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.