Sensitivity of regional dust modelling to the wind speed and the emissions

schemes: Impact on the hourly dust previsibility

Laurent MENUT (1), Cyril MOULIN (2), Isabelle CHIAPELLO (3)

menut @ lmd.polytechnique.fr

(1) Laboratoire de Meteorologie Dynamique, IPSL, France, (2) LSCE/IPSL France, (3) LOA, France

Abstract:The mineral dust emissions are highly sensitive to the surface characteristics and the wind speed. For global and climatological studies, the associated potential uncertainty may lead to large errors in the dust amount budget. The wind speed having really an hourly variability, this study first quantify the impact of using two meteorological datasets to model hourly dust emissions, over several months. Coupled to a sensitivity analysis on the emissions schemes themselves, quantification of the variabitily due to meteorological and emissions input parameters is quantified. The same modelling system was used for daily forecast in the framework of the AMMA experiment. Each day, the model CHIMERE-DUST forecasted dust concentrations over the whole North-Atlantic, Europe and northern Africa. First , an evaluation of the model is performed in analysis mode: hourly comparisons are done between surface AERONET optical thickness measurements, OMI satellite (aerosol index) measurements and concentrations and modelled optical thicknesses with CHIMERE-DUST. The accuracy and spread between measurements and model are quantified and discussed in term of the most important dust events observed during the first short observation period of the AMMA experiment, over the western Africa.Using the same comparisons criteria, the second step consist in the same type of calculations but with the 'forecasted' meteorological and dust concentrations fields. The model skill is evaluate in term of capability to forecast (i) the surface wind speed (the key process for dust emissions), (ii) the dust emissions (depending on the wind speed as well as numerous others uncertain parameters, including threshold values on the friction velocity) and (iii) the transport of aerosols from source to remote areas (depending of horizontal transport, convection etc.).

A09 - Airborne Mineral Dust:Sources, Emissions, DestinationsPaper Number: A43A-0270Thursday, 13:40 – Moscone North, Hall D

Development of the CHIMERE-DUST modelScientific goals:Better understanding and representation of long-range transport of thin aerosols layersQuantify the amount of mineral dust transported from Africa to Western Europe for a better aerosol forecast (at the regional scale)Methodology:Develop a mineral dust transport model with a high temporal resolution: 1h to account for realistic meteorlogical processesImprove the model parameterizations to reduce numerical errors, and validate with available dataPerform daily simulation (small scales processes) and long-term simulations (climatological studies)The forecast is necessary and also a good tool for research

Forecast principle:Forecast done every night using the last NCEP forecasted meteorological fieldsRun of MM5 and CHIMERE-DUST between [01:00] and [04:00]Update of the web site before [06:00]

Forecast use:Every day for systematic model validationWith specific outputs and scores for specific fields campaigns such as AMMA (African Monsoon experiment in 2006)

Forecast history:Forecast done every day since September 2005Output images and data in free access in http://www.lmd.polytechnique.fr/dust/

References:Menut L., I. Chiapello and C. Moulin, Previsibility of mineral dust concentrations: The CHIMERE-DUST forecast during the first AMMA experiment dry season, Journal of Geophysical Research - Atmospheres, under revisionVuolo M.R.,H.Chepfer, L.Menut and G.Cesana, Comparison of mineral dust layers vertical structures modelled with CHIMERE-DUST and observed with the CALIOP lidar, Journal of Geophysical Research - Atmospheres , under revisionMenut L., 2008, Sensitivity of hourly Saharan dust emissions to NCEP and ECMWF modeled wind speed, Journal of Geophysical Research - Atmospheres, 113, D16201, doi:10.1029/2007JD009522Menut L., G.Forêt and G.Bergametti, 2007, Sensitivity of mineral dust concentrations to the model size distribution accuracy, Journal of Geophysical Research, Atmospheres, 112, D10210, doi:10.1029/2006JD007766

Times series (m/s) of forecasted 10m wind speed:Every hour, one value for [D-1], [D+0], [D+1] and [D+2][D-1] represents the analyzed value[D+0], [D+1] and [D+2] represent the “forecasted” values

Important day to day variability for wind speed: what impact on emissions?

Variability of forecasted dust emissions:

[D-1]: analysis (yesterday)

[D+2]: forecast

Variability of forecasted surface dust concentrations:

Banizoumbou

Djougou

Rome

Comparisons model / observations for the 10 March 2006

OMI aerosol index

AOT CHIMERE-DUST model [D+2] forecast

Seviri Meteosat SG AOT

The forecast is very sensitive to the dust emissions, which are sensitive to the wind speed.

A sensitivity study is performed estimating dust fluxes with two meteorological models (NCEP/MM5 and ECMWF) and two emissions schemes.

Peaks of 22 February and 6 march 2004 are modelled but NOT by the two models.

The dust emissions fluxes are more sensitive to the wind speed (threshold process) than to the sandblasting scheme.