Quantification of Volcanic Ash Cloud-top Height using A-train Observations for the 2008 Chaiten Eruption

Evidence is presented showing the presence of thin (<1 km), low-level (<10 km) volcanic ash clouds during the explosive phase (2-9 May 2008) of the Chaiten (Chile) eruptions using data collected from sensors onboard the Afternoon-train (A-train) satellites. This is a significant finding as previous studies have only reported high-level (>10 km) ash during the eruption. In this study, volcanic clouds are identified using a reverse absorption technique applied to hyper-spectral measurements taken by the Atmospheric InfraRed Sounder (AIRS). Once volcanic clouds are identified, heights and thicknesses are derived from the 532 nm backscatter product from Cloud-Aerosol LIdar with Orthogonal Polarisation (CALIOP) measurements. As these two instruments are part of the same constellation of satellites (the A-train), many coincident retrievals are possible. Collocation of the data allows for detection of volcanic ash within CALIOP profiles. Once identified, a simple thresholding algorithm written in the Interactive Data Language (IDL) was used to report volcanic cloud-top height (VCTH) and thickness from the backscatter profiles. A total of ten cases of low-level ash have been identified. Of the ten cases, eight new VCTHs have been revealed. These heights range from 4-16 km and include corrected heights for three previous VTCHs measured by CALIOP. These heights were verified using back trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) dispersion model. The findings reported here demonstrate several cases where low-level ash was missed. This is significant, as volcanic ash that is present in the troposphere is subject to quite different dynamics than in the stratosphere and is potentially hazardous to aviation. These results have allowed new insight into the evolution of the 2008 Chaiten volcanic ash clouds.