety of different techniques for determining the height of the ash plumes emitted during the eruption of Redoubt Volcano in late March and early April 2009. Based on a comparison of these techniques, we have highlighted their strengths and limitations for determining the height of ash plumes in the North Pacific region.

The basic satellite temperature method has limited application in determination of plume height.

Early in the eruption, before significant dispersion occurs and when the plume is more concentrated, height retrievals will be more accurate; and nearer to the vent, where an ash plume is more likely to be highly concentrated and of greater optical depth, height retrievals will also be more accurate. This accuracy of retrieved plume heights may directly correlate to optical depth,

with plumes of greater optical depth producing more accurate temperature heights. These more accurate plume heights may correspond to a layer somewhere inside the plume, but likely not the plume top. Due to the temperature inversion at the tropopause, plume heights based on temperature will reach a maximum at the tropopause, ~10 km ASL in the NOPAC, unless the ambiguity can be resolved by other methods. Plumes will tend to have a minimum temperature retrieved height when meteorological cloud cover is present, and no minimum temperature retrieved height other than ground level in the absence of meteorological cloud cover.

Eruptions producing plumes that reach 10 km ASL and higher in the NOPAC are considered to through the tropopause. The basic temperature method will report the same altitude for these two types of plumes. Since even 10 km ASL constitutes a fairly large eruption, it is advised that when eruptions reach 9 or 10 km ASL in the NOPAC, dispersion should be forecast for the range of heights from 9 or 10 to ~20 km ASL, so as to predict the movement of particles at all these locations. If other, more reliable data are available, height information from those data should be utilized instead. In a best case scenario, data fusion would constrain the height of volcanic material, and more precise forecasts could be made. If only basic satellite temperature data are

available for height retrieval, an initial plume trajectory forecast must be quickly refined to a more precise forecast as soon as heights can be further constrained.

To accurately assess plume heights, additional methods other than the basic satellite temperature method are required. MISR is an excellent sensor which is unaffected by the presence of the tropopause because it does not rely on plume temperature for height retrievals. However, the temporal frequency and spatial coverage of the MISR sensor are limited. Still, if and when MISR stereo height data are available during an active eruption, they should be utilized. Certainly, MISR data should always be analyzed in retrospect, when available. Radar is also an excellent source of data; however, the range of ground-based radar is extremely limited. Where radar can be utilized, it is likely to be more reliable than temperature data in reporting plume tops.

Whatever the accompanying method to thermal satellite data, data fusion is currently the best way to assess an ash plume’s height most accurately. The following points give an assessment of the conditions under which each method best functions:

• For heavily populated areas, which are likely to have radar sensors, a ground-based radar system is best for time-sensitive detection of volcanic plumes. Such systems provide quick detection of activity and the assessment of the eruption column height needed to predict cloud movement and protect local communities.

• For remote locations, satellite methods provide the quickest analysis of plume height.

Either MISR stereo or the basic satellite temperature method can be utilized.

• For optically thick plumes, the basic satellite temperature method should be fairly reliable in close proximity to the vent, before the plume has had a chance to disperse, for plumes at or below the tropopause (~10 km ASL in the NOPAC). However, the height of plumes

which are optically thick and actually above the tropopause could be significantly underestimated if no other method is available to resolve the ambiguity due solely to the temperature inversion at the tropopause. Thus, these results should be supplemented, if possible, with MISR stereo analysis, or a method such as the shadow method.

• For optically thin plumes, MISR stereo analysis, if coverage is available, is the most reliable method for ash plume height determination. MISR can determine the heights of plumes for which the basic satellite temperature method will produce very poor results due to optical depth issues.

• For weak (low) plumes that are optically thick, basic satellite temperature analysis of height will suffice, while for weak (low) plumes that are optically thin, MISR stereo analysis will provide much more reliable results.

• For strong (high) plumes, the basic satellite temperature method will only suffice if the plume is below the tropopause (or if the true height can be resolved with another method) and optically thick. If either of these conditions is not met, MISR stereo or ground-based radar will provide much more reliable results.

Lastly, a more complete exploration of height determination methods, and especially temperature methods, which account for the imperfect and varying optical depth of plumes and clouds, is needed. Work along these lines has recently been described by Pavolonis (2010) who has developed a more robust approach for plume analysis using multiple infrared channels. We urge future investigation into such methods, in collaboration with the meteorology community, and the use of subsequently developed methods by volcano observatories for whom accurate heights, and accurate dispersion forecasts, are of the utmost importance.