Blue dust plume above nocturnal temperature inversion

Blue plume above nocturnal temperature inversion

2 August 2008 00:00 UTC

Blue dust plume above nocturnal temperature inversion
Blue dust plume above nocturnal temperature inversion

Blue dust plume above nocturnal temperature inversion observed in Meteosat RGB imagery.

Last Updated

25 May 2022

Published on

02 August 2008

By Kerstin Schepanski (University of Leeds), Jochen Kerkmann and HansPeter Roesli (EUMETSAT)

During the night of the 1 August 2008, a Mesoscale Convective System (MCS) formed over western Mali and south-eastern Niger producing a Haboob** during the early morning hours propagating into the Sahara. The MCS itself moved towards the south-west.

During the following day an optically thick dust plume covered the eastern part of Mali, represented by a strong magenta colour in the MSG RGB dust image (see animation, 1 Aug 06:00 UTC–2 Aug 18:00 UTC). Prevailing north-easterly Harmattan winds dispersed the dust plume in a south-westerly direction, while the dust plume thinned out and cooled especially at the edges.

With nightfall on 1 August 2008, a blue-coloured dust plume appeared radially around the magenta-coloured dust plume, increasing in radius with time (Figure 1). The MSG channel 9 (IR10.8) measured a brightness temperature of around 300K for the blue dust layer at 04:00 UTC on 2 August 2008 (Figure 2), whereas the brightness temperature of the ground surface is approximately 290K–10K less.

The brightness temperature difference between channel 10 and 9 (i.e the red component of the RGB composite) is crucial for the colour of dust in the RGB image. For positive differences, the dust plume is cooler than the surface and dust plumes are highlighted as magenta. This is the case for most dust plumes during day over the hot desert, when surface temperature is significantly higher than the temperature of the dust layer. Thin and old dust layers may have a similar thermal characteristic as the surface and, therefore, remain undetected in Dust RGB images.

Here, the thin dust layer which was invisible for the RGB dust index during day on 1 August 2008 suddenly appeared in a blue colour during night. A strong nocturnal surface temperature inversion developed and the dust layer aloft became significantly warmer than the surface. Consequently the temperature difference between channel 10 and 9 became negative in value (Figure 3) and the dust plume re-appeared in a blue colour.

However, the higher dust plume (indicated in the image) was cooler than the surface and the temperature difference between channel 10 and 9 was positive, thus, appearing with the known magenta colour.

Finally, this case of an aging dust plume impressively highlights the effect of strong nocturnal surface temperature inversions on RGB dust images. During the night, optically thick dust plumes at low altitudes inhibit the development of a nocturnal temperature inversion as they limit the radiative cooling.

Thin dust layers at higher altitudes, however, do not stop the nocturnal radiative cooling process and the subsequent development of a surface temperature inversion. Thus, these thin dust layers which may be invisible in the RGB dust index during the day may re-appear during the night when a strong temperature inversion occurs.

**Note: Dust squalls or Haboobs are a frequent phenomenon over the southern Sahara and Sahel region during the Monsoon season (July–September). Within the humid monsoonal air thunderstorm systems, also called Mesoscale Convective Systems (MCS) form, especially in the vicinity of African Easterly Waves (AEW). In downwind areas of the MCS, subsidence of air is enhanced by evaporative cooling (e.g by cloud/rain droplets which evaporate while falling through under-saturated air). A pool of cold and therefore dense air is generated, which rapidly accelerates towards the ground. When this cold pool or downburst reaches the ground, air spreads out radially (10-15m/s) forming an arc-like front edge (gust front). The gust front lifts up dust from appropriate dust sources forming a 'wall of dust' — the Haboob. Depending on density (temperature) gradient between the cold pool and surrounding air, the gust front can travel hundreds of kilometers.

Blue dust plume above nocturnal temperature inversion
Figure 1: Meteosat-9 Dust RGB, 2 August 2008, 04:00 UTC. Magenta colour indicates dust plume cooler than the surface; blue colour highlights thin dust layer warmer than the surface. Without text overlays
Animation (1 Aug 06:00 UTC–2 Aug 18:00 UTC)
Blue dust plume above nocturnal temperature inversion
Figure 2: Meteosat-9 Channel 09 (IR10.8), 2 August 2008, 04:00 UTC. Without text overlays
Blue dust plume above nocturnal temperature inversion
Figure 3: Meteosat-9 Brightness Temperature Difference IR12.0–IR10.8, 2 August 2008, 04:00 UTC. Without text overlays.

On Figure 3 white indicates positive differences as they occur for dust plumes cooler than the surface. Dark grey colours represent negative differences, which indicates thin dust layers which are warmer than the surface, e.g in the case of a strong nocturnal surface inversion.


Additional content

Full Northern Africa animation Dust RGB (modified) (AVI)
Fast forward/backward animation Dust RGB
Dust RGB with 925 hPa geopotential and winds (ECMWF analysis) (6:00 UTC)
Dust RGB with 925 hPa streamlines (ECMWF analysis) (6:00 UTC)