Satellite and radar analysis of the volcanic‐cumulonimbi at Mount Pinatubo, Philippines, 1991

Abstract

Several observers reported interesting convective phenomena in the months and years following the 15 June 1991 climactic eruption of Mount Pinatubo, Philippines. The observed phenomena included deep convection resulting from (1) lower level eruptions, (2) secondary phreatic explosions, and (3) enhanced surface heating. We have compared radar records and satellite imagery to obtain a more coherent understanding of these “volcanic‐cumulonimbi” from 17 June to 30 September 1991 and to identify observational challenges for future events. Geostationary satellite imagery analysis for the period shows that the dominant localized effect on convection following the eruption was to bring the afternoon/evening peak in the diurnal convective cycle over the mountain forward by 2–3 hours. We examined four cases in detail and identified a number of possible interactions between the volcano and the meteorological environment. For three of these cases, the volcanic emissions apparently mixed with cumulonimbi developing around the volcano; on 21 June 1991, microphysical analyses clearly show a strong reduction in cloud top effective particle radius caused by the additional aerosols in the cloud, while on the two other occasions, there was a slight or barely detectible reduction. In the fourth case, for a cumulonimbus developing above a major secondary phreatic explosion, no unusually small cloud top effective radii were found, perhaps because the ash injected into the cloud was dominated by giant cloud condensation nuclei. These cases verify that volcanic emissions can be released through the depth of the troposphere and lower stratosphere from relatively small eruptions. Future major tropical eruptions and their aftermaths should be studied using intensive ground‐ and satellite‐based observations to explore these phenomena further.