Retrieval of ash properties from IASI measurements

Abstract

Abstract. A new optimal estimation algorithm for the retrieval of volcanic ash properties has been developed for use with hyperspectral satellite instruments such as the Infrared Atmospheric Sounding Interferometer (IASI). The retrieval method uses the wavenumber range 680–1200 cm−1, which contains window channels, the CO2 ν2 band (used for the height retrieval), and the O3 ν3 band. Assuming a single infinitely (geometrically) thin ash plume and combining this with the output from the radiative transfer model RTTOV, the retrieval algorithm produces the most probable values for the ash optical depth (AOD), particle effective radius, plume top height and surface temperature. A comprehensive uncertainty budget is obtained for each pixel. Improvements to the algorithm through the use of different measurement error covariance matrices is explored, comparing the results from a sensitivity study of the retrieval process using covariance matrices trained on either clear-sky or cloudy scenes. The result exhibited that, due to the smaller variance contained within it, the clear-sky covariance matrix is preferable. However, if the retrieval fails to pass the quality control tests, the cloudy covariance matrix is implemented. The retrieval algorithm is applied to scenes from the Eyjafjallajökull eruption in 2010 and the retrieved parameters are compared to ancillary data sources. The ash optical depth gives an RMS difference of 0.46 when compared to retrievals from the MODIS instrument for all pixels and an improved RMS of 0.2 for low optical depths. Measurements from the FAAM and DLR flight campaigns are used to verify the retrieved particle effective radius, with the retrieved distribution of sizes for the scene showing excellent consistency. Further, the plume top altitudes are compared to derived cloud-top altitudes from the CALIOP instrument and show agreement with RMS values of less than 1 km.

The Complex Refractive Index of Volcanic Ash Aerosol Retrieved From Spectral Mass Extinction

“…The accurate quantification and retrieval of ash plume properties from satellite measurements requires detailed modeling of the radiative transfer through the atmosphere and ash cloud system (Kokhanovsky & de Leeuw, ; Ventress et al, ). Assumptions in these models, including the complex refractive index of particles, the assumption of spherical particles (and use of Mie theory), and assumptions about the size distribution of particles, propagate to uncertainties in retrieved plume parameters, such as the plume top height, optical path, and the effective radius of ash particles (Corradini et al, ; Wen & Rose, ; Western et al, ).…”

A New Parameterization of Volcanic Ash Complex Refractive Index Based on NBO/T and SiO2 Content

“…This calculation is provided in section . These were then used to calculate optical parameters that allow the retrieval of ash mass loadings from the Infrared Atmospheric Sounding Interferometer (IASI) satellite data using the optimal estimation algorithm of Ventress et al (). IASI is a nadir‐viewing Fourier Transform Spectrometer measuring in the infrared (3.62–15.5 μm).…”

“…IASI is a nadir‐viewing Fourier Transform Spectrometer measuring in the infrared (3.62–15.5 μm). Briefly, Ventress et al () implement an iterative optimal estimation retrieval algorithm to produce probable values for volcanic ash properties. Combining an infinitely thin (geometrically) ash plume with a fast radiative transfer model to simulate the ash plume, and analyzing the spectra from IASI, the scheme is able to retrieve the following parameters: ash optical depth (at a reference wavelength of 550 nm), ash effective radius (μm), ash plume top height (km), and brightness temperature (K), which can be used to infer the mass loading of the plume.…”

“…Combining an infinitely thin (geometrically) ash plume with a fast radiative transfer model to simulate the ash plume, and analyzing the spectra from IASI, the scheme is able to retrieve the following parameters: ash optical depth (at a reference wavelength of 550 nm), ash effective radius (μm), ash plume top height (km), and brightness temperature (K), which can be used to infer the mass loading of the plume. The results were compared with retrievals based on the Eyjafjallajökull ash refractive index used by Ventress et al (). An objective cost function, detailed in Ventress et al, , was used to determine whether using the calculated refractive indices provides an improvement over the use of a measured index of a different ash composition.…”

“…The results were compared with retrievals based on the Eyjafjallajökull ash refractive index used by Ventress et al (). An objective cost function, detailed in Ventress et al, , was used to determine whether using the calculated refractive indices provides an improvement over the use of a measured index of a different ash composition.…”