Claire McConnell

A new Saharan dust source activation frequency map derived from MSG-SEVIRI IR-channels

Background

• Chemical, physical & optical properties of dust particles vary with source and transport paths

• Adequate description of source areas is a prerequisite for estimates of effects of dust on climate, dust under changing climate, and dust forecast models

Previous Information on Dust Sources

• Aerosol Index (AI) from TOMS/OMI– Regions of high AI used as a proxy for

dust source areas, & agree with locations of enclosed topographic depressions (Prospero et al., 2002)

• “Deep Blue” algorithm (Hsu et al., 2004)– SeaWIFS and MODIS – blue part of

spectrum• Meteosat 1 IDDI (Infrared Difference

Dust Index) (Brooks & Legrand, 2000)

Sun synchronous over pass (OMI) at 13:45 local time

Time resolution = daily

Satellite Based Dust Retrieval from MSG

• MSG satellite located at 3.5ºW above equator• Sampling rate 15mins, resolution = 3km at nadir• SEVIRI (Spinning Enhanced Visible and Infrared Imager – see Schmetz

et al., 2002) brightness temperature differences (BTDs) used

• EUMETSAT website (www.eumetsat.int) offers a dust index product using BTDs:– 12 - 10.8μm

• dust decreases the BTD– 10.8 - 8.7μm

• dust decreases the BTD• thermal emissivity of desert sfc low at 8.7μm compared to 10.8μm

• Effective over desert and at night time

Evaluation of MSG Dust Product

• SEVIRI 0.6μm channel (visible) over dark ocean

• Daily emission from Bodele evident• AOD at AERONET stations –

Banizoumbou, Cinzana, Dakar• Visibility at WMO stations Faya-Largeau,

Abeche, Bilma, Nguigmi, Agadez and Zinder

• Good agreement

Source Detection

• High spatiotemporal resolution allows dust plumes to be traced back to their sources

• 1º by 1º source activation map created for the Saharan region

• Monthly frequency of source activation events for each cell calculated

• March 2006 – February 2007

Red Sea MtsBodele Depression active all year

Massif l’Air, Adrar Mts, Sebkhas Mekherrane and Azzel Matti

DJF

SONJJA

MAM

Catabatic flows around mountains are important for dust source activation

Strong sfc winds forced by convective clouds/squall lines are especially important in summer

Land-Sea regional circulations may activate dust sources in northern Libya

Seasonal changes in meteorology activates different dust sources

Comparison with OMI derived sources

• July 2006• OMI derived sources are

shifted downwind of MSG sources

• OMI samples at 13:45 local time

• Dust sources activated around morning or early evening, leading to a bias in OMI-retrieved sources

• OMI has difficulties in retrieving aerosol below 1.5km altitude – may not detect dust close to sources

MSG

OMI

Model Application

• Use LM-MUSCAT regional dust transport model for 5-10 March 2006]

• Dust emission based on Tegen et al. (2002)• Resolution 28km, 40 vertical layers• Set dust emission to zero for grid boxes where

activation <4 per year• Original model used as a reference case – uses

topographic depressions parameterized as hot spots of dust emission

• New source map improves modelled AOT compared to reference case

• Correlation between modelled and observed AOT increases from 0.7 to 0.9

Conclusions & Limitations

• High spatiotemporal resolution of MSG makes detection of individual dust events possible and geostationary satellite data allows detection of dust close to source

• Allows compilation of 1º by 1º dust surface activation frequency map for March 2006 - Feb 2007

• Detection of dust emission in the presence of clouds is limited

• (What about existing dust storms passing over a new source area?)

• Potential to improve performance of regional scale Saharan dust models compared to earlier source parameterizations

• Future plans – extend results for sources over upcoming years, additional plans for modelling comparisons