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. 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). 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. Use the dust scheme to quantify the seasonal dust deposition footprint to the Atlantic Ocean from the Sahara Desert.

Current Dust Modelling The current dust scheme has been produced by Stephanie Woodward. 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 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 – 17.8 μm The representative radii for the bins range from – 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 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. The dust is advected throughout the chosen domain (global in this case) by the model winds.

The DEAD scheme Another dust uplift scheme has been incorporated into the UM as produced by Zender et al (2003). 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. 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: 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.

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

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

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

Simplifying the model The model calculates the horizontal dust transport as a function of several variables such as friction velocity. The model calculates the horizontal dust transport as a function of several variables such as friction velocity. Model also uses the relative mass of dust in each size division on all land points derived from observations. Model also uses the relative mass of dust in each size division on all land points derived from observations. Latest version of the DEAD scheme fixes these relative masses over all land points. Latest version of the DEAD scheme fixes these relative masses over all land points.

New Profile Data Dust concentrations from observations are coloured. Dust concentrations from observations are coloured. Old model versions in dark lines. Old model versions in dark lines. New version given by red dashed lines. New version given by red dashed lines. Better agreement with observations. Better agreement with observations.

Size Distribution 1/N dN/dR 1/N dN/dR Observations in blue. Observations in blue. Original DEAD scheme plus Woodward scheme in Res/Black Original DEAD scheme plus Woodward scheme in Res/Black Output from latest version of DEAD in green. Output from latest version of DEAD in green. Again, much better agreement with observations. Again, much better agreement with observations.

Aerosol Optical Depth (AOD) The next phase of the comparison to observations is to look at the AOD. The next phase of the comparison to observations is to look at the AOD. Plot shows the AODs around midday on 14 th, 15 th and 16 th Feb Plot shows the AODs around midday on 14 th, 15 th and 16 th Feb Some slight differences. Some slight differences. The aim is to compare the AODs in these plots with those taken by Claire in DODO1. The aim is to compare the AODs in these plots with those taken by Claire in DODO1.

Conclusions The number concentration of small dust particles in the DEAD and Woodward schemes were initially far too high. The number concentration of small dust particles in the DEAD and Woodward schemes were initially far too high. Fixing the relative mass contribution of dust in each bin has reduced the number of small particles. Fixing the relative mass contribution of dust in each bin has reduced the number of small particles. Better agreement between model dust profiles and size distributions and those observed in DODO1 flight campaigns. Better agreement between model dust profiles and size distributions and those observed in DODO1 flight campaigns. Simplifying the model seems ho have helped make the output more realistic. Simplifying the model seems ho have helped make the output more realistic.

Future Work Compare the AODs measured on the flight campaign and from AERONET to the model output. Compare the AODs measured on the flight campaign and from AERONET to the model output. Begin to analyse the output for the summer DODO2 cases (August 2006) and compare with the flight campaign data. Begin to analyse the output for the summer DODO2 cases (August 2006) and compare with the flight campaign data.