Dynamics of volcanic and meteorological clouds produced on 26 December (Boxing Day) 1997 at Soufrière Hills Volcano, Montserrat

Abstract

Abstract The 26 December 1997 explosive activity of Soufrière Hills Volcano, Montserrat, provided an opportunity to study the evolution of a volcanic cloud by merging data from various satellites with wind-trajectory data. The activity involved a debris avalanche that descended SSW from the lava dome, to the coast, and a pyroclastic density current that traversed the coast and entered the sea. The slope failure and subsequent dome collapse occurred at c . 07:01 universal time (UT; 03:01 local time), lasted 15.2 minutes, and produced an upwardly convecting volcanic ash cloud that cloud temperatures suggest rose to c . 15 km. The volcanic ash cloud was unusual because the pyroclastic density current transported hot fine ash to the sea, where it rapidly transferred its heat to the sea water. The evaporation of large volumes of water produced a volcanogenic meteorological (VM) cloud that convected along with the volcanic ash cloud. The evolution of the volcanic and VM clouds was studied using an isentropic wind trajectory model and data from three satellite sensors: Geostationary Observational Environmental Satellite 8 (GOES 8), Advanced Very High Resolution Radiometer (AVHRR), and Total Ozone Mapping Spectrometer (TOMS). The high temporal resolution of the GOES 8 images filled many of the time gaps the other satellites left, and allowed quantitative retrievals to be performed using a two-band infrared retrieval method. The three-dimensional morphology of the volcanic cloud was reconstructed using GOES 8 data and by determining the heights of air parcels from wind-trajectory data. The volcanic cloud was estimated to contain up to 4.5 x 10 7 kg of silicate ash. Between c . 07:39 UT and 13:39 UT the ash signal of the volcanic cloud was masked by the VM cloud, which had a mass of up to 1.5 x 10 8 kg of ice. Ice forms when moist air is convected upwards to temperatures of less than -40°C and becomes saturated. Ice formation in volcanic clouds is especially likely when hot volcanic material is cooled by seawater rather than the atmosphere. The efficiency of evaporation of the seawater was calculated to be c . 5%, based on physical and GOES 8 data. TOMS data showed the SO 2 in the volcanic cloud rose higher than the ash in the volcanic cloud, as has occurred in several other eruptions. A comparison between GOES 8 and AVHRR data showed that AVHRR data retrieved higher fine-ash silicate masses and higher cloud areas than GOES 8 due to the finer spatial resolution of AVHRR images. The effect on retrieval data of the high water vapour content in the lower troposphere of the tropical atmosphere was quantified; the high humidity in the Montserrat region caused the characteristic ash signal to the infrared sensors to be depressed by up to 80%. This signal depression caused a corresponding underestimation of the mass and area of the volcanic cloud when the infrared brightness temperature difference retrieval technique was used.