Persistent fog/low cloud over Mediterranean Sea

By Carla Barroso (EUMETSAT), Joao Paulo Martins, (IPMA/LSA SAF)

Towards the end of the year 2021, the onset of an anticyclone over south-western Europe led to a mild period that continued in the first days of 2022. Some temperature records for this time of year were broken in Portugal and France, while other parts of Europe, such as the UK, Austria and Germany, experienced mild temperatures.

In France, Nîmes registered a maximum temperature of 20.9°C on 29 December and Marseille-Marignane reached 20.7°C on 30 December. Minimum air temperatures were also higher than average — 11.4°C in Mont-Aigoual on 31 December, 10.4°C in Puy on 29 December, 16.9°C in Perpignan on 29 December, 12.6°C in Auch on 29 December. In Portugal, Zambujeira the maximum temperature of 26.4°C on 31 December broke records for the weather station and was also a new extreme for the month of December in mainland Portugal since 1941.

This synoptic situation also favoured the development of a persistent fog/low stratus layer over the western Mediterranean Sea. Figure 1 shows an animation of Day and Night Microphysics RGB between 29 December 2021 12:00 UTC to 4 January 2022 12:00 UTC, where fog/low level water clouds are shown as pastel green.

During the night of the 28 to 29 December 2021, a ridge that extended from North Africa, began to intensify over the western part of the Mediterranean (Figure 2), setting up the conditions for low clouds/fog to form and gradually progress eastward over the next few days.

The intensification and persistence of the anticyclone allowed the advection of warm air, seen in green in the Airmass RGB image from 1 January 2022 (Figure 3) and Equivalent Potential Temperature at 850hPa (Figure 4).

For several days, although becoming less thick and with some dissipation and clearing near the coastal zones, the large area of low cloud was present during daytime. In the animation of the Snow RGB from 29 December 2021 to 4 January 2022 (Figure 5) the fog/low clouds with small water drops appear as white, while those with larger drops are light yellow/green.

By 2 January 2022, the low clouds covered a large area of the western Mediterranean, reaching the Adriatic Sea. Later the synoptic situation started to slowly change in the northeast Atlantic and the cloud layer began to dissipate, again from west to east.

The subsidence associated to the anticyclone over the area increased the temperature above the boundary layer through adiabatic compression, which, together with the corresponding relative humidity decrease, was responsible for a strong temperature inversion during the night (Figure 6).

Over land, the planetary boundary layer was very low during the night, which limited fog occurrence to rivers and low valleys (radiation fog). For the remaining land regions, the drier air reached the surface and, together with the higher air temperature, prevented fog formation inland. Moreover, land surfaces temperatures (LST), as measured by radiometry and corresponding to land surface thermal emissions, were abnormally high during these days, as indicated by the map in Figure 7, showing very high positive anomalies of minimum LST.

Marine stratus is a low-level cloud with a rather uniform cloud base that forms within the marine boundary layer. The cloud structure is similar to fog, except that it is above the ground. Radiative cooling at the cloud top generates upside-down convection, which enhances mixing within the boundary layer. The formation of fog is associated with the dynamic evolution of the marine boundary layer and this type of fog can be called dynamically-forced fog (where advection fog is one of the most familiar sub-type). Dynamically forced fog is generated primarily by the cooling of moist near-surface air by dynamic processes (may include advection and vertical mixing processes that lead to changes in the boundary layer temperature or moisture characteristics) and the radiative processes take a secondary role.

Sea Surface Temperature (SST) is an important ingredient and, sometimes, a large temperature difference between the water surface and the lowest levels of the atmosphere cools the near-surface air to saturation, while other times, depending on the underlying dynamical process, the temperature difference between the water and overlying air is near zero.

In this case, multiple processes, even radiative ones, see sounding from Murcia which shows radiation fog (Figure 8), possibly occurred, which led to the onset of the marine stratus and the fog, confirmed by METAR in Balearic Islands (table below).

On the 29 December, the initial conditions were SST values around 14-16⁰C over south of Spain (Figure 10), lower than those over Golf of Cadiz or even near Portuguese coasts, and the dew point temperature values were around 11-14⁰C (Figure 9), which were favourable to the development of the marine layer and possible fog. Furthermore, with a relatively moist boundary layer capped by drier air (intense ridge), the potential for fog was there.

*OSTIA uses satellite SST data provided by international agencies via the Group for High Resolution SST (GHRSST) Regional/Global Task Sharing (R/GTS) framework to produce SST analysis on a daily basis at the UK Met Office. OSTIA analysis uses satellite data from sensors that include the Advanced Very High Resolution Radiometer (AVHRR), the Spinning Enhanced Visible and Infrared Imager (SEVIRI), the Geostationary Operational Environmental Satellite (GOES) imager, the Infrared Atmospheric Sounding Interferometer (IASI), the Tropical Rainfall Measuring Mission Microwave Imager (TMI) and in-situ data from ships and drifting/moored buoys. This analysis is produced to be used as a lower boundary condition in Numerical Weather Prediction (NWP) models.

The absence of cloudiness in the Gulf of Cadiz, and in many land areas, such as the Iberian Peninsula, can also be seen in the example in Figure 11 with the Night Microphysics (Fog) RGB image, showing the low clouds/fog extending over the western Mediterranean Sea. Warm, thick fog/low cloud, with small droplets are shown in aqua shades, cloud-free sea and land are seen in shades of blue or pink, respectively. High, thin, ice clouds are also depicted as dark blue, while pink colours near the Baleares islands denote the presence of thick low clouds consisting mainly of large droplets.

The synoptic situation greatly contributed to the persistence of the event with stability, a moist boundary layer with strong inversion, light winds (Figure 12) and a relatively cool SST. The synoptic scale conditions are important for fog events. However, a major part of the spatial and temporal variability of fog events depends on mesoscale and microscale processes, which are often very difficult to predict with currently available numerical models.

Additional content

La fin d'année 2021 est la plus douce jamais mesurée (MeteoFrance, in French)
Boletim Climático Portugal Continental December 2021 (IPMA)
Group for High Resolution Sea Surface Temperature (GHRSST)

SYNOP and METAR station locations