Ocean colour, Airmass, Dust RGB, Total Aerosol Optical Depth, BSC Dust Forecast, Absorbing Aerosol Index, Total Attenuated Backscatter
A continental-wide plume of Saharan dust extended across the Mediterranean Sea up as far as northern Europe in March 2022.
21 February 2023
25 March 2022
By Federico Fierli (EUMETSAT), Miguel-Angel Martinez (AEMET), Jörg Asmus (Germany) and HansPeter Roesli (Switzerland)
The dust outbreak lasted for a week from 15 to 22 March, impacting a large swath of western, central and northern Europe, from Spain up to Scandinavia.
Figure 1 shows the evolution of the dust position in specific days of the event in Europe, from France on 15 March and central Europe on 17 March.
Such events are the result of specific meteorological patterns that favour a persistent northward long-range transport.
Saharan dust has several impacts in the populated areas of Europe — from flight security to air quality and impacts on solar radiation. There were reports of red skies, the effect of dust dimming radiation, and dust deposits across Europe, including on snow in the Alps, associated with record concentrations of atmospheric aerosol measured by the Spanish air quality network.
This case bring together near-real-time satellite observations from selected instruments and two reference operational dust forecasts from the WMO Dust Warning Advisory System (SDS-WAS) and the Copernicus Atmosphere Monitoring Service (ECMWF-CAMS). The case shows well the forecasts and observations complement each other and how reliable that combination is in an operational framework of warning and real-time monitoring. The impact of dust on cloud formation is also visible in this event and documented in this case study. The case also highlights possible perspectives in term of data needs to characterise the inter-annual and decadal variability and occurrence of dust transport.
Desert dust is the main source of atmospheric particles and plays a crucial role in climate, ecosystems and air quality. The Sahara is main dust source in the Northern Hemisphere and has clear impacts at an inter-continental scale, from fertilisation in South America (Prospero et al., 2020) to air quality in Europe (e.g. Dayan et al., 1991). In fact, dust is frequently transported in the Mediterranean basin area, but can also appear at higher latitudes in Europe (Gkikas, et al., 2018.). Transport is related to specific weather patterns that favour efficient northward transport in the troposphere (Bibi et al., 2019).
The event under scrutiny is related to a specific weather pattern associated with a low-pressure area above Spain (Storm Celia), a cut-off low located south west of the Iberian Peninsula, and a persistent high-pressure area above central northern Europe that provided an efficient pathway for northward advection.
Celia was documented by AEMET, the Spanish weather service, as the first storm with generalised rainfall after a long period of winter drought in the Iberian Peninsula (since January), and as one of the types of Mediterranean lows that can generate intense and extensive outbursts of Saharan dust over the Iberian Peninsula.
This event led to record concentrations of dust at ground level, especially in Spain — as documented by the air quality network, and reported by WMO Barcelona Dust Regional Center. PM10 concentration recordings were historically high at 107 of the country’s 471 measurement stations. Some of them exceeded average concentrations of 1,700µg/m3 and some stations stopped recording after reaching 1,000µg/m3. Due to the record values there was intense coverage in national media, such as El Pais which also reported that average daily exposure considered safe for health by WHO is under 45µg/m3.
In the Pyrenees and Alps, Saharan dust deposits on snow were widely documented, especially on social media.
Comparing dust forecasts
Figure 2 shows the CAMS forecast from 14 to 18 March. Predictions indicated a wide extension of the event for almost one week, with likely advection towards higher latitudes during a progressive penetration north. The forecast for 18 March showed remnants of the Saharan dust cloud could reach as far as Finland, plus new dust spells over the Mediterranean. The forecast provided by the SDS-WAS from AEMET and the Barcelona Dust Center (Figure 3) shows a coherent pattern of direct northward intrusion on the two-day forecast.
The forecasts allowed for timely alerts to be issued, and their accuracy is confirmed by both ground-based and near-real-time satellite observations.
Evolution of the event seen by satellite products
The evolution of storm Celia and cloudiness in Europe during the initial phase of the dust event is shown in in the Meteosat-11 Airmass imagery (Figure 4).
The formation of the cut-off low above Spain is visible with the bands of clouds on 13 and 14 March, bringing the first heavy rain to Iberian Peninsula since early January. The frontal system was continuously fed by air transported from Africa, and extended northward from 15 March, generating a sharp transition with clear skies over the Atlantic and UK, seen by Sentinel-3 (Figure 1).
Clouds from 16 March appeared as very bright white over the Iberian Peninsula, in particular over the coast, and near Greece.
In Meteosat-11 Dust RGB animation (Figure 5), the dust is visible as magenta areas, moving from the Iberian Peninsula to central Europe. The storm Celia and high-cloud remnants above Europe are visible as dark red areas, hiding the complete detection of the dust's eastward penetration. Dust could can be seen through occasional gaps in the cloud deck above central Europe/the northern Adriatic.
The Metop-C GOME-2 instrument provides the capability to identify aerosol particles below the clouds, making use of polarisation of UV signal. The spatial distribution on 17 March (Figure 6) complements the visible, indicating a high aerosol load above central Europe, confirming the agreement among forecast and observations. Moreover, GOME-2 shows that the extent of the dust event was continent-wide with a strong feeding from central Sahara and is — at the date of the event — the largest aerosol intrusion worldwide.
High-level clouds and role of dust
The joint presence of dust and the cyclonic storm Celia led to the generation of a dust-infused baroclinic storm (Dust‐Infused Baroclinic cyclone Storm clouds (DIBS) or 'dusty cirrus'. Fromm et al., 2016). The passage of dust through an extratropical baroclinic cyclonic storm can lead to effective uplift enhancing long-range transport in the upper-tropospheric layers. DIBS exhibit peculiar cirrus cloud tops, reflected and emitted radiance from the UV through thermal IR, involving positive UV absorbing aerosol index and muted visible reflectivity.
This event showed the likely signature and effects: clouds from 16 March appeared very bright over the Iberian Peninsula (in particular over the coast) and near Greece. A description of bright white bands of clouds reminiscent of DIBS is reported in a previous Saharan dust case from February 2021. The structure end extent of the DIBS as observed by the Meteosat is reported in Figure 7. The image is simultaneous to the one from Sentinel-3 in Figure 1 and indicates that the extremely sharp cloudiness gradient is due to the presence of a large DIBS over central Europe.
The subsequent evolution of this cloud system is shown in the Airmass RGB together with CALIPSO lidar data (Figure 8), to deduce the vertical structure. The DIBS are observed above Scandinavia, with filaments above central Europe and the Mediterranean area. CALIPSO data shows that DIBS occur close/at the tropopause level (1km) and have an observed structure similar to the one reported in Fromm et al. 2016 (see Figure 4). It is also worth noting that high-level clouds are efficiently transported eastward in the upper troposphere reaching Siberia. Additional analysis of Himawari imagery (not included in this case) showed a possible long-range advection to Eastern Asia.
The analysis undertaken for this case shows the importance and benefits of combining both forecasts and a variety of satellite observations for predicting and monitoring such high impact dust events.
New generation satellites (such as Meteosat Third Generation) will provide improved products, in terms of both resolution and information on aerosol and clouds, that will be beneficial to services and research. Already, specific attention is devoted to evaluating whether frequency of dust events may change due to climate natural variability and anthropogenic change, due to changes in the weather patterns, sources, rainfall etc. A recent analysis (Gavrouzou, et al., 2021) indicates that in the 2005-2019 period the frequency of dust observations from MODIS is doubled on the Mediterranean area. However, due to the interplay of multiple processes identification and attribution is complex; in addition identification of events from remote sensing data may suffer from several biases. For this, together with improvements in real-time monitoring, it is that reliable datasets and climate data records (especially data on aerosol optical properties), are made available to best support climate analysis.