Dust modelling in HiGAM Presentation for HiGEM meeting, Reading 31 st Jan 2008 Margaret Woodage Environmental Systems Science Centre University of Reading, U.K.

Plans for HiGAM dust paper(s) (Woodage, Woodward, Hodges, Slingo) Motivation : how does a (relatively) simple dust model (retuned HadCM3 scheme) behave in a high res global AGCM (HiGAM)? What does model do with dust ? What does dust do to the model (radiative feedback)? Context: other relevant studies on dust/climate interaction: Comparison of dust models with observations from surface (BoDEX, DABEX etc), and satellite (e.g. GERB, TOMS). Analysis of interaction between dust and African E Jet and E waves, tropical cyclones.

Dust expts performed with HiGAM ● Atmos component of HiGEM driven by AMIP2 SSTs in paired expts, both including dust aerosol. ● One is advanced using direct radiative effect of dust (‘active’ dust) ● Other is advanced ignoring radiative effect of dust (‘passive’ dust) but increments are calculated and output to estimate radiative forcing. ● Two experiment pairs have been run, first pair (20yrs), second pair (18yrs) starting from different initial fields. ● 76 years in all (!), but 20+ years is standard for dust impact studies because interannual variability is so high (in models and in reality) ● Suggest 2 short papers (to GRL) rather than one long one.

Paper 1 outline ● Brief description of model (ref main HiGEM paper) and dust tuning (ref Woodward 2001), and show that dust distributions are credible in space and time when compared with available observations. ● Focus on high resolution features such as dust storms and diurnal cycle, comparing with observations to see how well model performs.

20yr mean seasonal dust loadings

20yr seasonal zonal mean Xsect of dust concn

DJF and JJA Dust and Biomass loadings and AODs

Model dust storm 1-8 Feb, dust load and 925mb winds

Model dust storm July, dust load and 925mb winds

1-10 Feb Diurnal cycle for Bodele and Niamey T*, 925mb and 10m wind speed, dust emiss OLR, dust loading, cloud

Diurnal cycle for Bodele and Niamey OLR, dust loading, cloud 1-10 Feb (3hrly data) 21-30July

Impact of dust on diurnal cycle in OLR: comparison of HiGAM with GERB data for July (from Ruth Comer)

Paper 2 outline Intercomparison of the 4 HiGAM dust expts : Find differences between active and passive dust expts to examine feedback on model climatology Find differences between the 2 ‘ensembles’ (2 active and 2 passive dust expts) to assess and account for variation Examine impact of dust on Saharan Air Layer and mechanism of feedback on Easterly waves and Tropical Cyclones. Look particularly for differences in number and intensity of storms in runs with and without dust FX Explore correlations between storms, dust and SSTs in individual years

18yr mean dust load by size bin (1-6) for 4 expts ANN DJF MAM JJA SON

Ann mean dust loads and diffs for 4 expts

Ann zonal mean dust concn and diffs for 4 expts

Ann mean pptn and diffs for 4 expts

● African Easterly waves – look at 700mb meridional wind component June to Sept at 12hrly intervals across Atlantic ● Add 12hrly dust loading diagnostics for limited no. of years (4) in ‘active’ dust expt.

June-Sept time-longitude plot of V and dust load meaned between 15N-25N across Atlantic (Model Yr 1998)

● Results from Kevin Hodges’ storm tracking software indicate that easterly wave activity increases when dust rad FX are included (in both expt pairs, attributable to increased wind shear around the AEJ) ● BUT TC activity differs, increasing in first pair ad decreasing in second pair when dust FX included.

7yrs 850mb vorticity, +dustFX (upper), -dustFX (lower)

DIFFS in 850 mb storm density (upper) and strength (lower) xcdfr - xcdfg

DIFFS in 600mb storm density (upper) and strength (lower) xcdfr - xcdfg

Dust loadings (JJA, SON) and no of TCs for all expts

End of presentation (additional slides follow)

DJF and JJA dust and biomass profiles over Africa

5yrmn xcdfr vs obs surf dust

HiGAM orography over Africa