In late-September 2019 Mount Pico in the Azores was shrouded in clouds formed by the orographic effect.
By HansPeter Roesli (Switzerland)
With a height of 2351 m, Mount Pico is the highest mountain in Portugal. It is a stratovolcano and sits on Pico Island, one of the five central islands of the Azores archipelago.
Due to its very isolated position and dominant height, Pico is frequently (semi-) shrouded in orographically-induced clouds.
This was also the case on 25 and 26 September 2019, as can be seen on the two photos from 25 September at 16:49 UTC and 26 September at 18:37 UTC (Figure 1). The photos were taken looking east-south-east, from the town of Horta on the nearby Faial Island, with Mount Pico at a distance of around 20 km.
In late afternoon of 25 September (left-hand image) a thin cloud collar sat at the foot of the steep cone, topped by a multi-layered cloud that hid the peak. A similar situation was present in the early evening of 26 (right-hand image), though with a thicker and more extended cloud collar and a topping cloud leaving the peak clear.
Both images suggest that the cloud systems were supported (partially at least) by winds from the West-North-West-sector. This is confirmed by the radiosounding launched on 25 September at 12:00 UTC from Lajes Santa Rita on the nearby Terceira Island (left panel on Figure 2), the only measurements available in the Azores during the days of interest.
Profiles taken from GFS analyses at 12:00 UTC on both 25 and 26 September (middle and right-hand panels of Figure 2) confirm that on both days conditions were quite similar at the levels of Mount Pico (peak level indicated by green dots on the skewT plots), i.e. westerly to west to northwesterly winds in a humid maritime boundary layer with dry air above.
It is interesting to match up satellite imagery of the cloud situations with those shown in the photos. In order to get the timing right, the geostationary satellites, GOES-E (GOES-16) and Meteosat-11, would be the ideal choice. However, as shown by the closest time frames on both days (Figure 3), the cloud scenes around Mount Pico were relatively complex, although the photographs hinted an essentially cloud-free sky (besides the orographic clouds). The situations on these images get even more complicated due to the considerable parallax shifts and different (shallow) viewing angles that even the low clouds and the volcano peak undergo.
For GOES-E the shift factor is >3 per km of cloud height and for Meteosat-11 it is 1.8, which gives shifts of ~3 km/2 km for clouds below the peak, and of ~7 km/4 km for the peak itself in the East-North-East/West-North-West directions, respectively. Also, the GOES-E VIS0.64 band has better spatial resolution than the HRV channel of Meteosat-11, although GOES-E is farther from central Azores. Looking at the pixel size, VIS0.64 has roughly half the spatial resolution over HRV.
Polar-orbiting satellites like NOAA-20 and Sentinel-2/3 offer much higher spatial resolution, but their daytime overflights occurred earlier in the afternoon than the timing of the photographs.
On 25 September the best match, only 90 minutes too early, was found with NOAA-20. On Figure 4 the on-the-spot image from GOES-E VIS0.64 (right-hand panel) compares quite well to the Natural Color RGB at 375 m resolution from NOAA-20 (left-hand panel).
The differences are the lack of parallax and some more cloudiness on the NOAA-20 image, but the collar cloud with its topping part (seen on the photograph) can be identified. Overflights of Sentinel-3A/B (not shown) happened around noon and were hampered by even more cloud cover than at the time of NOAA-20 passing.
Although not shown in this case, on 26 September NOAA-20 (taken 15 minutes earlier than the previous day) hinted at the clouds semi-shrouding Pico from west-north-west. However, Sentinel-2A at 12:51 UTC offered the most spectacular view. At 10 m spatial resolution the True Colour RGB from the MSI radiometer (Figure 5) gave an overview (left-hand panel) and a very detailed zoomed view of the clouds upwind of the crater (right-hand panel). Consider that the image was taken almost six hours earlier than the photograph, but cloud cover and variability was less pronounced than on 25 September.
On 25 September another orographic cloud structure was also photographed. This time it was an anvil-like cloud sheet that appeared to flow out from above Cabeço Gordo, the central crater/caldera on Faial Island.
The photo (Figure 6) was taken at 14:39 UTC looking northwest from Monte da Gula, also on Faial Island. This very persistent ‘anvil’ was flowing in a south-westerly direction. Half an hour later the Natural Colour RGB of NOAA-20 confirmed this cloud wedge, indicated by white arrows on Figure 7.