Abstract
Volcanic ash is emitted by most eruptions, sometimes reaching the stratosphere. In addition to its climate effect, ash may have a significant impact on civilian flights. Currently, the horizontal distribution of ash aerosols is quite extensively studied, but not its vertical profile, while of high importance for both applications mentioned. Here, we study the sensitivity of the thermal infrared spectral range to the altitude distribution of volcanic ash, based on similar work that was undertaken on mineral dust. We use measurements by the Infrared Atmospheric Sounding Interferometer (IASI) instruments onboard the MetOp satellite series. The retrieval method that we develop for the ash vertical profile is based on the optimal estimation formalism. This method is applied to study the eruption of the Chilean volcano Puyehue, which started on the 4th of June 2011. The retrieved profiles agree reasonably well with Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP) measurements, and our results generally agree with literature studies of the same eruption. The retrieval strategy presented here therefore is very promising for improving our knowledge of the vertical distribution of volcanic ash and obtaining a global 3D ash distribution twice a day. Future improvements of our retrieval strategy are also discussed.
Highlights
We will compare our results for the selected test case with the ones from the infrared mineral aerosol retrieval scheme (IMARS) developed for desert dust retrieval from Infrared Atmospheric Sounding Interferometer (IASI)
In the IMARS algorithm, an emission temperature is retrieved, while mineral aerosol profiling from infrared radiances (MAPIR) provides a vertical profile of number concentration, from which we have obtained a mean altitude for the ash plume
The method detailed in this work allows the retrieval of vertical profiles of volcanic ash number concentration from IASI thermal infrared measurements, with almost global coverage twice a day
Summary
From 1100 to 1200 cm−1 , ice particles cause a flat reduction of radiation Because of this spectral signature of ice particles, which is somewhat opposite that of dust and ash, the mineral aerosol retrievals will be more difficult in the presence of ice, maybe even impossible if there is a large amount of ice particles (we are not yet able to quantify this). The conclusion of this paper is that it is possible to retrieve reasonable vertical information about ash plumes from IASI measurements, with the ability to separate two ash layers, at least for the case studied here This is a very promising result, considering the time and space coverage by IASI instruments, and the long-term availability of IASI data. To be able to generalize this work to any volcanic eruption, more work is required, as the sensitivity depends on the surface and atmospheric conditions and because the ash particles vary in size and composition for each eruption
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