Patrick Hamill

Physics Department, San Jose State University, San Jose, California, USA

The eruption of the Soufrière Hills Volcano on the Caribbean island of Montserrat on May 20, 2006 was observed by two satellite borne instruments, CALIPSO and OMI. The CALIPSO instrument is a satellite-borne down-looking lidar that measures the backscatter from clouds and aerosols. The OMI instrument measures a number of gases, including total column SO₂. The OMI science team created a “realtime” movie showing a cloud of SO₂ from the volcano as it drifted across the Pacific Ocean. This cloud slowly grew less dense as it diffused and was sheared into a much larger region of space. By June 6, 2006 the cloud was over Indonesia and about one week later it had largely dissipated. The CALIPSO satellite with the CALIOP lidar system on board was launched on April 28, 2006. The “first light” picture from CALIPSO on June 6 showed a layer of particles in the lower stratosphere over Indonesia. This layer was identified as a layer of sulfuric acid particles formed from the SO₂ injected by the volcano. We have carried out microphysical modeling to study the development of this layer. Our microphysical model incorporates the dissipation of the SO₂, conversion of SO₂ to sulfuric acid, the nucleation of sulfuric acid solution droplets, and the growth of these droplets by condensation and coagulation. The model carries out the appropriate calculations for each of these processes and predicts the size distribution of the sulfuric acid droplets as a function of time. The size distributions generated are fed into a Mie scattering model to estimate the backscatter from the aerosol layer. We find that the model estimate is in excellent agreement with the observations made by CALIPSO. Thus we show that using the lidar return of CALIPSO along with correlative measurements of gas phase species and a microphysical model can generate “closure” for lidar in space observations. Our presentation includes a description of the CALIPSO satellite and the CALIOP lidar, as well as the OMI measurements and the microphysical model used to achieve closure.