Enhancing weak transient signals in SEVIRI false color imagery: Application to dust source detection in southern Africa

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.

The influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust infrared imagery

“…For example, can we be certain that the locations where we first observe the pinkness characteristic of a dust storm are actually where the dust storms had their source? The background conditions and the typical dust type affect the image appearance, which means that there are regional differences in the applicability of the imagery: for example, Murray et al (2016) have recently shown that over southern Africa the desert dust scheme appears to be inferior to alternative renderings in terms of identifying precise dust sources and emission times.…”

Satellite‐Derived Characteristics of Saharan Cold Pool Outflows During Boreal Summer

“…An example of this is shown in Figure 1a. Brightness temperature difference (BTD) fields have been used in the past to pinpoint dust in this type of imagery (Ashpole & Washington, 2012; Bachl et al., 2012; Liu et al., 2012; Murray et al., 2016). One such field is the difference between the 10.8 and 8.7 μm channels; higher emissivity at 8.7 μm highlights regions of dust and rocky desert surfaces (Banks et al., 2018), the latter of which may be subtracted by calculating a BTD anomaly field relative to a 15 days moving window (Figure 1b) as in Ashpole and Washington (2012).…”

Responses to the referees

“…Estimating and measuring particle aspect ratios is currently an active area of research: for example, Dubovik et al (2002) set a range of plausible aspect ratios for typical dust particles between 1.6 and 2.2 based on retrieval errors, preferably between 1.8 and 2.0. During the Saharan Mineral Dust Experiment (SAMUM) in Morocco in 2006, Kandler et al (2009) took particle samples and found a median AR value of 1.6 for dust particles with radii greater than 250 nm, a value used also by Otto et al (2009). Meanwhile, during an aircraft campaign as part of the African Monsoon Multidisciplinary Analyses (AMMA) project, in the same year, Chou et al (2008) measured a median AR value of 1.7 using a threedimensional area projection calculation.…”