Montserrat: There and Back Again

Whilst Reading plunged into a gloomy autumnal daze, a group of researchers namely Geoff Wadge, Antonio Costa and Thomas Webb, from the Department of Meteorology decided to replace their warm coats with T-shirts, their wellies with flip-flops and don excessive amounts of suncream, heading for the tiny island of Montserrat in the West Indies (figure 1) to observe Soufrière Hills volcano. Pyroclastic flows and explosive eruptions from this stratovolcano have caused widespread damage for the island including the destruction of the capital city of Plymouth in 1996. The lava dome continues to grow, occasionally venting ash ahead of another eruption; though it was quiet during this expedition.

Figure 1: Map showing where Montserrat is in the West Indies.
Figure 1: Map showing where Montserrat is in the West Indies.

Tasked with the job of building the largest ground based differential interferometric radar dataset on any andesitic volcano, GPRI2 was hired from Gamma Remote Sensing in Switzerland. Large amounts of surface heating, a ridge of hills and the Trade Winds provided the perfect conditions for sporadic orographic precipitation and deluges that sometimes led to intense but localised thunderstorms. Ingenious improvisation prevented damage to the expensive equipment occurring in the field – GPRI2 was prepared for lightning strikes but was not completely waterproof and so cables were ‘baked’ in a kitchen oven to remove some of the rain water and a large plastic sheet had to be held in place over the rotating antennae during high precipitation events . Despite teething problems with GPRI2, the phase change between two radar signals was measured from four different locations around the volcano each pointing towards the lava dome and for several hours at a time, each measurement taking one minute. An example of differential phase measurement received is shown in figure 2. This phase change may, in future, correspond to deformation of the volcano due to eruptive processes (the main topic of the study). The volcano is not in a deforming state and so this is unlikely to be the signal here. Alternatively this phase change is caused by the atmosphere – the variation of water vapour content around the volcano which changes the refractivity of each radar signal passing through it.

GPRI2 points towards volcano from observatory.
Differential interferogram of same field of view.

Figure 2: (Top) GPRI2 points towards volcano from observatory. (Bottom) Differential interferogram of same field of view where the GPRI2 is to the left of this image. Phase changes on the slopes of the hills surrounding the volcano and particularly around the volcano dome are associated with water vapour as we don’t think the volcano is deforming currently. ‘X’ marks the same spot in both images.

Unless glanced by a tropical storm (Lahar in Belham Valley, 14 October 2012 – MVO), strong Trade Winds mean for the majority of the time clouds and water vapour pile up on the Eastern side of the island. The background water vapour was measured during the field study by a hand-held sun photometer showing the column water vapour at a point and by automatic GPS measurements (figure 3). In addition to the meteorological water there is the magmatic water (sulphur dioxide, carbon dioxide, hydrochloric acid) emitted by the volcano’s plume which complicates the picture. Over 70% of the volcanic plume rising from the top of the lava dome is made up of water vapour, mixing with the Trade Winds and causing a disparity in leeward concentration (western). You know if you’re underneath the plume anywhere on the island because of the strong smell of sulphur.

Figure 3: Outline of Montserrat.
Figure 3: Outline of Montserrat – 18km long and 12km across. Squares represent GPRI2 measurements of volcano, triangles GPS measurements and circles are places where sun photometer measurements were taken. ‘V’ is the location of Soufrière Hills volcano and ‘MVO’ is the location of the Montserrat Volcano Observatory.

During the expedition a numerical model was left to run at the University of Reading – an effort to capture the turbulent mixing of water vapour using the WRF model at a resolution of 200m. The winds generated by this model used to provide high resolution estimates of water vapour concentrations to correct ground and future space based differential interferometric images. Extensive GPS and sun photometer precipitable water vapour datasets, built up during this trip, are being compared with this numerical model using a ray tracing algorithm adapted from computer gaming to account for non-zenith lines of sight. More about this project can be found here.

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