How I Plan to Spend My Twenties: My Ph.D. Research in Plain English (Meghan Fisher)

I study explosive volcanic eruptions, specifically the mixing of ash clouds with the surrounding air (entrainment).  When an ash cloud entrains enough air, it allows the plume to rise; if it is unable to ingest enough air, the ash cloud will collapse.  The stability of an ash cloud is very important for assessing volcanic hazards.  If a fully formed ash cloud develops, hazards include roof collapse from falling ash and engine failure for planes if they encounter ash cloud.  If an ash cloud collapses, it produces an ash flow like that which entombed Pompeii in 79 AD.

The traditional mixing hypothesis is that air enters the ash cloud horizontally and in direct proportion to the vertical velocity (Fig. 1).  As the vertical speed of the ash cloud decreases, mixing decreases.  Atmospheric scientists have been able to show that the entrainment model for cumulus clouds is more complicated, thus prompting volcanologists to believe that the entrainment model for volcanoes is not entirely correct.

Figure 1. Schematic of volcanic eruption column (not to scale) illustrating regions of the plume divided by motion type. Green arrows indicate entrainment of ambient air into the column, proportional to vertical velocity (blue arrows). If enough air is entrained in the gas-thrust region, the plume will rise buoyantly as sketched here. If not enough air is ingested, the relatively dense eruption material will collapse and flow down the sides of the volcano as a dangerous pyroclastic density current. I am working to establish a more accurate entrainment model than the historic one illustrated by the arrows here, focusing on eddies (rotational flow) and mixing scales in the various regions of the plume. 

I plan on conducting experiments on analogue eruption columns produced by injecting colored salt water into a large tank of water.  Through observing and measuring the controlled, scaled eruption to determine how the two fluids mix, I hope to be able to map the different speeds and directions the cloud travels as it mixes with the water. 

I will analyze video taken of the analogue eruptions, using a modified version of FlowJ, software that tracks the change in position of the pixels in the video to determine their speeds. I will also use a technique called particle image velocimetry, in which a laser sheet in the tank illuminates the motion of small glass beads in the eruption column.  Post-processing of the images showing the illuminated particles allow us to calculate the velocities and generate a 2D vector field of the flow in the column. Using infrasound, an acoustic technique similar to sonar, I will record sub-audible sounds produced by the cloud as it mixes turbulently with the ambient fluid.  The frequencies of noise emitted will be mapped back to mixing eddy structures and velocities in the column.  From the data collected from all three methods, I hope to create a mathematical function describing entrainment throughout the eruption plume.

It is important to collect velocity data using the various methods to create a fully formed velocity field.  By combining the FlowJ output and particle image velocimetery data, I can map both the surficial and interior velocities.  Infrasound is a relatively new, innovative method for volcanic monitoring that I plan to further develop. I hope to develop this method of volcanic ash cloud monitoring because 1) it is fairly inexpensive 2) it allows scientists to monitor volcanoes from a safe distance instantaneously during an eruption, and 3) it does not require daylight or limited fly-bys to gather useable data.  To validate this method, I will compare the results of the infrasound study to the video and particle image velocimetry studies to identify links between eddy mixing scales and infrasonic frequencies.

The results of this study can be used in eruption simulations to track ash dispersion and deposition.  This information can be used by volcanic monitoring agencies for hazard assessment and response.

Remotely Gathered Data May Prevent Airline Delays

Photograph: JON GUSTAFSSON/AP

Grounded flights are a nightmare for travelers, especially when traveling internationally.  The 2010 eruption Eyjafyallajökull in Iceland resulted in a week-long travel ban across Europe.  Over 100,000 flights were cancelled, costing airlines US$1.7 billion.

Planes were grounded due to the destructive interactions between ash particles and jet engines, which could cause the engines to stall when the glassy ash melts in the engine.  The International Civil Aviation Association (ICAA) institutes a no-fly zone in areas where the ash concentration exceeds 4 milligrams per cubic meter of air space.  Currently, a conservative approach is taken for determining the no-fly zones, but it is expensive and may be overkill.  Better methods for determining the location of ash would allow for more tightly constrained bans and less interruption to aviation.

The ash concentration and the predicted spread of the ash cloud are determined from source parameters like the rate of eruption and ambient wind conditions.  These parameters are normally inferred from satellites, radar, and ground observations.  However, in the first hours of any eruption, accurate values of these parameters are usually unavailable and the destructiveness of eruptions means that we cannot directly sample them even if we could get a scientist there fast enough.  A paper published in Earth and Planetary Science Letters in March 2013 has addressed this problem by using infrasound and thermal imaging to determine these source parameters.

Infrasonic monitoring and thermal imagery are remote sensing techniques that allow scientists to safely monitor volcanoes without any direct contact with the volcano and ash cloud.  Infrasound records sound waves whose frequencies are too low to be heard with the human ear. Thermal imagery, or heat sensing, detects the heat emitted by an object.  Infrasound recordings were combined to identify consistent jumps in low-frequency sound that result from changing pressures at the vent.   These jumps in pressure are used to calculate the acoustic speed.  When combined with thermal imaging, they determine that the acoustic speed is equivalent to exit velocities at the vent. Knowing the exit velocity and the radius of the vent, the scientists were able to calculate the eruption rate.

By using remote sensing techniques, scientists can more quickly gather the data they need. The eruption rate is used determine the amount of volcanic material being erupted into the atmosphere.  The scientists can use atmospheric models to predict where the ash will go and how much of it will be there, allowing the ICAA to better constrain the no fly zone for planes – great news for all of us hoping to travel safely.

Original article: Ripepe, M., Bonadonna, C., Folch, A., Delle Donne, D., Lacanna, G., Marchetti, E., Huskuldsson, A., 2013. Ash-plume dynamics and eruption source parameters by infrasound and thermal imagery: The 2010 Eyjafjallajokull eruption. Earth and Planetary Sciences Letters. 366. 112–121.

Synopsis by Meghan Fisher 

Erupted grain sizes key to determining eruption column height

Photograph: CHRIS WEBBER (AP)

Despite the social and economic consequences caused by the 2010 Eyjafjallajokull eruption in Iceland was relatively small, emitting only 0.24 cubic kilometers of material, enough ash to fill 96 thousand Olympic swimming pools.  In comparison, the 1980 Mt. St. Helens eruption produced four times as much ash and an ash plume twice the size as Eyjafjallajokull’s. While no human lives were lost as a result of the eruption of Eyjafjallajokull, its economic impact was outsized: the eruption of Mt. St. Helens cost US$2.74 billion (by todays estimates) while Eyjafjallajokull cost the global economy a whopping US$5.13 billion.

So the big question is how did an eruption a quarter of the size of Mt. St. Helens cause so much economic destruction?

While there are many factors that contribute to effects of a volcanic eruption (e.g., volcano location, location relative to major aviation routes), the presence of a strong ambient wind had a major effect on Eyjafjallajokull.  The strength of the surrounding winds was much greater than the strength of the eruption column, causing it to bend.   During the Mt. St. Helens eruption, weaker ambient winds relative to a stronger eruption column allowed for the ash to rise without bending – it only began to sheer when the ash formed an umbrella cloud similar to the clouds from nuclear explosions. 

The jet stream blew the ash from Eyjafjallajokull over Europe, grounding hundreds of thousands of flights due to unsafe levels of ash in the air that could clog jet engines resulting in engine failure. To predict the ash concentration, scientists use height of the plume and rate of eruption to determine how the ash is transported.   The current model for determining plume height of bent ash columns requires the ambient wind velocity, velocity of the ash cloud, and the rate that air mixes with the plume.

A recent study published in the Journal of Geophysical Research: Solid Earth has modified the traditional model for bent ash clouds by incorporating the influence of ash size on the plume.  When grains are greater than a few millimeters, it can fall out of the ash cloud before it cools, resulting in the overall temperature of the ash cloud decreasing.  Since heat affects how high the plume rises, this loss of heat due to fallout plays an important role in determining the height of the plume.

These scientists tested their model against the 2010 Eyjafjallajokull eruption.  When the effects of grain size were included in the model, it more accurately predicted the plume height.  They also found that detailed local meteorological data was critical to model success.  It superseded the effects of tephra – without the atmospheric data, the simulated plume underestimated the height of Eyjafjallajokull.

It is crucial to develop these models for bent plumes because bent ash clouds are more often produced by smaller eruptions, which are more frequent than large, catastrophic ones.  By developing models to study bent eruption clouds, we can better predict where ash will spread during an eruption and more strictly constrain ‘no-fly’ zones, limiting their economic impact.

Original Article: Woodhouse, M. J., Hogg, A. J., Phillips, J. C., & Sparks, R. S. J. (2013). Interaction between volcanic plumes and wind during the 2010 Eyjafjallajökull eruption, Iceland. Journal of Geophysical Research: Solid Earth.

Synopsis by Meghan Fisher