Active and Passive Satellite Remote Sensing of Volcanic Clouds

This thesis presents new and novel research on the satellite remote sensing of volcanic clouds, which was motivated by the need to mitigate the associated impacts of volcanic eruptions on civil aviation and provide deeper understanding on the radiative effects of volcanic ash. Afternoon train (A-train) sensors were used to characterise the horizontal and vertical evolution of the 2008 Chaitén eruption from 2–10 May in southern Chile. The combination of the Atmospheric InfraRed Sounder (AIRS) measurements with the Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP) is demonstrated to be an effective synergistic technique capable of detecting volcanic ash in lidar backscatter returns. Geometrically thin (< 400 m) and low-level (< 10 km) volcanic ash clouds were identified. Ensemble forward trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) dispersion model were generated to demonstrate how the new data could be used to improve Volcanic Ash Advisory Centre (VAAC) operations.
   The CALIOP/AIRS technique has also permitted identification of stratospheric volcanic aerosols. The particulate lidar ratio (S_p) for two classes of volcanic aerosols; fine ash and sulphate has been determined using CALIOP observations. For the volcanic ash layers produced by the Puyehue-Cordón Caulle eruption (June 2011) mean and median particulate lidar ratios of 72 ± 15 sr and 69 sr, respectively, were obtained. For the Kasatochi and Sarychev sulphates, mean (and standard deviation) of the retrieved lidar ratios were 68 ± 21 sr (median 62 sr) and 68 ± 17 sr (median 62 sr), respectively. The volume depolarisation ratio for fine ash was generally much higher (δ_v up to 0.30) than that found for sulphates (δ_v from ∼0.05–0.10). However, the Sarychev observations revealed an exponential decay in δ_v from 0.25 to 0.05. This finding highlights the key importance of depolarisation measurements in understanding the compositional evolution of volcanic aerosols. It is suggested that a criterion of δ_v < 0.2 could be used to discriminate between stratospheric ash and sulphate layers in CALIOP observations.
   Finally, A-train satellite measurements have been used to characterise the immediate radiative effects of the Calbuco (April 2015) volcanic ash clouds. The Calbuco ash layers were estimated to have produced a strong SW cooling (−60 W/m^-2) at the top of the atmosphere (TOA). However, this cooling was offset by an even stronger LW warming (78 W/m^-2), which resulted in an net warming of ∼18 W/m^-2 at TOA. In contrast, a net cooling (−58 W/m^-2) was induced at the surface. The LW heating rates of the Calbuco ash clouds were estimated to up to 20 K/day, while maximum SW heating was an order of magnitude lower (∼3 K/day).