Abstract
A method is described to significantly enhance the signature of dust events using observations from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). The approach involves the derivation of a composite clear-sky signal for selected channels on an individual time step and pixel basis. These composite signals are subtracted from each observation in the relevant channels to enhance weak transient signals associated with either (a) low levels of dust emission or (b) dust emissions with high salt or low quartz content. Different channel combinations, of the differenced data from the steps above, are then rendered in false color imagery for the purpose of improved identification of dust source locations and activity. We have applied this clear-sky difference (CSD) algorithm over three (globally significant) source regions in southern Africa: the Makgadikgadi Basin, Etosha Pan, and the Namibian and western South African coast. Case study analyses indicate three notable advantages associated with the CSD approach over established image rendering methods: (i) an improved ability to detect dust plumes, (ii) the observation of source activation earlier in the diurnal cycle, and (iii) an improved ability to resolve and pinpoint dust plume source locations.
Highlights
To better understand the processes governing the availability and entrainment of dust into the atmosphere, dust sources and their associated activation patterns must be identified [Bryant, 2013]
The widely used red-green-blue (RGB) dust rendering scheme developed by Lensky and Rosenfeld [2008, hereafter LR2008] makes use of selected thermal channels measured by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI), the imager on board the Meteosat Second Generation series of geostationary satellites [Schmetz et al, 2002], to contrast the brightness temperature signal between surface, cloud, and dust in a false color rendering scheme in which dust appears magenta
This scheme has provided the basis for numerous studies of northern African dust sources and factors governing their activation [e.g., Schepanski et al, 2007, 2009, 2012; Ashpole and Washington, 2013], with the 15 min time resolution of the SEVIRI imagery proving an invaluable aid to tracking individual events
Summary
To better understand the processes governing the availability and entrainment of dust (mineral aerosols) into the atmosphere, dust sources and their associated activation patterns must be identified [Bryant, 2013]. While the high temporal resolution offered by SEVIRI is beneficial in studies of shortlived dust events, there are limitations associated with the existing RGB dust rendering (a fact further emphasized by Vickery et al [2013]), and it is worth improving geostationary imagery of dust With these factors in mind, the aim of this paper is to apply a relatively simple technique based on proven composite clear-sky schemes used in cloud property retrieval algorithms [e.g., Minnis and Harrison, 1984] to enhance the dust signal.
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