Interplay of atmospheric gravity waves over the South Atlantic off the African coast.
04 November 2020
26 June 2016
By HansPeter Roesli (Switzerland)
There are numerous published samples of atmospheric gravity waves, made visible to the eye by their typical cloud patterns.
On 26 June, another very complex event occurred over the South Atlantic, well off the coasts of Angola and Namibia, as can be seen in the True Color RGB MODIS imagery on the NASA Earth Observatory website.
In contrast to the seemingly crisp cloud lines on the MODIS image, the fine-resolution bands of VIIRS (375 m spatial resolution) on Suomi-NPP, flying over about an hour ahead of Aqua, showed the more granular pattern of the clouds that outlined the wave crests (Figure 1).
The typical temperature of the cloud tops can be gleaned from the strongly enhanced IR11.45-band in false colours (Figure 2).
The dark blue colours of many wave crests indicate values down to 285 K, whereas the orange/yellow sea-surface temperature was around 292 K, giving a difference of about 7 K — indicating rather low cloud tops.
As it is close to the sub-satellite point (SSP) at 0° longitude/latitude, Meteosat-10 also provided excellent imagery.
A time sequence from the HRV channel with 1 km spatial resolution at SSP, 26 June 08:00–15:00 UTC showed the dynamics of the various wave trains that were seen on the MODIS and VIIRS images. The area covered by the movie is outlined in red on Figure 3.
An unbelievably rich mixture of linear and arched wave patterns of different, and hardly identifiable, origins moved through the area, partially criss-crossing each other, driven by easterly winds.
One of the wave trains, marked by an arched cloud display that rushed through the scene more rapidly than any others, is very prominent in the Meteosat-10 HRV animation from 26 June 08:00–15:00 UTC.
With a speed of more than 60 km/h it looked very much like an undular bore, but again its origin (possibly from a distant convective outflow) remained hidden and its identification would require further research.
Although the origins were not readily identifiable, it is obvious that the waves were produced by events happening over the coast or even in land.
The Global Forecast (GFS) analysis of 12:00 UTC on 26 June, showed west-easterly cross-sections of wind, and the temperature (Figure 4) indicated a warm outflow of air from the African coast that was lifted over a cooler maritime boundary layer some 400 km from the coast.
A bit further downstream, after having picked up water vapour on the first 400 km over the sea, the lifting became sufficient enough to produce condensation along the wave crests.
In addition to the gravity wave structures, open and closed Bénard cells appeared to the north-west of the scene, in the area outlined in blue on the HRV image (Figure 3).
The beautiful display of Bénard cells stands out when zooming into the area on the fine resolution VIS0.64-band of VIIRS (Figure 5).
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