It was pointed out to me that the MODIS sensor captured the ash plume at 00:25 UTC, just minutes after the eruption, see below.
RGB image of the Zhupanovsky eruption. MODIS data, November 25, 2014 00:25 UTC
Given the suns angle, and the elevation of the ground surface above sea level, one is able to estimate the plume top. From the MODIS data, the suns angle was 74o. So using the figure sketch below, and assuming a flat ground (and this is not always true), one can estimate the height.
Schematic of shadow height estimation method.
The sun's angle was 74, or for theta this is 16. The imagery shows a shadow 25 km, and this leads to a plume height of 6.8 km above the ground level. Zhupanosky is 2.9 km above sea level, so a shadow method has the plume top of 9.7 km ASL.
Lets now look at the thermal infrared data. This can be used for young plumes, that are spectrally opaque in the thermal infrared to determine the altitude. The image below shows that the minimum temperature was around -50 - -52 C. If we use the local radiosonde to determine the height of the plume/cloud given it has equated to surrounding atmosphere, then the height would be between 7 - 13 km ASL.
Band 31, TIR at 11 microns, 00:25 UTC November 25.
Therefore, the thermal infrared method has a large range given that the plume had reached the tropopause. The height above sea level could then be constrained by the shadow method at 9.7 km ASL. Below is the webcam image that matches the satellite time..
00:25 UTC, November 25.
No comments:
Post a Comment