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

TL;DR

Spring 2020 saw record high solar irradiance in Western Europe, mainly due to unusually dry, cloud-free weather conditions during COVID-19 lockdown, with reduced aerosols playing a lesser role.

Contribution

This study quantifies the relative impacts of weather conditions and reduced aerosols on record solar irradiance during the COVID-19 lockdown in Western Europe.

Findings

Record high surface irradiance in 2020 surpassing previous records.

Dry and cloud-free weather contributed most to the irradiance increase.

Reduced aerosols and contrails had a smaller effect than weather conditions.

Abstract

Spring 2020 broke sunshine duration records across western Europe. The Netherlands recorded the highest surface irradiance since 1928, exceeding the previous extreme of 2011 by 13 %, and the diffuse fraction of the irradiance measured a record low percentage (38 %). The coinciding irradiance extreme and a reduction in anthropogenic pollution due to COVID-19 measures triggered the hypothesis that cleaner-than-usual air contributed to the record. Based on analyses of ground-based and satellite observations and experiments with a radiative transfer model, we estimate a 1.3 % (2.3 W m$^{-2}$) increase in surface irradiance with respect to the 2010-2019 mean due to a low median aerosol optical depth, and a 17.6 % (30.7 W m$^{-2}$) increase due to several exceptionally dry days and a very low cloud fraction overall. Our analyses show that the reduced aerosols and contrails due to the COVID-19 measures are far less important in the irradiance record than the dry and particularly cloud-free weather.