Dust that lifted from a dry Patagonian lake and reached the Atlantic could be seen on various satellite images in March 2020.
By HansPeter Roesli (Switzerland)
Patagonia is the only landmass exposed to southern westerlies and within the region there is a semi-dry lake, Colhué Huapí, which is a major dust source.
Lake Colhué Huapí is situated inland around 100 km northwest of Comodoro Rivadavia on the Atlantic coast and 300 km to the east of the southern section of the Andes. The lake, which covers a shallow depression, has a pronounced annual level cycle which changes from high (2 m deep) after the snow-melt, to dry in the warm season. For comparison the nearby Lake Musters has water all year round.
Figure 1 shows the two extremes of the last annual cycle: blue-water in August 2019, brown-dry in March 2020. Comparing the August 2019 state to this annotated satellite image shows that in 2019 the lake did not reach its maximum extent.
Subjective day-by-day inspection between January 2019 and May 2020 of visible MODIS/VIIRS imagery on NASA’s World View (https://worldview.earthdata.nasa.gov/) revealed that for the majority of the time the areas around the lake and east of it were shrouded in faint dust from the dry parts of the lake. Also, every month or so stormy westerlies lifted more dust than usual and spread it over the adjacent Golfo de San Jorge.
Dust events 2019-2020
List of dust outbreaks where dust streaks were well visible over the Golfo de San Jorge, subjective inspection of True Color RGBs from MODIS/VIIRS on WorldView.
2019: 28 Jan, 19 Feb, 17 Mar, 12 May, 17 & 18 May, 26 Jul, 26 Aug, 29 Sep *, 31 Oct, 03 Nov, 06 Nov, 15 Nov, 17 Nov, 18 Dec *
2020: 14 Jan, 25 Jan, 03 Feb, 14 & 15 Feb *, 07 Mar *, 04 Apr, 16 May
*major outbreaks with strong dust signal over water
In the warm season between October 2019 and March 2020, four of these events (marked with an asterisk) produced particularly dense and long dust streamers that reached over and into the South Atlantic.
On 7 March 2020 at 18:56 UTC a crew member on the International Space Station (ISS) took a photo of one such event, Figure 2 compares the ISS picture with a True Color RGB from GOES-East data, observed around the same time. For better comparison the ISS picture was rotated in order to have the dust streamers aligned with those on the GOES image. Obviously, the ISS photograph has better spatial resolution. It shows several distinct dust streamers taking off over the dry Lake Colhué Huapí (under two cloud banks on the left-hand ISS image border). Due to the very different points of observation (ISS from south-west at 400 km altitude, GOES-East from north at 36’000 km altitude) the dust streamers were sightly shifted north and southward with respect to the background.
Some of the streamers on the ISS picture were already visible in the early local morning at 13:08 UTC on a True Colour RGB from OLCI on Sentinel-3B (Figure 3).
In the following night, one day from full moon on 8 March at 05:10 UTC, the Day-Night Band (DNB) on NOAA-20 saw a dust plume straddling the 60°E longitude, some 700 km from its origin (Figure 4). The arched light-grey veil compares well to the pink dust signal on the adjacent Dust RGB image from GOES-East. The two clusters of lights to the south of the dust arc were most probably from fishing boats.
On 7 March the dust was lifted in the back of a deep depression (central pressure less than 964 hPa), centred just south of the Terra del Fuego (Figure 5). It influenced gale-force westerly winds across the Patagonian steppe, in the lee of the moderately high southern section of the Andes along the Chilean coast (Foehn effect).
In the area of interest the GFS analysis of the wind field at 18:00 UTC (Figure 6) indicated wind speeds of 40 kt to 50 kt at 950 hPa. Between 15:00 UTC and 17:00 UTC the METAR messages from the coastal Comodoro Rivadavia Aerodrome (SAVC, 87860) reported drifting dust with horizontal visibility down 5 km/3 km and wind gust up to 40 kt.
RGB image sequences from GOES-East show the evolution of the dust outbreak. The GOES-16 True Color RGBs (Figure 7) zoom in on the gulf area and the first hours of the event that started right after day break.
Figure 7: GOES-16 True Color animation, 7 March 11:00–22:30 UTC
On the Dust RGBs (Figure 8) the dust veil may be followed up to its ingestion into the depression that, at the end of the animation at 03:00 UTC on 8 March, had moved from Terra de Fuego towards the Weddell Sea.
Figure 8: GOES-16 Dust RGB animation, 7 March 11:00–8 March 03:00 UTC
By that time the dust head had travelled at least 1000 km eastward, which corresponds to an average speed of 45 kt over 12 hours, well in line with the analysed winds shown above.
Dust RGBs at six-hour intervals between 7 March 06:00 UTC and 8 March 12:00 UTC (Figure 9) give an idea of the event at synoptic scale.
On both animations (Figures 7 and 8) transient banks of altocumuli lenticularis stayed stationary over the coastal area. They signalled the presence of standing waves in a laminar atmosphere. NASA’s Earth Observatory website reported on a similar event in May 2013.