Haze veil on the Pakistani coast

Haze veil on the Pakistani coast

24 December 2010 00:00 UTC

Haze veil on the Pakistani coast
Haze veil on the Pakistani coast

The lower part of the Indus River (severely flooded during the last monsoon season) and the adjacent Rann of Kutch (seasonal salt marsh) have dried out after the monsoon, forming a potential source of dust.

Last Updated

04 May 2023

Published on

23 December 2010

By HansPeter Roesli (EUMETSAT)

On 23–25 December 2010 an episode of easterly surface winds picked up dust resulting in haze over the lower Indus valley and the adjacent salt marshes called Rann of Kutch and spread it over the Arabian Sea along the Pakistan coast towards the Sea of Oman (Figure 1).

Figure 1: Meteosat-9 Natural Colour RGB, 214 December 02:30-12:45 UTC-

By mid-morning on 24 December the haze veil was moving westwards at a speed of 7-8m/s, derived from a couple of Meteosat-9 images. This value compares well with the 10–20kts computed from the wind field deduced from the ASCAT instrument onboard Metop (Figure 2).

Haze veil on the Pakistani coast
Figure 2: Meteosat-9 Natural Colour RGB with Metop-A ASCAT wind flow and wind barbs overlaid, 24 December, 04:00-04:15 UTC.

The relative ASCAT wind field (black flow arrows in the same image) shows a sizeable wind convergence along the leading edge of the haze layer, giving rise to shallow convection and indicated by a string of small cumulus clouds well depicted by MODIS on Aqua (Figure 3).

Haze veil on the Pakistani coast
Figure 3: Aqua MODIS True Color RGB, 24 December. Source: NASA

One might now wonder about the constituents of the haze being driven across the sea. Remembering that extreme monsoon rains had inundated the lower Indus valley and the adjacent salt marshes called Rann of Kutch earlier in 2010, and that by December 2010 most water had evaporated, leaving behind areas with dried mud and salt, mineral dust would be considered as a likely candidate. However, this is not the case — the dust hardly shows up in the Dust RGB from Meteosat-9 (Figure 4).

Haze veil on the Pakistani coast
Figure 4: Meteosat-9 Dust RGB (left) v Natural Colour RGB (right), 24 December 05:00 UTC

Also, a sequence of Dust RGBs from Meteosat-8 taken during the night of 23–24 December does not reveal major dust outbreaks over the adjacent continental area.

Figure 5: Meteosat-8 Dust RGB, 23 December 16:45 UTC-24 December 02:15 UTC

On the other hand, the haze is easily detected in the solar channels (albeit in daylight only), in particular, in the morning when the Sun-satellite orientation gives rise to moderate forward scattering. From the mid-morning orbit of Metop (flying overhead the area, and thus less favourable to forward-scattering) the haze is more transparent. A mosaic of four images compares the Natural Colour RGB from both satellites shortly after 05:00 UTC (about 10:00 local time), both on 24 and 25 December (Figure 6)

Haze veil on the Pakistani coast
Figure 6: Meteosat-9 Natural Colour RGB comparison, 24 December 05:00 UTC-25 December 04:56 UTC

Looking more closely at the three SEVIRI solar channels , it is interesting to note the somewhat irregular behaviour of the reflectivity. VIS0.8 gives the strongest reflectivity values over the haze veil whilst VIS0.6 and (even more so) NIR1.6 reflect less in the early morning of 24 December. This higher value of VIS0.8 may point to a Mie-scattering regime, i.e scattering from particles with sizes similar to the wave lengths of the observing channels. However, particle sizes in the order of one micron are typical for suspended particles of mineral origin. Compare this to the smoke plumes from the forest fire near Haifa where the reflection strongly decreases with increasing wave length, suggesting a Rayleigh-scattering regime and due to much smaller smoke particles.

Haze veil on the Pakistani coast
Figure 7: Meteosat-9 VIS0.6, VIS0.8, NIR 1.6, 24 December 03:00 UTC

Thus, it appears that the haze consisted of many ingredients, but mostly non-mineral particles, yet of relatively large size when compared to smoke from hot wild fires. This analysis compares well with the conclusions of a paper on carbonaceous aerosols over the Indian Ocean (Novakov et al., 2000): "... that both biomass and fossil fuel combustion contribute to the carbonaceous particles, including light absorbing black carbon. However, the contribution of fossil fuel burning (from transportation, industrial and domestic sectors) is considerably larger and can be as much as 80% of the total." The same source in Table 2 gives a size less than 1.3 micron for the particles. Note however, that the measurements reported in the paper refer to flights around the southern tip of India.