Dust squall (haboob) over Argentina

By Claudio Mattio (Servicio Meteorologico Nacional (SMN)), Jose Prieto and Jochen Kerkmann (EUMETSAT)

A dust squall (also called haboob, from the Arabic word 'haboob' meaning strong wind) is a dust storm caused by convective downbursts. Haboobs are the true walls of dust and sand that most people think of as strong dust storms. Winds associated with the gust front of a dry downburst from a convective storm average 35 to 50kts and can easily generate a dust storm when they encounter an appropriate source area. Haboobs tend to be rather small, of the order of 100 to 150km, except in the Sahel area where they can reach horizontal extensions of several hundred kilometers.

The GOES and MSG images below show a spectacular case of a haboob over central Argentina that occurred in the early local afternoon of 29 October 2010. As reported on the news, the sudden appearance of a dense dust cloud close to Villa del Rosario, a town 100km east of Cordoba, made the visibility drop to some meters.

The gust front is very well visible in the GOES-12 visible image (Figure 1). The gust front is marked by yellow arrows, the dust cloud by a big red arrow (better visible with strong enhancement). Gust winds of 80km/h were recorded by the SMN. There were many fallen trees and severe road accidents happened in the area, one of them killing 10 people. Note that, although GOES has a higher spatial resolution than MSG in Argentina, the oblique view of MSG (Meteosat-9) makes it more suitable for daytime aerosol detection (smoke, dust, haze etc.) in South America, as shown in this GOES/MSG comparison (17:00 UTC).

Although over this part of Argentina the spatial resolution of the MSG-SEVIRI IR channels is around 12km (compared to the sub-satellite 3km resolution), the Dust RGB product from Meteosat-9 proved to be very useful to follow the northward displacement of the dust squall (Figure 3). The location of the road accident is marked by a yellow drawing pin. Note the semi-circular form of the leading dust squall, the large post-gust-front dust cloud and the convective cloud systems over the eastern parts of Argentina.

Dust aerosol is frequently detected in Argentina with satellite sensors. It has three major generation mechanisms:

1. Downslope winds from the high plains next to the Andes (viento zonda), typically from the north-west salt flats close to the triple border between Argentina, Chile and Bolivia.
2. Lake banks and eroded surfaces in the plain.
3. ASimilar in nature, but smaller in size, is the aerosol generated by man-made fires, after harvesting or due to deforestation. Around August and September it has extensive coverage over land with a definite impact on the less healthy members of the population.

Two of these source mechanisms are shown simultaneously in the MSG (Meteosat-9) Dust-RGB product at 18:00 UTC (Figure 4), taken three hours after the above image. The brighter magenta hue (left white oval) corresponds to dust generated in the high plateau of the Andes by Zonda winds. The weaker magenta one (right white oval), i.e. the dust cloud generated by the thunderstorm gust front, originated in lakes of the Cordoba region.

Only a few minutes after 18:00 UTC, the fleet of A-train satellites (Aqua, CloudSat, Calipso, Parasol and Aura) passed over the Argentinian dust cloud, which helped to verify the origin and the height of the dust cloud using Aqua (MODIS) and Calipso (CALIOP) data, respectively. To verify the vertical distribution of the dust cloud we have plotted the Calipso vertical profile (the total 532nm attenuated backscatter signal) in the upper right part of the 18:00 UTC MSG image. The Calipso track is given by the white line.

As expected for a haboob-type of dust cloud, the cloud stretches from the ground to a top at around 1km height. Note that the Calipso signal strength has been colour coded such that blues correspond to molecular scattering and weak aerosol scattering, aerosols generally show up as yellow/red/orange. Stronger cloud signals are plotted in gray scales, while weaker cloud returns are similar in strength to strong aerosol returns and coded in yellows and reds, see full Calipso granule (Source: NASA). For more information on Calipso and the CALIOP instrument, go to the: NASA Calipso Mission page.

The ultimate responsibility for this dust outbreak in central Argentina is put on the improper use of the soil with herbicides, lack of rotational harvesting, deforestation and the generalised use of the land for agriculture in recent years, which has degraded the humus quality and cohesion, providing large amounts of dust for wind transportation.

The Meteosat-9 image closest to the time of the worst accident (Figure 5) depicts isolated convective cells nearby, which raised the soil dust particles and reduced drastically the visibility. The station closest to the place of the accident (Pilar Observatorio, WMO no. 87349) reported increasing cloudiness and southerly wind of 30kts, with gusts of 45 kts (and visibility of 200m).

The Aqua MODIS high resolution Dust RGB product (Figure 6) was used to identify the dust sources of the above-described dust squall. The MODIS image shows two big dust sources which correspond to salty lakes in the Argentinian interior, from where two major dust streaks extend northward like two large air funnels. Strong southerly winds accelerate the dust against the gust front generated by convective downbursts. Note that MODIS has a spatial resolution in the order of 1km in the infrared channels, but being a near-polar orbiter, it does not provide repeated monitoring every 15 minutes.