Satellite detection of dust using the IR imagery of Meteosat: 1. Infrared difference dust index

Abstract

The Infrared Difference Dust Index (IDDI) is a satellite dust product designed for climatological applications, designed specifically for dust remote sensing in arid regions such as the Sahel and Sahara. It is based on the atmospheric response to dust, extracted from midday Meteosat‐IR imagery, and takes advantage of the impact of dust aerosols on the thermal infra‐red radiance outgoing to space. Simulations show a quasi‐linear relationship between satellite response to dust and shortwave optical depth, with a sensitivity depending on particle size distribution and radiative surface properties. Comparison of measured satellite response with photometric optical depth agrees with the simulations. Water vapor significantly affects the satellite signal for cases of large columnar amounts and oceanic air masses advected inland. Hence apart from possible coastal effects, the water vapor effect can be neglected in the Sahelian‐Saharan zone north of the Intertropical Convergence Zone, coinciding with the major regions of African dust emission and transport. The construction of the IDDI involves the processing of reference images, theoretically representing the outgoing radiance obtaining under clear‐sky conditions. Errors may arise from (1) dust remaining in the reference images and (2) seasonal shifts of the reference level; however, the latter error will be offset by averaging used in climatological processing. An error budget is presented for the station of Gao. A statistical comparison of IDDI data with visibility measured at synoptic stations results in (1) a validation of the product, and (2) a climatologically relevant visibility‐IDDI relation, valid for the arid regions of northern Africa. The latter relation is consistent with both simulations and photometric measurements. IDDI maps over Africa compare successfully with optical depth over adjacent ocean regions derived from Meteosat‐VIS imagery. The observed continuity of dust plumes across the African coast demonstrates the consistency between both products.

Improvement of the detection of desert dust over the Sahel using METEOSAT IR imagery

“…They vary over the studied period between 13.95 and 17.44 W.m −2 .sr −1 . The Infrared Difference Dust Index (IDDI), defined in Legrand et al (2001), is computed as the difference for cloud-free pixels between the satellite radiance of a midday METEOSAT IR [10.5-12.5 µm] image and the maximum radiance measured over a running reference period of 15 days. Ideally, this reference image is composed only of cloud-free and dust-free pixels.…”

“…The presence of a dust layer over land involves a radiative impact at the surface which modifies the surface temperature (Legrand et al, 2001). During the day time, the solar flux incident to the dust layer is partly backscattered to space and partly transmitted through this layer.…”

“…Because the AOT is the most popular measure of the aerosol amount, the IDDI index is compared to it. Examples of close correlation between IDDI and photometric AOT can be found (Legrand et al, 2001), but this is not systematic and cases with discrepancies and poor correlation can be found as well. Such an example is studied in Tanré and Legrand (1991) for measurements near Dakar, along the Atlantic coast of Africa.…”

“…In this case, the impact of water vapor has to be considered, in order that a corrected IDDI, closely correlated with photometric AOT, can be derived. Other sources of error related to the algorithm has been described and estimated in Legrand et al (2001). The IDDI algorithm creates a reference image, ideally dust-free, but indeed biased with residual amounts of dust.…”

“…Herman et al (1997) used the Total Ozone Mapping Spectrometer Absorbing Aerosol Index (TOMS AAI) in the UV. Legrand et al (2001) computed the Infrared Difference Dust Index (IDDI) in the TIR, using the IR channel of METEOSAT. Recently, Pierangelo (2005) used TIROS Operational Vertical Sounder (TOVS) data in the TIR to detect mineral dust.…”