forecasting resuspended volcanic ash clouds~ 0.2 tg . summary • name includes a resuspension...
TRANSCRIPT
Forecasting Resuspended Volcanic Ash
Clouds
F. M. Beckett
ADAQ: C.S. Witham, S.J. Leadbetter, M.C. Hort
University of Bristol: E.J. Liu and K.V. Cashman
NILU: A. Kylling
ADMLC, February 2017
Credit: Image courtesy of NASA. https://www.usgs.gov/media/images/ann
otated-satellite-image-southern-alaska-
showing-anchorage-and-kodiak-island
Credit: Taken from
Arnalds et al. (2016)
Valley of Ten Thousand Smokes, Alaska, 2010
Remobilisation of Volcanic Ash
Myrdalssandur, Iceland, 2015
Puerto Ibáñez, Chile, 2008
Credit: Taken from Wilson et al.
(2011)
• The remobilisation of particles from a deposit depends on:
• Meteorological Conditions
• Soil Moisture
• Terrain Roughness
• Characteristics of the Deposit
• NAME has a dust scheme, where emission is a function of:
• Soil moisture
• Vegetation fraction
• Clay fraction
• Wind friction velocity
Modelling Resuspension
But! Information on the spatially varying characteristics of
volcanic ash deposits is often unavailable.
Remobilisation occurs when the local wind friction velocity (U*) exceeds a
threshold friction velocity (U*T) and precipitation rates are less than 0.01 mm hr-1.
The Resuspended Ash Scheme in NAME
3**Strength Source TUUK
K is a dimensional constant used as a scaling coefficient
U*T is taken to be 0.4 m s-1
Since the eruption of Eyjafjallajökull in 2010 the Met Office has
provided daily forecasts to the Iceland Meteorological Office (IMO).
Operational Resuspended Ash Forecasts
The resuspension and subsequent
advection of volcanic ash particles
is modelled using NAME.
Source area defined as the deposits
from the eruptions of Eyjafjallajökull
in 2010 and Grímsvötn in 2011.
Forecasts show boundary layer 6-
hour averaged relative air
concentrations.
Challenges with Forecasting The
Remobilisation of Volcanic Ash
How Do We Quantify the Emission Rate?
How Do We Define the Source Areas?
What are the particle characteristics?
Ash Mists and Brown Snow, Iceland, March 2013
Resuspended ash was
deposited on top of new snow
in Reykjavik following a
blizzard on the 6th March 2013.
Ash Mists and Brown Snow, Iceland, March 2013
The timing and duration of
the resuspended ash cloud
agreed well with
observations.
Ash Mists and Brown Snow, Iceland, March 2013
Componentry of the Remobilised Ash
Morphological, Textural, and
Compositional Analysis:
~ 50% Eyjafjallajökull
~ 50% Grímsvötn
Model Output:
~ 99% Eyjafjallajökull
~ 1 % Grímsvötn
Reconciling the Model and the
Measurements
• Comparing dispersed model output
to a single sample
• Local Topographic Effects
• Source areas
Backscattered electron (BSE-SEM) images showing
the contrasting morphologies and internal textures of
the Eyjafjallajökull and Grimsvotn ash. Taken from
Liu et al. (2014)
The September 2013 Resuspension Event
Remobilisation of tephra
deposits in southern
Iceland during the 16-17th
September 2013 was
exceptionally well
observed in satellite
imagery
3**Strength Source TUUK
Satellite Imagery
From VIIRS (Visible Infrared Imaging Radiometer Suite)
Meteorological Observations
Surface winds were:
• up to 25 m s-1
• north-westerly
Temperature
Inversion at ~ 1500 m
asl (850 hPa)
Modelled Plume Heights
The BTD Signal
The Brightness Temperature Difference between IR bands can be
used to detect volcanic ash
A positive BTD signal
was used to identify
the volcanic ash
Comparing NAME Forecasts to VIIRS Retrievals
Red Lines = VIIRS
Blue Lines = NAME
Quantifying The Emission Rate
Retrieved Mass Loadings from VIIRS
Quantifying The Emission Rate
Calibrating the Emission Rate in NAME
Considering the
difference between the
mode of the VIIRS
retrieved mass loadings
and the model output
suggests K = 1E3 to 1E4
Quantifying The Emission Rate
Calibrated NAME Forecasts
The Total Mass of Ash
Resuspended was
~ 0.2 Tg
Summary
• NAME includes a resuspension scheme for volcanic ash and is used to provide
daily forecasts of remobilised ash storms to the Iceland Met Office.
• To forecast resuspended ash storms the source area and the emission rate of the
particles must be known.
• This is challenging as deposits evolve, erode and are re-vegetated.
• The timing and location of forecast ash clouds have been shown to compare well
to observations.
• We have calibrated the emission rate to satellite retrieved mass loadings. The
calibration is uniquely related to the event studied, but the approach allows us to
consider how the emission rate is varying with time since the ash was deposited.