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

Duncan AckerleyDuncan AckerleyEllie HighwoodEllie HighwoodMark HarrisonMark HarrisonClaire McConnell Claire McConnell

Project AimsProject Aims

To develop the UM dust scheme further To develop the UM dust scheme further for real time and climate forecasting.for real time and climate forecasting.

Provide constraints to the model using Provide constraints to the model using data from the DODO flight campaign data from the DODO flight campaign (Claire).(Claire).

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

Current Dust ModellingCurrent Dust Modelling

The current dust scheme has been produced by Stephanie The current dust scheme has been produced by Stephanie Woodward.Woodward.

The scheme we are currently using divides the dust mass The scheme we are currently using divides the dust mass concentration into six size bins (although there is a version concentration into six size bins (although there is a version with nine).with nine).

The representative radii for the bins range from 0.0316 – The representative radii for the bins range from 0.0316 – 17.8 17.8 μμmm

The threshold friction velocity is calculated for each The threshold friction velocity is calculated for each individual size bin (depending on surface characteristics) individual size bin (depending on surface characteristics) such that emissions may occur from e.g. bins 3-6 but not such that emissions may occur from e.g. bins 3-6 but not bins 1 and 2.bins 1 and 2.

The dust is advected throughout the chosen domain (global The dust is advected throughout the chosen domain (global in this case) by the model winds.in this case) by the model winds.

The DEAD schemeThe DEAD scheme

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

Scheme is known as the Dust Entrainment And Scheme is known as the Dust Entrainment And Deposition (DEAD) scheme and has been used by Deposition (DEAD) scheme and has been used by other institutions. other institutions.

The DEAD scheme has two major differences from The DEAD scheme has two major differences from the original scheme:the original scheme:– Once the threshold friction velocity is reached particles Once the threshold friction velocity is reached particles

are emitted from all bins.are emitted from all bins.– The scheme requires a preferential source term (based The scheme requires a preferential source term (based

on the surface reflectivity) to constrain the dust on the surface reflectivity) to constrain the dust emissions.emissions.

Preferential Source Preferential Source TermTerm

‘‘Squared’ and ‘Linear’ Squared’ and ‘Linear’ source term fields source term fields used with the DEAD used with the DEAD scheme.scheme.

Both are based on Both are based on satellite satellite measurements of measurements of surface reflectivity.surface reflectivity.

The squared term is The squared term is used in the model as used in the model as it provides a stronger it provides a stronger gradient between gradient between strong and weak strong and weak sources.sources.

‘Linear’ Source Term

‘Squared’ Source Term

DODO Winter CaseDODO Winter Case

The next set of case studies under investigation The next set of case studies under investigation are from February 2006 over West Africa and the are from February 2006 over West Africa and the Atlantic Ocean.Atlantic Ocean.

The flights B173, B174 and B175 occurred on the The flights B173, B174 and B175 occurred on the 1414thth, 15, 15thth and 16 and 16thth February 2006. Model initialised February 2006. Model initialised with zero dust close to the 14with zero dust close to the 14thth February 2006. February 2006.

Information about the size distribution and Information about the size distribution and number concentration are being used to compare number concentration are being used to compare with output from DEAD and the Woodward with output from DEAD and the Woodward schemes.schemes.

The current focus is on the vertical dust number The current focus is on the vertical dust number concentration and the dust size distribution.concentration and the dust size distribution.

Initial ProblemsInitial Problems

Dust number Dust number concentrations for the concentrations for the DODO observations DODO observations (colours).(colours).

Dust number Dust number concentrations from the concentrations from the both model schemes are both model schemes are in black.in black.

Further analysis shows far Further analysis shows far too much dust in Bin 1.too much dust in Bin 1.

Over estimation also Over estimation also occurs in the summer occurs in the summer case too.case too.

Simplifying the modelSimplifying the model

The model calculates the horizontal The model calculates the horizontal dust transport as a function of several dust transport as a function of several variables such as friction velocity.variables such as friction velocity.

Model also uses the relative mass of Model also uses the relative mass of dust in each size division on all land dust in each size division on all land points derived from observations.points derived from observations.

Latest version of the DEAD scheme Latest version of the DEAD scheme fixes these relative masses over all fixes these relative masses over all land points.land points.

New Profile DataNew Profile Data

Dust concentrations Dust concentrations from observations from observations are coloured.are coloured.

Old model versions Old model versions in dark lines.in dark lines.

New version given New version given by red dashed lines.by red dashed lines.

Better agreement Better agreement with observations. with observations.

Size DistributionSize Distribution

1/N dN/dR1/N dN/dR

Observations in blue.Observations in blue.

Original DEAD scheme Original DEAD scheme plus Woodward scheme plus Woodward scheme in Res/Blackin Res/Black

Output from latest Output from latest version of DEAD in version of DEAD in green.green.

Again, much better Again, much better agreement with agreement with observations.observations.

Aerosol Optical Depth Aerosol Optical Depth (AOD)(AOD) The next phase of the The next phase of the

comparison to comparison to observations is to look observations is to look at the AOD.at the AOD.

Plot shows the AODs Plot shows the AODs around midday on 14around midday on 14thth, , 1515thth and 16 and 16thth Feb 2006. Feb 2006.

Some slight differences.Some slight differences.

The aim is to compare The aim is to compare the AODs in these plots the AODs in these plots with those taken by with those taken by Claire in DODO1.Claire in DODO1.

ConclusionsConclusions

The number concentration of small dust particles The number concentration of small dust particles in the DEAD and Woodward schemes were in the DEAD and Woodward schemes were initially far too high.initially far too high.

Fixing the relative mass contribution of dust in Fixing the relative mass contribution of dust in each bin has reduced the number of small each bin has reduced the number of small particles. particles.

Better agreement between model dust profiles Better agreement between model dust profiles and size distributions and those observed in and size distributions and those observed in DODO1 flight campaigns.DODO1 flight campaigns.

Simplifying the model seems ho have helped Simplifying the model seems ho have helped make the output more realistic.make the output more realistic.

Future WorkFuture Work

Compare the AODs measured on Compare the AODs measured on the flight campaign and from the flight campaign and from AERONET to the model output.AERONET to the model output.

Begin to analyse the output for Begin to analyse the output for the summer DODO2 cases the summer DODO2 cases (August 2006) and compare with (August 2006) and compare with the flight campaign data. the flight campaign data.