Forecasting Resuspended Volcanic Ash Clouds

Transcription

1 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

2 Remobilisation of Volcanic Ash Valley of Ten Thousand Smokes, Alaska, 2010 Myrdalssandur, Iceland, 2015 Credit: Image courtesy of NASA. otated-satellite-image-southern-alaskashowing-anchorage-and-kodiak-island Credit: Taken from Arnalds et al. (2016) Puerto Ibáñez, Chile, 2008 Credit: Taken from Wilson et al. (2011)

3 Modelling Resuspension 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 But! Information on the spatially varying characteristics of volcanic ash deposits is often unavailable.

4 The Resuspended Ash Scheme in NAME 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. SourceStrength * * U U 3 K T K is a dimensional constant used as a scaling coefficient U* T is taken to be 0.4 m s -1

5 Operational Resuspended Ash Forecasts Since the eruption of Eyjafjallajökull in 2010 the Met Office has provided daily forecasts to the Iceland Meteorological Office (IMO). 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 Forecasts show boundary layer 6- hour averaged relative air concentrations.

6 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?

7 Ash Mists and Brown Snow, Iceland, March 2013 Resuspended ash was deposited on top of new snow in Reykjavik following a blizzard on the 6 th March 2013.

8 The timing and duration of the resuspended ash cloud agreed well with observations. Ash Mists and Brown Snow, Iceland, March 2013

9 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 Backscattered electron (BSE-SEM) images showing the contrasting morphologies and internal textures of the Eyjafjallajökull and Grimsvotn ash. Taken from Liu et al. (2014) Comparing dispersed model output to a single sample Local Topographic Effects Source areas

10 The September 2013 Resuspension Event Remobilisation of tephra deposits in southern Iceland during the th September 2013 was exceptionally well observed in satellite imagery SourceStrength * * U U 3 K T

11 Satellite Imagery From VIIRS (Visible Infrared Imaging Radiometer Suite)

12 Meteorological Observations Surface winds were: up to 25 m s -1 north-westerly Temperature Inversion at ~ 1500 m asl (850 hpa)

13 Modelled Plume Heights

14 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

15 Comparing NAME Forecasts to VIIRS Retrievals Red Lines = VIIRS Blue Lines = NAME

16 Quantifying The Emission Rate Retrieved Mass Loadings from VIIRS

17 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

18 Quantifying The Emission Rate Calibrated NAME Forecasts The Total Mass of Ash Resuspended was ~ 0.2 Tg

19 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.