Anak Krakatau triggers volcanic freezer in the upper troposphere
A. T . Prata, A. Folch, A. J. Prata, R. Biondi, H. Brenot, C. Cimarelli, S. Corradini,
J. Lapierre & A. Costa
Anatomy of the Anak Krakatau convective anvil plume. (a) Atmospherically-corrected true colour MODIS-Terra observation at 03:05 UTC on 23 December 2018 (processed using the SatPy package83). Darwin VAAC (Volcanic Ash Advisory Center) volcanic ash advisory (VAA) from surface (SFC) to flight level 550 (FL550; 55,000 ft) at 02:45 UTC on 23 December is over-plotted. (b) Same as (a) but for MODIS-Terra 11.0 μm channel. (c) Same as (b) but for the level 2 cloud product (MOD06) of ice particle effective radius using the combined 1.6 and 2.1 μm retrieval. (d) Himawari-8 thermal infrared ice particle effective radius retrieval using channels 8.6 and 11 μm (see Methods) at 03:00 UTC on 23 December 2018 (with MODIS true colour underplotted). (e) TROPOMI SO2 retrieval (product shown is for a 1 km box vertical profile centred on 15 km asl and 7 × 3.5 km2 horizontal resolution84) at 07:05 UTC on 23 December 2018. (f) Histograms of the 1.6–2.1 μm (MODIS) and 8.6–11.2 μm (Himawari-8) retrievals shown in (c,d) respectively. All histograms correspond to retrievals inside the VAA polygon (shown in a). Maps and satellite imagery were generated and processed by the authors using Matplotlib 3.0.382 and Python 3.6.7 (https://www.python.org/).
Volcanic activity occurring in tropical moist atmospheres can promote deep convection and trigger volcanic thunderstorms. these phenomena, however, are rarely observed to last continuously for more than a day and so insights into the dynamics, microphysics and electrification processes are limited. Here we present a multidisciplinary study on an extreme case, where volcanically-triggered deep convection lasted for six days. We show that this unprecedented event was caused and sustained by phreatomagmatic activity at Anak Krakatau volcano, Indonesia during 22–28 December 2018. Our modelling suggests an ice mass flow rate of ~5 × 106 kg/s for the initial explosive eruption associated with a flank collapse. Following the flank collapse, a deep convective cloud column formed over the volcano and acted as a ‘volcanic freezer’ containing ~3 × 109 kg of ice on average with maxima reaching ~1010 kg. Our satellite analyses reveal that the convective anvil cloud, reaching 16–18 km above sea level, was ice-rich and ash-poor. cloud-top temperatures hovered around −80 °C and ice particles produced in the anvil were notably small (effective radii ~20 µm). our analyses indicate that vigorous updrafts (>50 m/s) and prodigious ice production explain the impressive number of lightning flashes (~100,000) recorded near the volcano from 22 to 28 December 2018. Our results, together with the unique dataset we have compiled, show that lightning flash rates were strongly correlated (R = 0.77) with satellite-derived plume heights for this event.
Aerial photographs of the Anak Krakatau eruption on 23 December 2018. (a,b) show two angles of the sprawling phreatomagmatic cloud. Plume extends toward the east over Panjang Island. (c,d) Cock’s tail jets characteristic of Surtseyan volcanic activity. All photographs used with permission from the copyright owner Dicky Adam Sidiq/kumparan.
Tropical thunderstorms can be triggered in a variety of ways. Common triggering mechanisms include solar heating, convergence of surface winds and the flow of wind over topography1. A less studied mechanism is in the case of an erupting volcano where the input of heat at the surface initiates deep convection2–4. Intense heating at ground surface and entrainment of moist air generates positive buoyancy5, which rapidly transports volcanic gases and ash particles up to the tropopause and beyond. Here we present the first detailed account of tropical deep convection triggered and sustained by magma-seawater interactions at an island volcano.
Anak Krakatau (‘Child of Krakatau’) is an island volcano located in Indonesia’s Sunda Strait (6 °06′07″S, 105 °25′23″E) between the islands of Java and Sumatra (Fig. 1). The volcano first appeared in January 1927 having formed in the caldera left behind by the famous cataclysmic eruption of Krakatau in 18836. On 22 December 2018, Anak Krakatau underwent a major explosive eruption after experiencing six months of intense Strombolian to Vulcanian activity. The eruption resulted in a flank collapse on the southwestern side of the volcano7,8, which generated a deadly tsunami that hit the coasts of Java and Sumatra at 21:27 LT (14:27 UTC)9. The flank collapse marked the beginning of sustained phreatomagmatic activity at the volcano and led to the formation of a deep convective plume recorded by satellite for about six days.