Abstract
Abstract. This study presents an analysis of TROPOMI cloud heights as a proxy for volcanic plume heights in the presence of absorbing aerosols and sulfur dioxide for the 19 February 2018 eruption plume of the Sinabung volcano on Sumatra, Indonesia. Comparison with CALIPSO satellite data shows that all three TROPOMI cloud height data products based on oxygen absorption which are considered here (FRESCO, ROCINN, O22CLD) provide volcanic ash cloud heights comparable to heights measured by CALIPSO for optically thick volcanic ash clouds. FRESCO and ROCINN heights are very similar, with the only differences for FRESCO cloud top heights above 14 km altitude. O22CLD cloud top heights unsurprisingly fall below those of FRESCO and ROCINN, as the O22CLD retrieval is less sensitive to cloud top heights above 10 km altitude. For optically thin volcanic ash clouds, i.e., when Earth's surface or clouds at lower altitudes shine through the volcanic ash cloud, retrieved heights fall below the volcanic ash cloud heights derived from CALIPSO data. Evaluation of corresponding Himawari-8 geostationary infrared (IR) brightness temperature differences (ΔBTs) – a signature for detection of volcanic ash clouds in geostationary satellite data and widely used as input for quantitative volcanic ash cloud retrievals – reveals that for this particular eruption the ΔBT volcanic ash signature changes to a ΔBT ice crystal signature for the part of the ash plume reaching the upper troposphere beyond 10 km altitude several hours after the start of the eruption and which TROPOMI clearly characterizes as volcanic (SO2 > 1 DU – Dobson units – and AAI > 4 – absorbing aerosol index – or, more conservatively, SO2 > 10). The presence of ice in volcanic ash clouds is known to prevent the detection of volcanic ash clouds based on broadband geostationary satellite data. TROPOMI does not suffer from this effect and can provide valuable and accurate information about volcanic ash clouds and ash top heights in cases where commonly used geostationary IR measurements of volcanic ash clouds fail.
Highlights
Monitoring airborne volcanic ash is of crucial importance for aviation planning, as volcanic ash is an environmental hazard that can cause damage to avionics systems, abrasion of exposed airframe parts, engine damage, and even engine failure (Prata and Rose, 2015)
The volcanic plume had dispersed over an area with an approximate diameter of 200 km, while some parts of the volcanic ash cloud sufficiently thinned so that cumulus clouds lower down in the atmosphere could be identified in VIIRS imagery
Guided by the absorbing aerosol index (AAI) and SO2 contour lines, the ash cloud can be identified in the FRESCO cloud height and ROCINN cloud pressure – in particular for cloud tops above 10 km – as well as in the FRESCO and O22CLD scene pressures (Fig. 1b), but not in the FRESCO cloud fraction (Fig. 1b), probably because of light absorption by ash
Summary
Monitoring airborne volcanic ash is of crucial importance for aviation planning, as volcanic ash is an environmental hazard that can cause damage to avionics systems, abrasion of exposed airframe parts, engine damage, and even engine failure (Prata and Rose, 2015). An important aspect of EUNADICS-AV is to verify how well various satellite instrument are capable of monitoring volcanic eruptions and volcanic ash clouds and how to integrate various satellite data products aboard a variety of satellites. TROPOMI provides measurements with a better signal-to-noise ratio and much better spatial resolution (factor 10 or more, depending on the satellite that it is compared with) This allows for a much better and more detailed characterization of volcanic ash and SO2 plumes. Due to a better spatial resolution and better instrumental signal-to-noise ratio, TROPOMI is expected to provide improved height retrievals of volcanic ash clouds and volcanic SO2, important for parameter monitoring purposes (WMO, 2015). The 19 February 2018 Sinabung eruption provides one of the first possibilities to study the quality of TROPOMI data for volcanic cloud monitoring, because there was a fortunate overpass of the CALIOP instrument on the CALIPSO satellite. TROPOMI-based volcanic ash cloud heights are compared with measurements from the CALIPSO satellite overpass
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