Record high solar irradiance in Western Europe during first COVID-19 lockdown largely due to unusual weather

In spring 2020, Western Europe experienced exceptionally sunny and dry conditions that coincided with COVID-19 lockdowns and large reductions in anthropogenic emissions. The Netherlands set all-time records in sunshine duration and surface solar irradiance, with unusually low diffuse fractions. This context raised the question of how much the irradiance extreme was driven by unusual weather (notably reduced cloud cover and atmospheric dryness) versus reduced anthropogenic aerosols and aviation-related contrails due to lockdown measures. The study aims to quantify the individual contributions of weather (clouds, water vapour) and aerosols to the spring 2020 irradiance extreme, hypothesizing that in already relatively clean Western European air, weather anomalies dominated while aerosol reductions provided a smaller, secondary contribution. Understanding these contributions provides insight into lockdown effects and the drivers of surface irradiance extremes relevant for climate and solar energy.

Prior work documents multi-decadal changes in surface solar radiation, including a transition from “global dimming” to “brightening,” largely attributed to declining aerosol burdens over Europe, which increase surface irradiance. Contrail-cirrus is known to have a small net shortwave cooling effect over Western Europe but can substantially enhance diffuse irradiance. Weather patterns, especially atmospheric blocking, are linked to irradiance variability; however, trends in the frequency and persistence of circulation types and blocking events remain debated and sensitive to classification methods and domains. These strands of literature frame expectations that both aerosol reductions and circulation anomalies could influence irradiance, with uncertainties in the extent and trends of weather-related drivers.

The study combines observations, reanalyses, and radiative transfer modeling focused on The Netherlands (Cabauw and Veenkampen stations) for spring (March–May) 2020, with comparisons to 2004–2020 climatologies. Key elements: (1) Observations: Baseline Surface Radiation Network (BSRN) data at Cabauw for global, direct, and diffuse irradiance; long-term records at Veenkampen; ground-based aerosol optical depth (AOD) at 499 nm from a precision filter radiometer processed at PMOD/WRC; sunshine duration records. (2) Reanalyses and satellite/model products: ERA5 reanalysis for atmospheric variables (e.g., precipitable water, total cloud cover, geopotential height anomalies at 500 hPa, and 250 hPa conditions for contrail formation); CAMS McClear clear-sky irradiance; CAMS aerosol product for hourly AOD at 550 nm; OpenSkyNetwork flight departure data as a proxy for aviation activity. (3) Contrail/cirrus assessment: Manual inspection of Terra/Aqua MODIS high-resolution imagery (NASA Worldview) over the Netherlands and surrounding regions near local noon to count cirrus occurrence and contrail contamination, and evaluation of meteorological favorability for persistent contrails at ~250 hPa via ERA5. (4) Weather regime context: German objective weather type classification used to quantify anticyclonic and dry regimes relative to 1980–2019 climatology. (5) Radiative transfer modeling: Employed RTE+RRTMGP to simulate surface irradiance using hourly ERA5 atmospheric profiles (pressure, temperature, water vapour, cloud fields, ozone) on a 0.25° grid, sampling 100 vertical cloud realizations with specified overlap assumptions. Four experiment configurations isolate contributions: (i) clouds present, no aerosols; (ii) clear-sky, no aerosols (clouds removed); (iii) clear-sky, no aerosols, no water vapour (dry); and (iv) clear-sky with aerosols via McClear. Contributions are quantified by differences between configurations: cloud radiative effect (clouds vs clear-sky no aerosol), aerosol direct effect (clear-sky no aerosol vs clear-sky with aerosols), and water vapour effect (clear-sky no aerosol vs dry). Model skill was validated against observed time series and ERA5 surface irradiance, and sensitivity of direct aerosol effect to AOD was checked. Box plots of hourly precipitable water, cloud cover, and AOD (2004–2020, 5–18 UTC) put 2020 in climatological context.

- Spring 2020 in Western Europe, particularly The Netherlands, set records: daily mean integrated global horizontal irradiance (GHI) at Veenkampen was 206 W m^-2 for March–May, exceeding the previous 2011 record by 25 W m^-2; diffuse fraction was a record low 38% (vs 49–58% in other top-10 springs). Sunshine duration reached 805 h vs 517 h normal. Clouds reduced daily irradiance by only 22% relative to clear-sky, compared with a 2004–2020 mean reduction of 36%. - Persistent anticyclonic blocking led to dry, cloud-free conditions: 500 hPa height anomalies showed blocking centred over the UK; total cloud cover in spring 2020 had the lowest mean, median, and percentiles since 2004 (mean 0.50 vs 0.66 climatology; 0.05 lower than 2011). - Atmospheric dryness was exceptional: ERA5 precipitable water during 22–26 March averaged 4.0 ± 1.2 kg m^-2 vs 11.5 ± 4.2 kg m^-2 (1981–2019 mean for that period). Radiosondes corroborated very low moisture. - Aerosols: Spring 2020 had the lowest median hourly AOD since 2004 (CAMS), but minima and 5th percentiles were not unprecedented; the reduction manifested as frequent low-AOD hours rather than new extremes. Separating aerosols from thin cirrus is challenging in sun-photometer/satellite data; CAMS was used to contextualize AOD. - Aviation/contrails: Flight departures dropped sharply in late March. MODIS imagery indicated about twice as much cirrus and 50% more contrail contamination in 2011 and 2015 compared to 2020. Contrail-cirrus likely contributed to the exceptionally low diffuse fraction in 2020, but had minor effect on global irradiance. - Radiative transfer attribution (relative to 2010–2019 means): cloud reduction increased surface irradiance by +30.7 W m^-2 (+17.6%); reduced aerosols increased irradiance by +2.3 W m^-2 (+1.3%); reduced water vapour enhanced clear-sky irradiance by +1.5 W m^-2. At noon on a case day (29 April), removing clouds increased irradiance by ~250.7 W m^-2, removing water vapour by ~130.8 W m^-2, while aerosols reduced it by ~12.5 W m^-2. - Overall, the extreme irradiance was dominated by exceptionally cloud-free, dry weather; aerosol and contrail reductions during COVID-19 played a much smaller role and were not necessary for the record to occur.

The study directly addresses whether lockdown-related reductions in anthropogenic aerosols and aviation materially drove the record springtime irradiance. The quantitative attribution shows that anomalously low cloud cover associated with blocking and a dry atmosphere overwhelmingly controlled the irradiance increase, with aerosol reductions providing an order-of-magnitude smaller enhancement. While reduced contrail-cirrus plausibly lowered diffuse fractions, its impact on global surface irradiance was minor compared to clouds. The findings align with the broader brightening trend in Europe stemming from longer-term aerosol reductions, yet they emphasize that synoptic-scale weather anomalies can be decisive for seasonal extremes. Debates persist over trends in European circulation and blocking; given current uncertainties, the study cannot confirm a trend toward more sunshine-favourable patterns. Nonetheless, with already low aerosol levels in Western Europe, future seasonal irradiance records will likely hinge more on weather variability than further aerosol changes, whereas larger aerosol effects may occur in more polluted regions. The results are relevant for understanding radiative impacts of abrupt emission changes and for solar energy forecasting under extreme synoptic regimes.

Spring 2020 delivered unprecedented surface solar irradiance in Western Europe, primarily due to exceptionally low cloudiness and dry atmospheric conditions linked to blocking. Radiative transfer experiments attribute a ~30.7 W m^-2 increase to reduced clouds versus only ~2.3 W m^-2 to lower aerosols and ~1.5 W m^-2 to reduced water vapour relative to recent climatology, demonstrating that weather dominated over COVID-19-related aerosol/contrail reductions. Given ongoing decreases in European aerosol emissions, weather is poised to be the key driver of future spring irradiance records. Future research should quantify rapid cloud adjustments and aerosol–cloud microphysical effects omitted here, explore potential circulation responses to abrupt emission changes with longer-term global modeling, and assess regional contrasts where higher pollution levels could lead to larger aerosol-driven irradiance changes. Continued multi-platform observations will help refine attribution as more pandemic-period data become available.

- The radiative transfer framework quantifies instantaneous clear-sky direct effects and cloud radiative effects but does not capture rapid adjustments or aerosol–cloud microphysical indirect effects, which could modify cloud properties and circulation. - Attribution relies on differences between model experiments using ERA5 and McClear; uncertainties in reanalysis cloud fields, overlap assumptions, and aerosol inputs can influence results. - Separation of aerosol signals from thin, homogeneous cirrus is challenging in sun-photometer and satellite retrievals; CAMS AOD was used for consistent statistics but carries its own uncertainties. - Contrail assessment is based on manual inspection of twice-daily MODIS imagery near noon and a regional domain, potentially missing thin or transient features. - The study focuses on a single season and region; proving a circulation response to emission changes is not feasible with this dataset alone.