Validation of the absorbing aerosol height product from GOME-2 using CALIOP aerosol layer information

Within the framework of aviation safety, knowledge on the location and height of volcanic ash layers is of extreme importance. Several ground based instruments (such as lidars) can provide detailed information on the height and vertical extent of these ash layers, however with a limited spatial coverage. The biggest advantage of satellite instruments is their ability to have near daily global coverage which makes them the perfect candidate for locating and tracking aerosol layers around the globe. Since the Global Ozone Monitoring Experiment 2 (GOME-2) instrument is carried on the MetOp series of operational satellites, it is designed to cover a long time period from 2007 until 2022 (and beyond) and global coverage is achieved within one day. The GOME-2 Absorbing Aerosol Height (AAH) is a new product for aerosol detection, developed by the Royal Netherlands Meteorological Institute (KNMI) which uses the Absorbing Aerosol Index (AAI) to detect the presence of absorbing aerosol and derives the actual height of the absorbing aerosol layer in the O2-A band using the Fast Retrieval Scheme for Clouds from the Oxygen A band (FRESCO) algorithm. The first results of a quantitative validation of the AAH product focusing on case studies of volcanic eruptions will be presented here. For a total of 15 different volcanic eruptions, GOME-2 AAH data are compared to the minimum and maximum aerosol layer height provided by Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) for pixels within 100 km distance from each other. For GOME-2A and -2B, about 50 to 60 % of the AAH pixels are within the EUMETSAT threshold requirements (for layers which are located lower than 10 km, the maximum absolute difference should be within 3 km; for layers which are located higher than 10 km, the maximum absolute difference should be within 4 km), while for GOME-2C this is about 70 %. The optimal requirement threshold (for layers which are located lower than 10 km, the maximum absolute difference should be within 1 km; for layers which are located higher than 10 km, the maximum absolute difference should be within 2 km) is reached for GOME-2A, GOME-2B and GOME-2C in 17 %, 28 % and 41.5 % of the cases. If only tropospheric aerosol species are studied, the results improve. This can also be seen when looking at the mean error of GOME-2. GOME-2A, GOME-2B and GOME-2C are able to represent the minimum CALIOP layer height with a mean error of −2.5 ± 5 km, −1.2 ± 5.9 km and −2 ± 5.8 km respectively. If the stratospheric aerosol layers are removed from the data, the errors obtained are −0.2 ± 3.6 km, −0.1 ± 5.4 km and −0.8 ± 3.8 km for GOME-2A, GOME-2B and GOME-2C respectively (for the minimum CALIOP layer height). The results from two specific case studies (i.e. the Calbuco eruption in 2015 and the Sarychev Peak eruption in 2009) are highlighted and show that GOME-2 underestimates the height of volcanic ash layers. Especially if the layers are located at altitudes above 15 km, since GOME-2 is not able to detect these layers due to the loss of sensitivity of the FRESCO algorithm at these algorithms