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Numerical Modelling of Dust Events for DODO and the Development of a New Dust Scheme in the UM

Numerical Modelling of Dust Events for DODO and the Development of a New Dust Scheme in the UM. Duncan Ackerley Ellie Highwood Mark Harrison Claire McConnell . Project Aims. To develop the UM dust scheme further for real time and climate forecasting.

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Numerical Modelling of Dust Events for DODO and the Development of a New Dust Scheme in the UM

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  1. Numerical Modelling of Dust Events for DODO and the Development of a New Dust Scheme in the UM Duncan Ackerley Ellie Highwood Mark Harrison Claire McConnell

  2. Project Aims • To develop the UM dust scheme further for real time and climate forecasting. • Provide constraints to the model using data from the DODO flight campaign (Claire). • Use the dust scheme to quantify the seasonal dust deposition footprint to the Atlantic Ocean from the Sahara Desert.

  3. Current Dust Modelling • The current dust scheme has been produced by Stephanie Woodward. • The scheme we are currently using divides the dust mass concentration into six size bins (although there is a version with nine). • The representative radii for the bins range from 0.0316 – 17.8 μm • The threshold friction velocity is calculated for each individual size bin (depending on surface characteristics) such that emissions may occur from e.g. bins 3-6 but not bins 1 and 2. • The dust is advected throughout the chosen domain (global in this case) by the model winds.

  4. The DEAD scheme • Another dust uplift scheme has been incorporated into the UM as produced by Zender et al (2003). • Scheme is known as the Dust Entrainment And Deposition (DEAD) scheme and has been used by other institutions. • The DEAD scheme has two major differences from the original scheme: • Once the threshold friction velocity is reached particles are emitted from all bins. • The scheme requires a preferential source term (based on the surface reflectivity) to constrain the dust emissions.

  5. Preferential Source Term ‘Squared’ Source Term • ‘Squared’ and ‘Linear’ source term fields used with the DEAD scheme. • Both are based on satellite measurements of surface reflectivity. • The squared term is used in the model as it provides a stronger gradient between strong and weak sources. ‘Linear’ Source Term

  6. Case Study: Mar 8th 2006 Woodward scheme • Dust AOD from the Woodward and DEAD schemes. • Both show dust front on 9th March orientated West-East over North Africa. • The Woodward scheme has a gap in the dust front (circled) whereas the output using the DEAD scheme does not. • Which is more realistic? DEAD 0.0 0.0001 0.001 0.01 0.1 1.0

  7. March 8th 2006 • AOD taken at Capo Verde, Dakar and Niamey. • Woodward scheme in black, DEAD in red and AERONET data as plusses. • Good agreement with both schemes at Capo Verde and Dakar. • Underestimation of the dust concentration at Niamey for both schemes but DEAD does a better job of showing the passage of the dust front.

  8. DODO Winter Case • The next set of case studies under investigation are from February 2006. • The flights B173, B174 and B175 occurred on the 14th, 15th and 16th February 2006. • Information about the size distribution and number concentration are being used to compare with output from DEAD and the Woodward schemes. • The current focus is on the vertical dust number concentration.

  9. Current Issues • Dust number concentrations for the DODO observations (colours). • Dust number concentrations from the both model schemes are in black. • Further analysis shows far too much dust in Bin 1. • Over estimation also occurs in the summer case too.

  10. Conclusions • The DEAD scheme does a reasonable job of capturing the March 8th 2006 dust plume and does slightly better than the Woodward scheme. • The number concentration of small dust particles in the DEAD and Woodward schemes are far too high compared to the observations from the DODO flight campaigns B173-175. • The model needs further constraint to produce more realistic results before deposition can be considered.

  11. Future Work • Investigate further methods to constrain the concentration of dust in bin 1. • Further analysis and comparison to the data from the DODO flight campaign. • Decide which model to use (DEAD vs Woodward) based on further experiments and comparing ‘real time’ forecasts from the Met Office • Decide on the resolution to use for quantifying the dust deposition footprint to the Atlantic ocean.

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