Broad Azores high

By Ivan Smiljanic (DHMZ)

High atmospheric pressure prevailed around wider Azores Islands area, mid-December. Due to an associated air subsidence mostly lower and mid-level stratiform stratocumulus cloud patches prevailed in this statically stable area.

Typically, it is not easy to observe anticyclonic movement in the atmosphere because of the subsidence and divergent motion of the air.

In these cases there are no clouds, i.e. ‘tracers’, to follow in order to map the movement of atmosphere using satellite imagery.

in exceptional cases you can actually observe high-level clouds that show the anticyclonic rotation of such a system. See this previous case study .

Low and mid level clouds were present in this case and were easily detected using satellite images.

Airplane contrails and lee flow effects, i.e. Von Karman vortex, over Madeira Island are also observed in the same area.

As seen from the Airmass RGB image (Figure 1) this area was enclosed by the jet stream from the west and the north (indicated by the red colour in the image), and the pressure in the middle of the system reached values of more than 1040hPa.

Warm subtropical air was introduced to the system (green in the image) which influenced the temperatures in the west of the Europe in the future time steps.

Low and mid level stratiform clouds prevailed in this area, thus the rotation of the system was easy to follow via satellite imagery.

The animation of Meteosat-10 infrared 10.8µm channel , 15 December 15:00 UTC–17 December 12:00 UTC, with the Azores Islands in the middle of domain, revealed these clouds and their smooth and scattered texture. In this channel they appeared in almost uniform grey shades and their motion/rotation was easily visible.

High frontal clouds in the west and north appeared in very bright white colour in this IR 10.8 imagery, due to tuning of the grey scale range towards warmer temperatures.

To verify the general motion of the system Figure 2 shows an IR10.8 image with NWP wind field over sea surface (10m), overlaid with the swaths of the ASCAT instrument wind field. The visualisation was taken from ePort.

Aside from the prevailing low and mid clouds, airplane contrails, to the west of the coast of Portugal, could be seen on Dust RGB imagery (Figure 3). Since they are very high, narrow and thin icy clouds they appear as black stripes in Dust RGB images.

A small Von Karman vortex sheet pattern is also visible in the Meteosat-10 broadband high resolution visible loop, 18 December 08:00–14:45 UTC, forming behind the Madeira Island in the north-east flow.

A similar lee follow pattern can also be observed behind Canary Islands.